Life Sciences

▪ 2009

Introduction

Zoology

      In 2008 several zoological studies provided new insights into how species' life-history traits (such as the timing of reproduction or the length of life of adult individuals) are derived in part as responses to environmental vagaries. The findings had implications for both short- and long-term evolutionary responses of animals to global climate change, harsh natural environments, and infectious disease. Anne Charmantier of the University of Oxford and colleagues reported on their examination of the behavioral adjustments of a wild-bird population of great tits (Parus major) that had been studied since 1961. The long-term data set included information on seasonal temperature changes, the timing of the emergence of a vital prey (larvae of the European winter moth, Operophtera brumata) for the birds' young, and the reproductive success of the bird population. By 2008 the average date on which the female birds laid eggs had shifted to about two weeks earlier than in the 1970s, a gradual change that tracked an increase in the environmental temperatures that preceded egg laying over the same time period. The timing of peak abundance of winter-moth larvae had also shifted in response to environmental temperatures. In order for the birds to capitalize on the availability of this key prey for their young, the females had to adjust when they laid eggs each year, since the optimal time changed annually in response to early spring temperatures. On the basis of analyses of the annual timing of the birds' egg laying and rearing of young in response to environmental temperature fluctuations, the investigators concluded that the population responded successfully to regional climate change by adaptive phenotypic plasticity of individual birds rather than by a genetically based response.

      Curtis A. Deutsch, Joshua J. Tewksbury, and Raymond B. Huey of the University of Washington at Seattle and colleagues constructed thermal performance curves for terrestrial insects from around the world through the use of a global data set that related population growth rates of insects to environmental temperatures. The investigators then used the performance curves to predict the direct impact that rising environmental temperatures might have on insect fitness at different latitudes. Even though greater increases in environmental temperatures were expected in temperate regions, the smaller warming in tropical regions was predicted to have greater impact on insects because tropical species lived at close to their optimal temperature and had limited capacity to adjust to change. Species living at temperate latitudes generally operated at conditions appreciably cooler than their optimal temperature, a situation in which an increase in temperatures might enhance fitness. One conclusion from the analyses was that the greatest risk of extinction from global warming would occur in species living in the world's regions of greatest biological diversity, the tropics.

      Among living tetrapods—amphibians, reptiles, birds, and mammals—virtually all species live one year or more after they are hatched or born, and females typically reproduce several times in their lifetime. In a dry desert region of Madagascar, Kristopher B. Karsten of Oklahoma State University and colleagues discovered an unusual chameleon that lived most of its life in the egg stage and whose females reproduced only once in their lifetime. The investigators found that all individuals of the chameleon, Furcifer labordi, were the same age. The entire population hatched from eggs in November. They mated about two months later, and after the females laid their eggs, both sexes became senescent. The adults died within five months of hatching—the shortest postembryonic life span ever reported for a tetrapod. The entire species then persisted for at least six months each year solely in the egg stage. It was uncertain how such an unusual life-history pattern might have evolved, but presumably it was one strategy for a species that lived in an extremely harsh and unpredictable seasonal environment where high adult mortality led to the evolution of shorter life spans. The confirmation that some chameleons were naturally short-lived had important implications to conservation programs that held animals in captivity to form groups known as assurance colonies for later release into the wild.

      Menna E. Jones of the University of Tasmania and colleagues investigated changes in the life-history traits of populations of the Tasmanian devil (Sarcophilus harrisii), a carnivorous marsupial endemic to Tasmania. Tasmanian devil populations were being devastated by a contagious cancer called devil facial tumour disease (DFTD). The disease produced large tumours around the head and mouth that interfered with eating and invariably led to death within a few months. Researchers first noted DFTD among Tasmanian devils in 1996. By 2007 it was present in at least one-half of the populations of the species, and some infected populations had declined by about 90%. Susceptibility to DFTD was believed to be a consequence of low diversity in the genes that facilitated the animal's immune responses to tumours, and the spread of the infection was promoted by the physically aggressive biting behaviour among individuals during the mating season. The investigators examined demographic data of Tasmanian devil populations from five locations before and after the appearance of the disorder, and they determined that the proportion of animals that were more than three years old in a given population was greater before than after the onset of the disease. Also, in most populations before the onset of the disease, a majority of females produced several litters between ages two and four, and no females bred before then. After DFTD became prevalent, the number of females that bred early increased by 16 times on average. Despite an unprecedented shift by most females in the population to begin breeding at significantly earlier ages, the spectre of extinction of Tasmanian devils continued to be a major conservation concern. Plans to save the species included developing a vaccine against DFTD, keeping healthy Tasmanian devils in zoos and breeding programs under quarantine, and building fences to protect healthy populations in the wild from infected animals.

      Many animals communicate with others of their species for reproduction, and the challenges in such communication range from situations in which being too quiet is ineffective to situations in which being too loud can be dangerous. A study by Ryo Nakano of the University of Tokyo and Takuma Takanashi of the Forestry and Forest Products Research Institute, Tsukuba, Japan, and colleagues in Japan and Denmark reported on a moth that produced ultrasonic sounds during courtship. The male Asian corn borer moth, Ostrinia furnacalis, directed the low-intensity sounds toward a nearby female. Predators or other males that might compete for the same mate could not detect the quiet sound. Yet the nearby female could hear the courtship sounds, which enhanced the male's opportunity for mating. The investigators determined that the male produced the sound by rubbing specialized scales on the wings against the thorax. Further investigation revealed that production of low-intensity ultrasonic sounds during courtship was common among a variety of species in other families of moths.

      Jun-Xian Shen of the Chinese Academy of Sciences, Beijing, and colleagues discovered another type of ultrasonic communication—in an amphibian. During ovulation female Chinese torrent frogs, Odorrana tormota, produced ultrasonic sounds that signaled when they were ready to mate. After ovulation, the females did not produce the call. The males gave advertisement calls during the mating season, but the female calls were distinctive in having a higher frequency and shorter duration. The call of the female informed males that she was ready to mate and indicated her location in a densely forested habitat. Male torrent frogs had a hyperacute ability to detect the call amid high ambient noise levels created by stream waters and to determine the female's location precisely. The production of high-frequency sounds by females and the males' ability to pinpoint their source were most likely adaptations for communicating in the noisy habitat of torrential streams.

      One of the oddest vertebrates is the platypus (Ornithorhynchus anatinus), a type of mammal called a monotreme. Platypuses lay eggs like reptiles and birds but have fur and feed their young milk produced from lactate glands with no nipple. Other unusual features of the platypus include the presence of a bill with electrosensory pits, the absence of teeth in adults, and—in males—the production of venom, which they apply through spurs on the hind feet. Geneticist Wesley C. Warren of the Washington University School of Medicine, St. Louis, Mo., and an international consortium sequenced the entire genome of the species to assess the evolutionary relationships between platypuses, other mammals, birds, and reptiles. Comparative investigations of protein-coding and non-protein-coding genes and the reading of some 26.9 million DNA sequences revealed information on the genomic evolution of mammals. The findings showed that the venoms of reptiles and monotremes evolved independently as the result of convergent evolution and that the milk-producing genes were conserved from a mammalian ancestry. The study also confirmed that marsupials and placental mammals are more closely related to each other than either is to monotremes.

J. Whitfield Gibbons

Botany

      In 2008 progress was made in creating genetically modified ( GM) plants to produce pharmaceutical drugs. The production of pharmaceuticals derived from GM plants had proved to be efficient on a large scale, but little research had been done in using GM plants for vaccines against cancer and other chronic diseases. In one report Alison McCormick of Touro University California's College of Pharmacy and colleagues described new plant-made vaccines that they had developed for treating non-Hodgkin lymphoma cancer. The researchers were able to use the GM plant technique to make vaccines tailored to individual patients, which was important because the molecular signature of the lymphoma tumour cells differed from patient to patient. The researchers created the vaccine by isolating the antibody to each patient's tumour and inserting the gene for that antibody into a modified version of the tobacco mosaic virus, which was then used to infect a tobacco plant. The virus carried the gene into the plant's cells, where the antibody was produced, and after a few days the antibody was extracted and purified. Only a few plants were needed to make enough vaccine for each patient. The results of a phase 1 clinical trial showed that 70% of the patients developed an immune response to the plant-made vaccine.

      In another study South Korean researchers showed that the tomato plant held promise as a suitable plant for producing a possible oral vaccine against Alzheimer disease. The researchers produced GM tomatoes engineered with the human gene for beta-amyloid, a peptide that was believed to be one of the major components of Alzheimer disease. The gene was introduced into the tomato plants by infecting them with a genetically engineered bacterium belonging to the genus Agrobacterium. When mice were fed soluble extracts from the plants, the beta-amyloid triggered an immune response. The researchers hoped that it would eventually be possible to reduce the accumulation of beta-amyloid in the human brain in this way and thereby inhibit the degeneration of neuron cells.

      Scientists discovered how a gene known as SUN controlled the shape of fruit. The fruit of the wild ancestral tomato plant was small and round, but cultivated varieties came to have a wide range of shapes and sizes. After investigating the molecular basis of the SUN gene's effect on elongation, Esther van der Knaap and colleagues at Ohio State University and Michigan State University reported that a duplication of a DNA sequence in the SUN gene had increased the gene's expression and had led to the elongated shape of the fruit. The gene-duplication event might have been caused by a DNA element called a retrotransposon, which inserted itself within the plant's genome, or genetic code, and increased the expression of the gene. The authors said that their findings demonstrated that retrotransposons might be a major driving force in genome evolution, especially in plants. The discovery might also help unravel the mystery behind the huge differences in shape among fruits and vegetables and might provide new insights into the basic mechanisms of plant development.

      More evidence came to light concerning the effects of climate change on plants. Researchers from AgroParisTech in France surveyed 171 species of forest plants across six Western European mountain ranges by reviewing about 8,000 plant surveys that had been collected between 1905 and 2005. The researchers found that more than two-thirds of the species had climbed in elevation over those 100 years and that the average increase in their optimum elevation was 29 m (95 ft) per decade. The shift to higher elevation was greater for plant species whose habitat was restricted to mountains. Average temperatures in Western Europe rose by nearly 1 °C (1.8 °F) during the 20th century, and these results added to the growing body of evidence that increasing temperatures were leading to the migration of plants in search of cooler climates. The study also showed that quick-breeding grasses had moved up mountains more quickly than slower-growing trees. This disparity raised concerns that communities of plants would disintegrate and possibly affect the animals that relied on them for food and shelter.

      Flowers typically used scents to attract their pollinators, but a new study revealed that tobacco flowers used a mixture of both attractants and repellents to regulate their pollination and defend themselves. A team of botanists led by Ian Baldwin at the Max Planck Institute for Chemical Ecology in Jena, Ger., found that tobacco flowers produced nectar with both benzyl acetone, which had a sweet smell, and nicotine, which had a bitter taste and was poisonous. The study selectively blocked the production of each scent to see how they affected the plant's pollination. The nicotine repelled predatory insects that tried to rob the nectar or eat the flowers. The nicotine also prevented pollinators from lingering too long at any one flower and thereby caused them to visit more flowers and increase the chances of cross-pollination. The proper dose of both attractant and repellent chemicals was needed to optimize pollination by enticing pollinators to the flower and then persuading them to leave shortly afterward. “This … shows just how sophisticated a plant can be in using chemistry to get what it wants,” commented Baldwin.

      A team led by Sarah Sallon of the Louis Borick Natural Medicine Research Center at Hadassah Hospital in Jerusalem managed to germinate a Judean date-palm seed that was thought to be at least 2,000 years old. It was the oldest seed to have been successfully germinated. The seed was found at Masada, the hill fortress overlooking the Dead Sea that was besieged by the Romans in AD 72–73. The scientists treated the seed with hormones, and after eight weeks it began to sprout. It grew over 26 months into a healthy sapling 1.5 m (4.9 ft) tall, which was comparable to modern date seedlings. Radiocarbon dating of fragments of the seed's shell that clung to the plant's roots when it was transferred to a larger pot pinpointed the age of the seed. “The exceptionally dry and hot climatic conditions at Masada may have prevented it from disintegrating and preserved its viability, but this still says a lot about the ability of seeds to survive,” said Sallon. The study of the viability of such ancient seeds was important for understanding conservation techniques for seed banks, and it might also help in modern date-palm cultivation and breeding. (See Environment: Sidebar (Seed Banks-Preserving Crop Diversity ).)

Paul Simons

Molecular Biology and Genetics

The Genetics of Stress Response.

      Physical traits often run in families. Tall parents tend to have tall children; short parents tend to have short children; blond-haired parents tend to have blond-haired children; and so forth. Emotional or behavioral traits also tend to run in families, although these traits can be more complex and difficult to quantify. Anxiety disorder (the tendency to experience excessive anxiety relative to a stimulus) is a behavioral trait that demonstrates 40–60% heritability. This level of heritability indicates that environmental factors, such as stressful conditions, and genetic factors, such as those that influence how stress is perceived and accommodated, are both very important in contributing to the etiology of the disorder. A study published in April 2008 by a team of researchers led by David Goldman of the U.S. National Institutes of Health was an important step toward dissecting the genetic factors that contribute to anxiety disorder. It provided insights into the basis not only of the disorder but also of the normal variations in responses to stress.

      The study consisted of several components. One component explored the functional significance of normal genetic variation in the gene NPY, which encodes a 36-amino-acid peptide called neuropeptide Y. The peptide is expressed at high levels in regions of the brain that are associated with arousal and emotional response to a stress-inducing challenge. Previous studies had demonstrated that neuropeptide Y is released in the brain in response to stress and that its release helps to control characteristic fight-or-flight hormonal and metabolic responses to stress, such as an increase in heart rate. The researchers hypothesized that natural genetic variation in the NPY gene might lead to variation in the expression of neuropeptide Y, which in turn might correlate with variation in stress response from individual to individual (a characteristic called trait anxiety). To test their hypothesis, the researchers identified seven naturally occurring variations in the human NPY gene sequence. They then took DNA samples from a large number of study volunteers and characterized the samples with regard to these variations. The resulting data enabled them to classify the NPY alleles into haplotypes (groups of alleles defined by the presence and absence of specific DNA-sequence markers). Since humans carry two copies of most genes—one maternally inherited and one paternally inherited—the volunteers in the study could be further categorized by the diplotype (set of two NPY haplotypes) each person happened to carry.

      The researchers then tested the possible impact of NPY diplotype on the expression of neuropeptide Y by measuring the level of neuropeptide-Y messenger RNA (mRNA) in lymphoblast cells from 47 volunteers whose NPY diplotype had been determined. The results demonstrated a threefold range in neuropeptide-Y mRNA levels and a clear correlation between NPY diplotype and the expression level of the NPY mRNA. A similar correlation between NPY diplotype and neuropeptide-Y mRNA levels was observed from studies of 28 postmortem brain samples and from an independent study of neuropeptide-Y levels in plasma samples derived from a separate study of 42 subjects.

      Next, the researchers sought to test whether NPY diplotypes associated with low, medium, or high neuropeptide-Y expression levels might also correlate with brain responses to emotion and stress. They applied a technique called functional magnetic resonance imaging (fMRI) to detect amygdala and hippocampal activation in 71 study volunteers who were subjected to transient stress by showing them images of threatening facial expressions. The fMRI provided real-time and noninvasive measurement of small changes in the blood flow or oxygenation levels of tissues. Since the amygdala governs arousal, emotional response, and autonomous responses to fear and the hippocampus functions in establishing memory and is influenced by stress, small changes in the blood flow or oxygenation levels of these regions of the brain served as quantifiable markers for the emotional recognition of and response to stress.

      The results were striking. Amygdala activation in stressed study volunteers with a diplotype associated with low NPY expression was significantly higher than in study volunteers with a high NPY-expression diplotype. Indeed, NPY diplotype accounted for 9% of the variance observed in amygdala activation among the volunteers. Studies of task-related hippocampal activation also demonstrated a significant correlation with NPY diplotype.

      To extend their work from imaging studies to trait anxiety, Goldman and colleagues used the Tridimensional Personality Questionnaire to characterize 137 study volunteers on various measures of harm avoidance. From these data the researchers found statistically significant, although modest, correlations between an individual's NPY diplotype and both fear of uncertainty and anticipatory worry, but they found no correlation between NPY diplotype and either shyness with strangers or fatigability and asthenia (loss of strength). Considering the multitude of factors that influence emotional perception and response, it was remarkable that normal, naturally occurring sequence variations in one gene, NPY, could be demonstrated to have such an impact.

Seasonal Susceptibility to Influenza.

      Despite efforts to promote widespread immunization, every year in the United States and many other countries, 5–20% of the population becomes infected with influenza (flu) virus and experiences symptoms such as high fever, headache, fatigue, nasal discharge, sore throat, muscle aches, gastrointestinal upset, and general misery. In addition, many thousands of people die every year from influenza or its complications.

      Influenza is generally spread by aerosol transmission, particularly when an infected person coughs or sneezes in proximity to others. Influenza can also be transmitted when a person touches a surface contaminated with the virus from an infected person and then inadvertently touches the mucous membranes of the nose or mouth with the contaminated hand or finger.

      A notable characteristic of influenza infection in the Northern Hemisphere is that it is seasonal. Influenza peaks in the winter, and the months from November to March are typically considered to constitute the flu season. Although the seasonal epidemiology of influenza infection was long recognized, it was poorly understood. In 2007, however, experiments were reported that convincingly demonstrated that temperature and humidity affect flu transmission, and in 2008 a study emerged that provided clear evidence of a mechanism to explain this effect.

      This study, by Joshua Zimmerberg and colleagues from the U.S. National Institutes of Health, concerned the properties of substances, called phospholipids, that make up the influenza viral envelope. The researchers used a methodology called proton magic-angle spinning nuclear magnetic resonance to probe the ordered-versus-disordered arrangement of the phospholipids at different temperatures. At cool to cold temperatures (temperatures below 22 °C [72 °F]), the phospholipids formed an ordered gel phase, which the researchers believed would protect the virus from the elements and thereby extend its survival during transmission. At warmer temperatures, such as those common in the summer, the phospholipid envelope melted into a liquid phase, which the researchers believed would not protect the virus effectively against the environment. Thus, its survival and the range of its transmission would be limited.

      The study not only offered a logical explanation for the seasonal nature of the epidemiology of influenza but also presented new approaches to preventing influenza transmission. For example, compounds might be designed to disrupt the organization of the phospholipids in the viral envelope at cool temperatures. The results of the study also suggested that other viruses that use a phospholipid envelope to shield themselves from the environment during transmission might demonstrate similar properties.

Judith L. Fridovich-Keil

Paleontology

      In 2008 scientists from The Netherlands, the United States, and Yemen published a report on the first dinosaur trackways discovered on the Arabian Peninsula, a region in which dinosaur finds were extremely rare. At a site in Yemen, more than 100 footprints were found in two sets of tracks. One was made by a single bipedal ornithopod and the other by a herd of 11 quadrupedal sauropods. A new dromaeosaur, Mahakala omnogovae, was reported from fossil remains discovered in Mongolia. The small size of the animal's body, together with its phylogenetic placement, indicated that extreme miniaturization was typical in ancestral members of the clade that contained Dromaeosauridae, Troodontidae, and Avialae (which included all birds). A report presented at the annual meeting of the Society of Vertebrate Paleontology in late 2007 reevaluated the status of four species of pachycephalosaurs—dinosaurs with distinctive thick skulls with bony spikes along their snouts. The paper argued that two of the species were actually juveniles of the largest species, Pachycephalosaurus wyomingensis, and that the skulls of juvenile pachycephalosaurs changed considerably as they matured.

      The presence of collagen proteins in soft-tissue samples of Tyrannosaurus rex was reported in April 2007 by researchers who analyzed material that had been discovered inside a Tyrannosaurus femur (thighbone) two years earlier. A study published in January 2008 by a second group of researchers examined the collagen peptide sequences in the samples and determined that the reported Tyrannosaurus collagen peptide sequences were not valid. A response paper by the authors of the 2007 report claimed that the methods that were used in the second study were less accurate than mass spectrometry, the method that they had used. A third report, published in August 2008, claimed that the blood vessels and soft-tissue cells that had been described in the Tyrannosaurus were instead biofilms formed by bacteria that had invaded the fossils after death. At year's end the issues remained in dispute.

      A newly studied fossil of a bat from the Early Eocene (about 50 million years ago) of Wyoming exhibited features that were more primitive than those of other known bat fossils. The fossil indicated that the bat was capable of flight, like other Eocene bats, but the ear structure showed that the bat would have lacked the ability to use echolocation. This research supported the hypothesis that flight evolved in bats before echolocation.

      Although whales were known to be related to artiodactyls (even-toed ungulates, such as pigs and deer), the first fossil of an early artiodactyl that was similar morphologically to early whales was not reported until late 2007. The report described a fossil of Indohyus, a small raoellid artiodactyl from the Eocene of southern Asia, and it showed that the animal was closely related to whales. Unlike other artiodactyls, Indohyus had an ear structure and premolars that were similar to those of early whales, and the limb-bone density and stable-oxygen-isotope composition of its teeth were also similar. The study concluded that the raoellids were aquatic waders and that an aquatic lifestyle developed in this lineage prior to the origin of whales.

      The closest living relatives of the elephants were sea cows and hyraxes, but the three groups were highly divergent from their common ancestor. A study of fossil teeth from Moeritherium, a 37-million-year-old elephant from Egypt, suggested that these animals spent most of their time in the water and that their lifestyle was therefore more similar to that of sea cows than modern elephants. The study further found that the carbon and oxygen composition of the teeth of Moeritherium was more similar to that of modern aquatic mammals than terrestrial animals.

      The earliest-known primitive ungulates had long been types of condylarths from the Early Paleocene of Montana. A recent report, however, documented an earlier condylarth specimen—from the late Cretaceous. An isolated molar from lake deposits of central India not only extended the fossil record of condylarths back into the Mesozoic Era but also showed that early condylarths had been more widely distributed than was previously thought.

      The sparse fossil record of the monotreme duck-billed platypus was greatly extended back in time when a misidentified specimen was reidentified. A computer tomographic scan of fossil jaws that were found in Australia and were thought to belong to Teinolophos trusleri indicated that the enigmatic fossil was actually a fossil platypus. Dated to 112 million years ago, the fossil was by far the oldest monotreme specimen discovered to date. The next-oldest-known specimen was a 62-million-year-old fossil tooth from Patagonia. (Although the adult platypus lacks teeth, juveniles have distinctively shaped compressed teeth.)

      A newly described specimen of the Permian xenacanthid shark Triodus had two larval temnospondyl amphibians and a small acanthodian fish preserved in its digestive track. This unusual find provided evidence of a three-level food chain. The acanthodian fish had been ingested by one of the temnospondyls, which in turn had been eaten by the shark.

      A newly described fish from the Late Devonian Gogo Formation of Australia represented the oldest record of a live-bearing vertebrate in the fossil record. The new ptyctodontid placoderm, Materpiscis attenboroughi, preserved an intrauterine embryo connected by a permineralized umbilical cord. A second ptyctodont, Austroptyctodus gardineri, from the same formation showed three small embryos in the same position.

      The origin of modern amphibians (frogs, salamanders, and caecilians) was controversial primarily because of the large gaps in the fossil record between modern forms and two possible ancestral groups, the Temnospondyli and Lepospondyli. A newly reported temnospondyl fossil, Gerobatrachus hottoni, from the Early Permian of Texas showed characters found in both frogs and salamanders and therefore partially bridged the gap between modern amphibians and their Paleozoic ancestors.

       Antarctica had long been the coldest place on the Earth, but a team of scientists found fossil evidence that it was once warm enough for tundra plants and animals. The scientists reported finding 14-million-year-old fossil stems and leaves of a semiaquatic moss in deposits of an ancient alpine lake in one of Antarctica's Dry Valleys. The most unusual thing about the fossils was that they were freeze-dried; when rehydrated they resembled living specimens. The bodies of ostracods (seed shrimp) with intact soft parts were also collected.

      The Paleozoic machaeridians were an enigmatic group of wormlike organisms known mainly from shell plates found in ocean-bottom assemblages. At various times over the past 150 years, they had been considered to be related to barnacles, echinoderms, mollusks, or annelid worms. A newly described machaeridian fossil from Morocco was found with rarely preserved soft parts, including parapodia and chaetae. The presence of these structures indicated that the machaeridians were clearly related to the annelids. Another example of an exceptionally preserved fossil was a jellyfish described from Middle Cambrian deposits of Utah. The specimen, found in the Marjum Formation, showed soft-tissue structures such as tentacles and subumbrellar and exumbrellar surfaces.

      Carnivorous fungi from 100 million years ago were found preserved in Cretaceous amber from southwestern France. The fungi contained structures called hyphal rings that had trapped small nematode prey, which were also preserved. On the basis of the type of hyphal rings found in the fossils, the fungi did not belong to any group of living carnivorous fungi. The authors suggested that the hyphal structures for trapping prey developed independently in various lineages of fungi through geologic time.

William R. Hammer

▪ 2008

Introduction

Researchers studied the origin of cat domestication, variations in dog size, hormone structure in plants, and chemical changes in the proteins that package DNA. Genetic engineering created novel strains of rice and mosquitoes to combat malaria, and rice strains with human genes began to be grown commercially.

Zoology

      Zoological research in 2007 provided new insights into the domestication of cats. Cats were known to have been associated with humans as early as 9,500 years ago from archaeological evidence on Cyprus, but their evolutionary line from wild ancestors and the region where they were first domesticated had been uncertain. To determine the origin of cat domestication, Carlos A. Driscoll of the Laboratory of Genomic Diversity, Frederick, Md., and colleagues obtained and analyzed DNA from 851 individual nondomestic (wild) and domestic cats to determine their genotypes. The nondomestic cats included European cats, Near Eastern wildcats, central Asian cats, southern African wildcats, and Chinese desert cats, all of which were considered subspecies of Felis silvestris. The sample of domestic cats included both feral domestic cats and recognized breeds of housecats. A separate wild species, the sand cat (F. margarita) of North Africa and the Middle East, was chosen as the closest outgroup (group belonging to a separate evolutionary branch). Genetic mixing is extensive between feral domestic cats and wildcats throughout their geographic ranges, but the genetic evidence supported the conclusion that cats were first domesticated in the Middle East, presumably during the rise of agriculture in the Fertile Crescent. The investigators estimated that the common ancestor for the Near Eastern wildcat subspecies (F. s. lybica) and domestic cats lived approximately 131,000 years ago.

 Nathan B. Sutter and Elaine A. Ostrander of the National Human Genome Research Institute, Bethesda, Md., and colleagues determined that a single allele (gene variant) is the major genetic determinant of body size in domestic dogs. The gray wolf (Canis lupus) is the accepted ancestor of the domestic dog (C. familiaris), but as a consequence of centuries of selective breeding, the latter had one of the greatest ranges of body size among terrestrial vertebrates. The largest dogs weighed 50 times more than the smallest. Several genetic explanations for the observed variability had been suggested, but none had been confirmed. The investigators located genetic sequences related to size on a section of chromosome 15 in the Portuguese water dog, a recognized domestic breed with a wide range in size. They discovered that an allele of the gene that encodes the insulin-like growth factor 1 (IGF-1) was present in small dogs but typically absent in large ones. The genetic association between body size and the IGF-1gene was also apparent in 14 small breeds of dogs and typically absent in 9 that were classified as giant breeds. IGF-1 had been shown in earlier studies to affect the body size of mice and humans. The findings helped clarify the genetic origin of size diversity among domestic dogs and also revealed how natural selection on a single gene could lead to rapid evolution in body size in species undergoing adaptive radiations.

      Andrew F. Russell of the University of Sheffield, Eng., and colleagues provided insight into previously unrecognized benefits of cooperative bird-breeding systems in which nonbreeding helper males assisted in providing food for offspring. In such species the young are given more food when helper males assist with feeding, and helper males are presumed to benefit from kin selection (by being closely related to the offspring) or group augmentation (such as a greater efficiency in acquiring resources by being associated with other individuals). How females or offspring benefit from the presence of helper males, however, had been difficult to assess because when offspring received additional food, their fledgling size and survival were often unaffected. The investigators compared breeding units of the superb fairy-wren (Malurus cyaneus) of Australia. Some of the breeding units consisted only of breeding pairs, and others contained helpers. In cooperative breeding units with helpers, the young received 19% more food than in breeding units without helpers. When helpers were present, however, mother wrens laid smaller eggs that had reduced nutritional content and produced smaller chicks. The benefits that offspring received when helpers were present were thereby concealed by the overall reduction of the females' investment in their eggs. Experiments in which eggs laid by a female in a group with helpers were substituted for eggs laid by a female in a breeding pair and vice versa gave further confirmation that the birds compensated for the presence or absence of helpers by adjusting egg and hatchling size. For example, chicks from helper-group eggs incubated and raised only by breeding pairs exhibited reduced growth and survival. (The incubation period and the time that the superb fairy-wren chicks remained in the nest did not vary.) The investigators showed that the advantage for females when helpers were present was the reduction in their reproductive investment, which increased their fitness and probability of breeding again.

      Two independent teams of researchers reported on two major genome-sequencing studies. The Rhesus Macaque Genome Sequencing and Analysis Consortium, under the leadership of Richard A. Gibbs of Baylor College of Medicine, Houston, sequenced the genome of the rhesus macaque monkey (Macaca mulatta). The species had been used as the premier nonhuman primate in biomedical research for decades, including studies on viruses that caused flu, polio, and AIDS, and it was the species in which the blood protein known as the Rh factor was first identified. Using a genetic map with an estimated 20,000 genes, researchers expected to be able to target particular traits expressed in individuals and use genetic pathways to identify and manipulate specific genes that were responsible for the trait. In addition to the value of the genome sequence for biomedical research, the sequence provided unprecedented opportunities for examining at the genome level the evolutionary relationships and changes between humans, chimpanzees (the closest living relative of humans), and rhesus macaques, which had a common ancestor 25 million years ago. A team led by Tarjei S. Mikkelsen of the Massachusetts Institute of Technology sequenced the genome of the South American gray short-tailed opossum (Monodelphis domestica) in the first such work on a marsupial. The species had been used frequently in genetic research and in the fields of immunology and neurobiology. Comparison of the genome of a metatherian ( marsupial) with those genomes available for eutherian (placental) mammals offered the prospect of insight into genomic function, organization, and evolution among mammal lineages. An initial finding was that in the opossum only about 1% of the genetic regions that code for amino-acid proteins, compared with about 20% for noncoding regions, had evolved since the divergence of metatherians and eutherians 180 million years ago.

      Life in the deep-ocean portions of the Southern Ocean, which encircles Antarctica, had been poorly explored compared with other oceans and with the shallow-water habitats of the Antarctic region. Angelika Brandt of the Zoological Museum, Hamburg, and colleagues provided the first overview of the zoological diversity of these deep-sea communities, based on their investigations in the Weddell Sea. A variety of sampling techniques, including underwater photography, bottom coring, and bottom and midwater trawling, were used during three expeditions between 2002 and 2005, at depths as great as 6,348 m (20,827 ft). The zoological distinctiveness and unexplored nature of the deep waters of the Southern Ocean were apparent in several ways. The collection of samples of more than 13,000 crustaceans known as isopods yielded 674 species, of which 86% had previously been unknown, and the number of isopod species found was 1.8 times greater than that known from the shallower depths of the entire Antarctic continental shelf. Numerous species were found among other major taxonomic groups, including foraminifers (158), nematodes (57), ostracods (more than 100), polychaete worms (more than 200), shelled gastropods and bivalves (160), and sponges (76). At least 20% and for most of the groups more than 50% of the species collected were new to science. The investigators noted several biogeographic trends. For example, among their deep-sea samples of isopods, ostracods, and nematodes—organisms that typically disperse poorly—there were species characteristically associated with the continental shelf and many not known outside the Southern Ocean. Organisms that were more likely to disperse, such as foraminifers, and that were found at great depths were more closely related to fauna found in other oceans, in particular the Atlantic Ocean. The observations dismissed an earlier perception that species diversity in the deep areas of the Southern Ocean is low, and they offered new opportunities for exploring the zoogeographic patterns and evolutionary relationships among the deep-sea and continental fauna.

      Shannon L. LaDeau and Peter P. Marra of the Smithsonian Migratory Bird Center, Washington, D.C., and A. Marm Kilpatrick of the Consortium for Conservation Medicine, New York City, used long-term records from the North American Breeding Bird Survey program to assess the impact of the West Nile virus on 20 species of birds. Adjustments were made for anticipated changes in population levels caused by climatic and ecological factors. Noticeable declines that coincided with the arrival of the virus in 1999 in New York were found in seven species. The greatest impact was observed in the population of American crows, which declined by as much as 45%. All of the bird species, including American robins and blue jays, were commonly associated with urban and suburban areas. The findings had implications concerning links between birds and humans, who were also susceptible to West Nile virus and other bird-transmitted pathogens.

J. Whitfield Gibbons

Botany

      In 2007 genetically modified (GM) plants that incorporated human genes began to be grown commercially for the first time. The plants, strains of rice modified to express human protein in their seeds, were made by Ventria Bioscience, a biotech company headquartered in Sacramento, Calif. Commercial planting of the rice in designated fields in Kansas was approved in May 2007 by the U.S. Department of Agriculture. The human proteins produced by the GM rice strains were lysozyme, lactoferrin, and serum albumin, which are commonly found in human breast milk. The company claimed that the rice-manufactured proteins could be taken orally to combat diarrhea and anemia, a potentially important application for less-developed countries. Before products made from the rice could be sold to consumers, however, further regulatory approvals would be needed. Critics of the project feared that the strains could contaminate normal rice strains and people would then unwittingly be exposed to the GM proteins by eating them in food. The journal Nature Biotechnology compared growing such pharmaceuticals in crops to packaging pills in candy wrappers. The USDA believed that the rice would not escape into the environment or enter the food supply, thanks to such safeguards as planting the GM rice more than 480 km (300 mi) away from any other rice farms.

      GM rice also showed promise for producing human vaccines. Japanese scientists engineered a strain of rice that carried a vaccine for cholera, which is caused by Vibrio cholerae bacteria. When the rice was fed to mice in scientific trials, it produced antibodies that targeted the vulnerable mucosal sites where cholera infection first occurs. In contrast, conventional vaccines that are administered via injection are less targeted. The rice-based vaccine also had the advantage that it could be stored at room temperature; conventional vaccines required refrigeration. Similar types of GM plants developed for vaccines could be particularly useful against other viruses that attack mucosal tissues in the body, such as human immunodeficiency virus (HIV).

      Zhixiang Chen of Purdue University, West Lafayette, Ind., and colleagues genetically modified plants to resist infection from the Cauliflower mosaic virus (CaMV), a virus that attacks many agriculturally important plants, including cauliflower, broccoli, and cabbages. CaMV uses reverse transcription to create copies of itself and spread the infection. In reverse transcription, the virus's RNA is copied into DNA after it latches onto a victim cell. In their investigation the researchers infected the plant Arabidopsis thaliana with cauliflower mosaic virus and found that reproduction of the virus makes use of cyclin-dependent kinase C complex (CDKC), a protein complex found in the plant cells. By blocking CDKC the scientists were able to make the plant completely resistant to infection from CaMV. This research might lead to ways of combating HIV, because HIV also uses reverse transcription and the same protein complex (referred to in human cells as positive transcription elongation factor b) to multiply and spread the infection.

      For the first time, scientists determined the structure of a plant hormone receptor and the means by which the receptor interacts with a hormone. A team led by Ning Zheng of the University of Washington School of Medicine studied the hormone receptor, TIR1, a type of enzyme known as a ubiquitin ligase, and its interaction with indole-3-acetic acid and two other plant hormones that are collectively known as auxin. Auxin plays an essential role in the growth and development of plants, and there was much conjecture about the nature of the receptor-hormone binding. The scientists extracted TIR1 from the plant Arabidopsis and purified it into crystals. Using X-ray images of the crystals, they determined the enzyme's three-dimensional structure. The crystals were then soaked in auxin and X-rayed again, which revealed that auxin functions as a “molecular glue” that improves the ability of TIR1 to bind its peptide target. In the absence of auxin, TIR1 does not bind its target as tightly. This discovery not only was a major advance for plant biology with important potential implications for agriculture but also might lead to new treatments of human cancers, because TIR1 is similar to human ubiquitin ligases that are involved in cancer. The scientists expected that these human enzymes might be affected by small molecules like auxin and that chemists might be able to synthesize such molecules as a new type of cancer drug.

 The parasitic plant Rafflesia arnoldii has the world's largest single flower, a red-and-white bloom that measures up to 1 m (3.3 ft) in diameter and stinks of rotting flesh to attract the small flies it needs for pollination. The classification of this and the other 20 or so species of rafflesias had long baffled scientists. A team led by Charles Davis at Harvard University examined eight genes of R. arnoldii and determined that the plant belongs to the Euphorbiaceae family, which includes poinsettias and bells of Ireland as well as commercially important crops such the rubber tree, castor-oil plant, and cassava. Given the inferred phylogeny, the scientists estimated that flower diameter among the plants in the evolutionary lineage of R. arnoldii underwent about a 73-fold increase over about 46 million years, one of the most dramatic cases of evolutionary size increase reported for any plant or animal.

      Biologist Santiago Ramírez and his colleagues at Harvard University identified the first fossilized remains of an orchid. The finding allowed them to work out the origins of orchids and solve a long-standing dispute over their evolution. Although orchids formed the largest family of flowering plants, they rarely fossilized. The fossilized remains that were found were particles of orchid pollen that covered a bee found preserved in fossil amber that was 15 million–20 million years old. The pollen was identified as belonging to an orchid from the orchid subtribe Goodyerinae. The scientists compared the fossil pollen with pollen of modern-day plants and reconstructed an evolutionary tree for orchids. On the basis of the assumption that the plants underwent a relatively constant rate of evolution, the oldest common ancestor of the orchid family dated from 76 million to 84 million years ago, in the Late Cretaceous Period. “The dinosaurs could have walked among orchids,” said Ramírez.

Paul Simons

Molecular Biology and Genetics

Epigenetics Takes Centre Stage.

      The genome is often called the blueprint of life, but it is the epigenome—the way the genome is modified chemically and packaged—that defines how the information in the blueprint is read and applied. Genetic information is encoded in the sequences of nucleotides that make up the DNA that is passed from parents to offspring. In contrast, epigenetic information, though heritable by cells or organisms, is not specific to the nucleotide sequence of the transmitted nucleic-acid genome. The field of epigenetics, the study of the epigenome and its functional significance, has recently exploded, revolutionizing the fields of genetics and developmental biology. For example, in 2007 researchers led by Keji Zhao of the National Institutes of Health, Bethesda, Md., and by Eric Lander and Bradley Bernstein of the Broad Institute, Cambridge, Mass., reported how they were able to identify and map key genomewide epigenetic modifications in mammalian cells.

      Epigenetics is known to involve a number of possible chemical modifications to DNA and to the proteins called histones that package the DNA into a complex substance, called chromatin, inside a cell. One principal type of modification is DNA or histone methylation. Methylation can be transient and change rapidly during the life span of a cell or organism, or it can be largely permanent once set early in the development of the embryo. (Other largely permanent chemical modifications also play a role; these include histone acetylation, ubiquitination, and phosphorylation.) The specific location of methylation on a histone protein can be important. For example, Zhao and colleagues identified specific histone modifications that distinguish actively expressed regions of the genome from repressed regions and found histone modifications that correlate with chromosome banding patterns. Lander and Bernstein similarly determined specific histone modifications that distinguish actively expressed genes, genes poised for expression, and repressed genes in different kinds of cells.

      Epigenetic changes not only influence the expression of genes in plants and animals but also enable the differentiation of distinct cell types from pluripotent stem cells early in development. In other words, such changes allow cells to become specialized as liver cells, brain cells, or skin cells, for example, even though the cells all share the same DNA and are ultimately derived from one fertilized egg. As the mechanisms of epigenetics have become better understood, researchers have recognized that the epigenome also influences a wide range of biomedical conditions. This new perception has opened the door to an understanding of normal and abnormal biological processes and promises interventions that might prevent or ameliorate certain diseases.

      Epigenetic contributions to disease fall into two classes. One class involves genes that are themselves regulated epigenetically, such as the imprinted (parent-specific) genes associated with Angelman syndrome or Prader-Willi syndrome. Clinical outcomes in cases of these syndromes depend on the degree to which an inherited normal or mutated gene is or is not expressed. The other class involves genes whose products participate in the epigenetic machinery and thereby regulate the expression of other genes. For example, the gene MECP2 encodes a protein that binds to specific methylated regions of DNA and contributes to the silencing of those sequences. Mutations that impair the MECP2 gene can lead to Rett syndrome.

      Many tumours and cancers are believed to involve epigenetic changes attributable to environmental factors. These changes include a general decrease in methylation, which is thought to contribute to the increased expression of growth-promoting genes, punctuated by gene-specific increases in methylation that are thought to silence tumour-suppressor genes. Epigenetic signaling attributed to environmental factors has also been associated with some characteristics of aging by research that studied the apparently unequal aging rates in genetically identical twins.

      One of the most promising areas of recent epigenetic investigation involves stem cells. It has been understood for some time that epigenetic mechanisms play a key role in defining the “potentiality” of stem cells. Only recently, however, as those mechanisms have become clearer, has it become possible to intervene and effectively alter the developmental state and even the tissue type of given cells. The implications of this work for future clinical intervention for conditions ranging from trauma to neurodegenerative disease are profound.

A New Weapon in the War on Malaria.

      Malaria is an infectious disease that affects more than 350 million persons each year, killing more than a million. It results from infection by any of four species of the protozoan parasite Plasmodium: P. falciparum, P. vivax, P. ovale, and P. malariae. The parasite has a complex life cycle that proceeds through distinct phases in an infected human host and an infected Anopheles mosquito. The mosquito acquires Plasmodium protozoa when it sucks blood from an infected person. The mosquito harbours the parasites as they proceed through stages of reproduction and maturation. The mosquito subsequently transmits them back to humans when it bites and injects saliva contaminated with Plasmodium.

      The struggle to reduce or eliminate malaria worldwide has been long, costly, and at times controversial. The measures have included the reduction of local mosquito populations, reduction of mosquito access to humans (by using insecticide-treated bed nets, for example), and medications, such as quinine or primaquine, that combat the infection in a human host. Efforts to devise a safe and effective vaccine continue but have yet to bear fruit. In recent years, however, a new weapon has emerged that might prove the best solution of all—a genetically modified Anopheles mosquito that is itself resistant to infection by Plasmodium.

 Since the mid-1990s, several groups have produced transgenic mosquitoes that are resistant to Plasmodium infection. One group, for example, modified mosquitoes to express a small amount of a substance called SM1 dodecapeptide in cells that line the salivary gland and gut of the mosquito. This peptide binds to the same cell-surface receptors used by Plasmodium to recognize and invade mosquito cells. Overexpression of this peptide in target tissues therefore competitively inhibits entry of the Plasmodium parasite. A key question was whether transgenic mosquitoes could thrive in the wild among natural mosquito populations, since the genetic manipulations that were required for establishing Plasmodium resistance could render the resulting mosquitoes less “fit” than their wild-type (normal) peers. In 2004 a team of researchers directed by Marcelo Jacobs-Lorena from Johns Hopkins University, Baltimore, Md., reported a major breakthrough. The team had identified a line of transgenic mosquito that expressed the SM1 peptide (and consequently was resistant to Plasmodium infection) yet remained as fit as its wild-type peers when fed on the blood of mice that had not been infected by Plasmodium. Equal fitness, however, was no guarantee of success in the field, especially given the much greater number of wild-type mosquitoes.

      In the spring of 2007, Jacobs-Lorena and colleagues reported the striking observation that their transgenic mosquitoes demonstrated a clear fitness advantage—relative to wild-type mosquitoes—when fed on the blood of mice that were infected by Plasmodium. To test the relative fitness of their transgenic mosquitoes in the context of Plasmodium infection, the researchers conducted a series of experiments in which initial populations that were made up of equal numbers of wild-type and transgenic mosquitoes were allowed to interbreed and expand for 13 generations. One hundred individual mosquitoes from each new generation were tested to ascertain the relative proportion of wild-type and transgenic subpopulations. The results clearly demonstrated a slow but steady increase in the proportion of transgenic mosquitoes, with a plateau of about 70% transgenic mosquitoes reached in each population by the ninth generation.

      These results were exciting for three reasons. First, if resistance to Plasmodium infection provided a fitness advantage to mosquitoes, then even a small number of transgenic mosquitoes released into a Plasmodium-infested area would increase until such mosquitoes provided an effective deterrent to human transmission. Second, if the vast majority of mosquitoes in currently Plasmodium-infested areas could be rendered disease-free, malaria might be controlled or eliminated without the need for widespread use of chemicals, deforestation, draining of wetlands, or other environmentally destructive measures. Finally, this strategy might be directed at other mosquito-borne infectious diseases, such as yellow fever, dengue, and West Nile virus encephalitis.

Judith L. Fridovich-Keil

Paleontology

 One of the most significant events in paleontology in 2007 was a report of the discovery in New Mexico of a grouping of fauna from the Late Triassic Period (228 million to 200 million years ago) that included fossils of both true dinosaurs and dinosauromorphs—animals that were related to dinosaurs but were more primitive. It had previously been thought that the first dinosaurs quickly replaced dinosauromorphs, through either competition or the extinction of the older species. The new finding indicated that the replacement was much more gradual.

      A newly described dinosaur, Turiasaurus riodevensis, from the Jurassic (200 million to 146 million years ago) of Spain represented one of the largest-known terrestrial animals. The dinosaur was estimated to have had a mass of 40,000 to 48,000 kg (88,000 to 106,000 lb), and its humerus (upper foreleg bone) was about 180 cm (70 in) in length. Analysis of the specimen suggested that the animal belonged to a previously unknown group of primitive European sauropods. Another newly discovered dinosaur species, Gigantoraptor erlianensis, was a giant birdlike dinosaur from the Late Cretaceous (100 million to 65 million years ago) of China. Although the dinosaur was estimated to have weighed about 1,400 kg (3,100 lb), a phylogenetic study placed it among the oviraptors, a group of small feathered theropod dinosaurs that generally had a body mass of less than 40 kg (88 lb). The large size of the new species was unusual because there was a general evolutionary trend toward decreasing size in this and other theropod lineages closely related to birds.

      Analyses of soft tissue that was discovered in 2005 inside a well-preserved Tyrannosaurus rex femur (thighbone) that was about 70 million years old revealed that protein was still present in the bone. The results showed the presence of small amounts of collagen, the main organic tissue in bone. Such molecular studies, until recently not believed to be possible with old fossil material, might eventually contribute to the understanding of evolutionary relationships between the dinosaurs. In another study, Velociraptor was discovered to have quill knobs. The presence of the bumplike structures, found on the forearm of a fossil specimen from Mongolia, indicated that the dinosaur had feathers and that the feathers were attached to the bone by follicular ligaments.

      The discovery of two fragmentary specimens of fish from about 420 million years ago in the Late Silurian shed light on the origin of the osteichthyans (bony fish). A lower jawbone of Andreolepis from Sweden and an upper jawbone of Lophosteus from Germany established these two genera as the oldest positively identified osteichthyans. The report in late 2006 of a lamprey, Priscomyzon riniensis, from about 360 million years ago—35 million years earlier than previously known specimens—led to a reanalysis of early jawless vertebrate relationships. The fossil was a soft-body impression of an animal that was nearly identical to the modern lamprey. Because lampreys have a cartilaginous skeleton, their fossil record is very poor, and lampreys were previously thought to have evolved from the ostracoderms (extinct armoured jawless vertebrates). The new finding suggested that lampreys evolved from other types of Paleozoic jawless vertebrates that lacked bony armour and that the ostracoderms were more closely related to the first primitive jawed fish than they were to lampreys.

      Two new fossil penguin species—a 1.5-m (5-ft) giant penguin, Icadyptes salasi, and a smaller species, Perudyptes devriesi—from Peru challenged traditional ideas about the role of climate change in penguin evolution. It had been suggested that penguins originated in high latitudes and migrated to equatorial regions after the climate had cooled near the end of the Eocene Epoch (about 34 million years ago), but the find showed that penguins arrived in lower latitudes at least 30 million years earlier than previously thought.

      A newly described Mesozoic mammal from northeastern China, Volaticotherium antiquum, represented a new group of insectivorous gliding mammals. The discovery not only pushed back the known origin of gliding by millions of years, but it also showed that early mammals had more diverse lifestyles than previously thought. A study of mammal assemblages across the Cretaceous-Tertiary, or K-T, boundary disputed the widely held assumption that mammalian faunas rapidly increased in diversity after the extinction event at the end of the Cretaceous Period about 65 million years ago. The study concluded that mammalian diversification rates barely changed across the K-T boundary and that they did not increase rapidly until the Eocene and Oligocene epochs (from 56 million to 23 million years ago).

      A newly described eutriconodont Mesozoic mammal, Yanoconodon allini, added to the understanding of the evolution of the mammalian inner ear. Detachment of the three middle-ear bones from the mandible occurred during the transition between reptiles and mammals. The new specimen showed the middle-ear bones still to be connected to the mandible but only by an ossified piece of cartilage. This feature is similar to the ear structure in embryos of modern monotremes and placentals, but in the eutriconodonts it apparently represents an embryonic feature that was not lost in adults.

      A piece of Cretaceous amber from Myanmar (Burma) yielded the oldest-known fossil bee, Melittosphex burmensis. Since bees are one of the most important pollinators of angiosperms (flowering plants), the discovery, reported in late 2006, of the 100-million-year-old bee is significant because it existed near the time of the first appearance of angiosperms. The previously oldest-known fossil bees were 35 million–45 million years younger. In another study bacteria, fungi, algae, and protozoans were found preserved in Triassic amber from Europe. The amber consisted of hardened resin droplets only about 1 mm (0.04 in) in size. Scientists were able to assign the microorganisms to living genera, which suggested that these lineages had not changed morphologically during the past 220 million years.

      In a rare event about 300 million years ago, a major earthquake caused an ancient forest of 1,000 ha (4 sq mi) to drop below sea level and undergo rapid burial. The newly described remains of this nearly intact ecosystem were uncovered in an Illinois coal mine. The finding allowed the scientists to analyze the distribution of a variety of extinct plants, including lycopsid trees, across the forest floor. The identity of 385-million-year-old fossilized tree stumps at a site near Gilboa, N.Y., had been a mystery since they were discovered in the late 19th century. Although considered to represent the earliest forest, no leaf or other type of photosynthetic structure had been found that was associated with the stumps. Newly described tree specimens from the area, however, showed that the stumps pertained to fernlike trees that grew leafy twiglike branches out of a vertical trunk. The specimens, classified in the genus Wattieza, grew to a height of 8 m (26 ft) or more.

      A report early in 2007 suggested that the 600-million-year-old globular microfossils from the Doushantuo Formation in China were not animal embryos as previously thought. The report instead found parallels between the microfossil structures and structures formed during cell division in modern sulfur bacteria and suggested that the microfossils were cell clusters of giant specimens of such bacteria. Several months later, however, another report described additional microfossils from the same formation that supported the original hypothesis. The new Doushantuo fossils were found encased in large organic enclosures with a complex structure, which indicated that the fossils were likely eukaryotic cells and therefore not bacteria. The study concluded that the fossils represented early cleavage-stage embryos inside egg cysts.

William R. Hammer

▪ 2007

Introduction

Animal studies indicated that apes could plan into the future and that meerkats helped teach their young to hunt prey. Researchers discovered that the sexual reproduction of moss plants had unexpected help and that an unusual form of inheritance known only in plants also occurred in mice. A fossil fish called Tiktaalik revealed close links to early land animals.

Zoology

      A series of experiments reported in 2006 by Nicholas J. Mulcahy and Josep Call of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Ger., provided evidence that animals other than humans could plan ahead by selecting and transporting tools for anticipated future use. In one experiment bonobos (pygmy chimpanzees, or Pan paniscus) and orangutans (Pongo pygmaeus) first learned how to use a plastic tool to obtain a reward (grapes) from a container in a test room. For each trial of the experiment, an ape was taken into the test room, where both the correct and unsuitable tools for obtaining the reward were placed on the floor. It was then taken to an adjacent waiting room, which had a window through which it could see the container with the reward and watch as the tools were removed. The ape was kept in the waiting room for one hour and then led back into the test room, where it could not get the reward without the correct tool. The only way for the ape to get the reward on future trips to the test room was to select and pick up the tool, carry it to the waiting room, and then return with it to the test room. Of three bonobos and three orangutans in the experiment, all learned within seven trials to pick the correct tool and return with it to the test room. In 16 trials one orangutan left and returned with the correct tool 15 times. The six experimental animals left the room with a tool 70% of the time, and the choice of a correct tool, compared with an unsuitable tool, was made a statistically significant proportion of the time. In another, similar experiment with one of the bonobos and one of the orangutans, the test animal remained in a waiting room overnight for 14 hours between its access to the tools and its return to the test room. The two animals successfully carried the proper tool when they left the test room in 19 of 24 trials, and they returned to the test room with the tool 15 times. Demonstrably, apes could choose, keep, and return with a tool that was appropriate for future use. The researchers concluded that an ability to plan for future needs had evolved at least 14 million years ago, when bonobos and orangutans had a living common ancestor.

      Numerous animal species, such as dolphins, seals, and honeybees, were noted for their learning abilities, yet documentation that individuals of any nonhuman species intentionally taught other individuals was rare, especially for animals in the wild. Alex Thornton and Katherine McAuliffe of the University of Cambridge provided convincing evidence that wild meerkats (Suricata suricatta) in the Kalahari desert of South Africa taught 30- to 90-day-old pups how to handle live prey, including scorpions with venomous stingers. The investigators defined teaching as activity in which an older, experienced member (teacher) of a group changed its behaviour and received no immediate benefits when a younger, inexperienced member (pupil) was present and, as a consequence of the teacher's behaviour, the pupil gained knowledge or useful skills. Meerkats are carnivorous animals that eat small vertebrates and invertebrates, and very young meerkat pups, which cannot find their own prey, are fed by older meerkats called helpers. The investigators observed the feeding behaviour of helpers toward pups of different ages. The helpers taught pups how to handle prey by providing them with opportunities to do so, sometimes nudging prey to draw the pups' attention to it and sometimes retrieving live prey that tried to escape. Before giving prey to pups that were still young, helpers usually killed the prey or disabled it, such as by removing the stinger of a scorpion. As the pups became older, the behaviour of helpers gradually changed, and for the oldest pups—which had learned how to handle prey—the helpers provided mostly intact prey to allow the pups to practice and perfect their skills. Experiments confirmed that the handling skills of the pups improved as a result of exposure to live prey. The investigators concluded that teaching did not need to be cognitively complex and that it might be common among many kinds of animals but difficult to document unequivocally.

 Many new marine species had been documented as part of the ongoing Census of Marine Life, a 10-year international scientific collaboration. (See Special Report.) One discovery announced in 2006 was of a new species (Kiwa hirsuta) of crustacean from hydrothermal vents of the Pacific-Antarctic Ridge about 1,500 km (930 mi) south of Easter Island. The species, which was described by Enrique Macpherson of the Centre for Advanced Studies of Blanes (Spain) and colleagues, belonged to a previously unknown genus and family (Kiwaidae). It was approximately 15 cm (5.9 in) in length and had a distinctive shell shape that differentiated it from other families of crabs and lobsters. The so-called Yeti crab had reduced eyes but was sightless, and its legs had a dense covering of setae that had the appearance of hair. The investigators used molecular studies to establish the relationship of the Kiwaidae to other crustaceans, which confirmed its uniqueness as a family.

      In a study that involved two invasive predators to New England coastal waters, Aaren S. Freeman and James E. Byers of the University of New Hampshire demonstrated a case of rapid evolution by a native prey species, the blue mussel (Mytilus edulis). The two invasive predators, the green crab (Carcinus maenas) and the Asian shore crab (Hemigrapsus sanguineus), crush the shells of mussels before eating them. The green crab was introduced to the United States from Europe in 1817. It reached southern New England more than a century ago and northern Maine at least 50 years ago. The Asian shore crab was introduced to the mid-Atlantic coast in 1988. It moved northward as far as southern New England but at the time of the investigation had not yet reached northern Maine. Therefore, mussels in southern New England had been exposed to both species of crab, but the mussels in northern Maine had encountered only the green crab. An effective defense mussels use when they become aware of the presence of a predatory crab is to grow a thicker shell that is difficult or impossible for the crab to break. The researchers conducted laboratory and field experiments to compare the shell-thickening response of mussels from northern Maine and southern New England when exposed to the two crab predators. Mussels from both locations were raised for three months in water that flowed downstream from cages that contained either green crabs or Asian shore crabs so that the water carried signs of their presence. A control group of mussels was also used in which no crabs were upstream. The researchers found that the northern Maine mussels developed significantly thicker shells when exposed to the water tainted by green crabs but showed no shell-thickening response in water from Asian shore crabs. The southern Maine mussels, however, developed thicker shells in the presence of Asian shore crabs. Similar responses were obtained in an experiment in which mussels from each location were placed in floating platforms in natural waters where both crab species lived. The results supported the interpretation that blue mussels evolved a shell-thickening response to the presence of green crabs within 50 to 100 years, and the observed morphological response to Asian shore crabs by the southern but not the northern mussel populations represented a case of rapid evolution (within 15 years) by mussels to an invasive species of predator.

 When Saharan desert ants (Cataglyphis fortis) forage, they return home in a straight line, though their outbound route in search of food is typically circuitous across flat desert with no visible landmarks. To determine a straight path home, the ants keep track of the direction of their travel (through celestial orientation) and keep a measure of distance traveled in various directions. How the ants measured distances was uncertain, but they were known to be able to assess how far they had walked even in the dark. One hypothesis was that the ants somehow measured distance traveled by registering their leg movements. To test the hypothesis, Matthias Wittlinger of the University of Ulm, Ger., and colleagues conducted experiments in which ants were trained to walk from a nest to a feeder along a 10-m (33-ft) channel that was open so that directional information could be obtained from the sky. Prior to releasing ants to return home in a parallel test channel, the researchers modified the gaits of two groups of ants. They lengthened the gait of one group by attaching pig bristles to their legs to function as stilts, and they shortened the gait of the ants in the second group by severing the outer part of each leg. After the treated ants had taken food, they were released to return home. Ants with stilts took longer strides and consistently walked beyond the point where their home site would have been, whereas the ants with shortened legs did not go far enough. When the ants with stilts or stumps later walked from the home site to the feeder, they accurately assessed the return distance home, owing to the same stride length in the outbound and home-bound trip. The investigators concluded that the ants measured the distance traveled by some mechanism that counted the number of steps taken.

J. Whitfield Gibbons

Botany

      In 2006 concerns about the unintentional spread of genetically modified (GM) plants were raised when plants of creeping bentgrass (Agrostis stolonifera) that had been genetically modified for possible use on golf courses were found as far as 3.8 km (2.7 mi) outside a test site in Oregon. It was the first GM perennial plant known to have escaped into the wild in the United States. The grass had been modified to be impervious to the herbicide glysophate (Roundup) so that golf courses with the grass could be sprayed to kill off weeds without harming the grass. Unlike GM crops such as corn (maize) and soybeans, the GM bentgrass was able to produce viable seeds. The U.S. Department of Agriculture ordered a full environmental audit of the spread of the grass and its impact on wildlife and flora.

      In research to identify commercially useful plant genes, a team of scientists at the Victorian AgriBiosciences Centre in Melbourne discovered a group of frost-resistant genes in the Antarctic hair grass (Deschampsia antarctica). The grass was one of only two flowering plants that grew in Antarctica, and it was able to withstand temperatures down to −30 °C (−22 °F). The resistance genes produce a remarkable protein that inhibits the growth of ice crystals, which in most plant species rupture cells and ultimately kill the plant. The scientists planned to use the gene to breed frost-resistant wheat and barley plants. As a test they successfully transferred the gene sequence into Arabidopsis thaliana (thale cress), which then withstood subzero temperatures. Another group of researchers identified a rice gene variant called Sub1A-1 that allows rice plants to survive completely submerged in water for up to two weeks. Most varieties of rice die after only a few days of complete submersion. The researchers, David Mackill of the International Rice Research Institute in the Philippines and colleagues, reported that the gene seemed to affect the way the plants responded to hormones such as ethylene and gibberellic acid. The researchers introduced the gene variant into a widely grown high-yield rice variety that was intolerant to being submerged in water and determined that the resulting plants were able to tolerate flooding.

      Legume plants, such as peas, beans, and clovers, form “fertilizer nodules” on their roots when they are invaded by rhizobia bacteria. In a symbiotic partnership the plant then provides the bacteria in the nodules with shelter, oxygen, and food, and in return the bacteria fix atmospheric nitrogen into substances that the plant needs for growth. By mutating a gene in the plants that produce a key messenger chemical called CCaMK (calcium/calmodulin-dependent protein kinase), plant geneticists at the John Innes Centre in Norwich, Eng., and the University of Århus, Den., tricked legume plants into producing the nodules without the aid of rhizobia. The researchers hoped that if the modified gene could be transferred to nonlegume crops such as wheat or rice, the plants could be coaxed into producing their own nodules for rhizobia that would then help nourish the plants and reduce the need for artificial fertilizer.

      In 2006 a large international team of researchers finished a four-year project of sequencing the DNA code of the black cottonwood poplar (Populus trichocarpa). It was only the third plant genome to be deciphered, after Arabidopsis and rice. The possible total number of genes in the tree genome was more than 45,000. The project laid the groundwork for improving the fast-growing tree as a source of cellulose for use as a feedstock for cellulosic ethanol, a potentially important form of renewable energy. Researchers planned to use genetic engineering to make the poplars grow wider trunks and to increase their proportion of cellulose to lignin.

 The rapid emergence and dominance of flowering plants, the angiosperms, about 130 million years ago had long perplexed scientists; Charles Darwin once described it as “an abominable mystery.” In 2006, however, Amborella trichopoda, a plant found only on New Caledonia, was declared a likely missing link between angiosperms and gymnosperms (which include conifers). William Friedman of the University of Colorado at Boulder used a combination of laser, fluorescence, and electron-microscope images to reveal a unique structure that housed the egg cell in Amborella flowers. He found one extra sterile cell in the embryo sac that accompanied the egg cell in the female sex organ, a unique configuration that was reminiscent of gymnosperms and was thought to be a relic of the time when the angiosperms diverged from gymnosperms. According to a perspective piece that accompanied Friedman's research article, the discovery was “akin to finding a fossil amphibian with an extra leg.”

      After more than a century of speculation by biologists, the mystery of the sex lives of mosses was solved. The eggs in moss plants are fertilized by swimming sperm, which need to stay moist. Sperm can swim from a male to a nearby female moss tuft, but in some cases the sperm was found to travel 10 cm (3.9 in) or more—too far for swimming or for being splashed by rain. Nils Cronberg at Lund (Swed.) University and colleagues set up an experiment in which they separated male and female plants of a common moss, Bryum argenteum, with barriers of plaster to absorb moisture and thereby prevent any sperm from swimming or being splashed from plant to plant. The result was a complete absence of fertilization. When mites or wingless insects called springtails—which are often found crawling around mosses—were introduced to the plants, fertilization was successful. The researchers suggested that the sperm hitchhiked on these animals by sticking to their cuticles.

      In their study of the orchid Holcoglossum amesianum, LaiQiang Huang at Tsinghua University, Shenzhen, China, and colleagues discovered a previously unknown form of pollination in a flower. The orchid, which grows on tree trunks in woodlands in China, blooms during the dry season, when there is no wind or flowing water and there are few available insects to act as couriers for transporting pollen. Instead, the orchid pollinates itself, using a bizarre procedure. A flexible stalk lifts two sacs of pollen at its tip and then bends outward and downward in a 360° arc around a protuberance on the flower to carry the sacs of pollen upward into a receptive stigma cavity. The technique ensures that no pollen is transferred to other flowers, even on the same plant.

Paul Simons

Molecular Biology and Genetics

Pushing Beyond Mendelian Genetics.

 In 2006 Minoo Rassoulzadegan from the University of Nice–Sophia Antipolis, France, and colleagues reported the first indication that a kind of non-Mendelian genetic inheritance originally described in the 1950s for plants also occurred in mammals. The potential implications of the finding were profound, because they concerned the overall understanding of genetic inheritance and how genes are expressed.

      The simplest forms of genetic transmission follow a set of rules originally described in the mid-1800s by the Austrian monk Gregor Mendel. From his studies of the garden pea, Mendel realized that the visible traits of peas correspond to invisible, discrete bits of information (genes) that are passed from parents to offspring. These bits of information come in pairs, and the alleles (individual units) in each pair are sometimes identical and sometimes not. When the alleles for a given trait are identical, the organism is said to be homozygous with respect to that gene, and the appearance of the corresponding trait is assured. When the two alleles are not identical, the organism is said to be heterozygous, and one allele or the other—or sometimes both—determines the trait that appears.

      The conclusions that Mendel reached from his studies can be given as two rules known as Mendel's laws. The first, called the law of segregation, states that in the formation of gametes (sex cells such as eggs and sperm), the alleles in each pair of genes segregate randomly, so that one-half of the gametes carry one allele and the other half carry the other allele. The second rule, called the law of independent assortment, states that for any one gamete, the distribution of inherited alleles is random.

      Many traits in species that range from plants to humans are inherited in a Mendelian fashion, but by the early 21st century, it was clear that most traits in most species follow so-called complex (not strictly Mendelian) patterns of inheritance. Complex traits can result from the combined effects of multiple individual genes, combinations of genetic and environmental influences, or various molecular effects such as DNA instability or histone methylation (a chemical change in a chromosome protein). The pattern of non-Mendelian inheritance found by Rassoulzadegan and her fellow researchers was called paramutation and involved a case in which a trait but not its corresponding allele was passed from a heterozygous parent to its offspring.

      The researchers worked with a strain of wild-type (normal) mice and related heterozygous mice that carried an engineered mutant allele of a gene called Kit. Each wild-type mouse had a tail that was uniform in colour; the heterozygous mutants had spotted tails. According to Mendel's law of segregation, the expected outcome of a cross (mating) between a normal (homozygous) mouse and a heterozygous mutant mouse would be a litter in which one-half of the pups had spotted tails and the other half did not. Instead, the researchers found spotted tails on all of the pups, including those that carried a pair of normal Kit alleles. These mice were paramutated—they showed the trait for the mutant Kit allele even though they did not carry it.

      Determined to uncover the mechanism of the paramutation, the researchers tested and ruled out a number of logical possibilities, such as DNA or histone methylation. They then explored the levels and structures of Kit mRNA (messenger RNA that was copied from the Kit gene) in the wild-type mice, heterozygous mice, and the paramutated mice. To their surprise, the researchers saw diminished levels and degraded forms of Kit mRNA in the tissues of both the heterozygous and paramutated animals. The researchers surmised that this effect was transmitted from a heterozygous parent to a paramutated pup through both eggs and sperm, because the effect appeared to be transmitted equally well from females and males.

      Further study showed that Kit RNA, which was not found in the mature sperm of the wild-type mice, was present in the mature sperm of the heterozygous animals, and it suggested that the RNA in the sperm consisted of small RNA fragments called microRNA, which was known to target corresponding full-length mRNAs for degradation. As a test, the researchers injected a solution of Kit microRNAs into otherwise wild-type one-cell-stage mouse embryos. The pups that developed were paramutated, and even the offspring of the paramutated pups had spotted tails. The fact that microRNAs in early embryos could cause a permanent and heritable change in gene expression meant that this unusual mechanism might account for some fraction of the as-yet poorly understood diversity of traits observed in humans and other animals.

New Insights into Evolution.

      With the increasing availability of full genome sequences for many animals, plants, and microorganisms, new data and insights emerged that were helping to confirm evolution and begin to explain the molecular mechanisms that drive it. One challenge in understanding evolution concerned the internal complexity and interdependence of biological systems. The ligand-receptor biological system, for example, requires that two types of molecules—a ligand (such as a hormone) and its receptor (a protein that recognizes and responds to the ligand)—work only in combination with each other. Mutations and natural selection, however, would presumably have worked upon each part of such a system individually.

      One study of a dual ligand-receptor system published in 2006 by Joseph Thornton and colleagues of the University of Oregon offered a plausible explanation. This system incorporates the mineralocorticoid receptor (MR)—which is stimulated by the hormone aldosterone and regulates electrolyte homeostasis and blood pressure—and the glucocorticoid receptor (GR)—which is stimulated by the hormone cortisol and regulates metabolism, inflammation, and immunity. Comparative genetic studies of a variety of species demonstrated that the MR/GR dual-receptor system arose through the duplication of a common ancestral gene into a two-gene system more than 450 million years ago. The ancestral gene, named AncCR (for ancestral corticoid receptor), was not found in jawless fish such as lampreys but did exist in cartilaginous fish and in bony fish and their descendants—the tetrapods (four-limbed vertebrates, including humans).

      The question the investigators asked was, How did this gene pair evolve to produce two receptors that recognize and respond to different ligands? The question was key because only tetrapods produce aldosterone (the MR ligand), so the MR/GR dual-receptor system must have originated and achieved genetic stability in the ancestral vertebrate lineage before the appearance of tetrapods and aldosterone. The investigators postulated that the AncCr gene must have responded to a different ligand, such as 11-deoxycorticosterone (DOC), a hormone present in living jawless fish. To test this hypothesis the investigators inferred what the DNA sequence of the AncCR gene must have been and then synthesized and expressed the gene in cultured cells. The AncCR receptor created in this way was found to respond well not only to DOC but also to aldosterone and—to a lesser extent—to cortisol.

      Thornton and his colleagues suggested that the original AncCR receptor responded to a number of ligands, including cortisol and DOC. After the gene changed into a two-gene system, one of the two genes would have continued to respond to DOC (and later aldosterone), while the other underwent mutations that would have improved its ability to recognize and respond to cortisol. The investigators identified two stepwise mutations that fit this scenario, and they were able to re-create the mutations to verify their predicted effect.

      By specializing the function of the MR and GR receptor-ligand systems, higher vertebrates acquired the ability to regulate the endocrine stress response and electrolyte homeostasis separately. Surely this greater specificity and flexibility was of benefit to species in navigating the changing environmental landscape. Just as surely this example was not unique but only the first of many that remained to be uncovered.

Judith L. Fridovich-Keil

Paleontology

      A significant new transitional fossil, Tiktaalik roseae, was described in April 2006. The fossil, discovered in Late Devonian deposits of the Canadian Arctic, was a new sarcopterygian (lobe-finned) fish that linked this group with the most primitive tetrapods (animals with four limbs). In particular, the front fin of Tiktaalik was determined to be both structurally and functionally transitional between a sarcopterygian fin and a tetrapod forelimb, and the pattern of bones and joints in the fin would apparently have made the fish capable of a range of postures and movements, including propping itself up on a solid surface. Geologic evidence from where the fossil was found suggested that Tiktaalik lived in a shallow-water environment of meandering streams.

      New studies of two sarcopterygian fish of the Middle Devonian also revealed several tetrapod-like features. A study of the fossil skull of Panderichthys from the Lode Formation of Latvia identified a breathing structure—including a gill-slit opening, tubular passageway, and adjacent bone—whose characteristics suggested that it might have evolved into the structure of the middle ear in tetrapods. A similar finding was reported in a study of a well-preserved fossil of Gogonasus from western Australia.

      A newly described Early Devonian fish, Meemannia eos from Yunnan, China, was reported to have features of both sarcopterygian and actinopterygian (ray-finned) fish and was considered the best candidate known to date for the common ancestor of the two lineages of bony fish. A fossil from the Late Devonian of Quebec revealed for the first time the gill structure of an extinct jawless fish. The fish, Endeiolepis, was found to have a gill structure similar to that of living (jawless) lampreys, but it had an unusually large number of gill pouches.

      In the world of dinosaurs, a newly described Late Jurassic small theropod (a bipedal flesh-eating dinosaur) from a site in southern Germany was one of the best-preserved, most-complete nonavian predatory dinosaurs known from Europe. Identified as a primitive coelurosaur, it had large portions of the integument (skin) preserved along the tail. There was no indication of feathers, even though the specimen was clearly related to feathered theropods from other parts of the world. Another newly described dinosaur found in Germany, from the Kimmeridgian marine beds, was a Late Jurassic sauropod (a plant-eating dinosaur) called Europasaurus holgeri. It was represented by 11 individuals that ranged in total length from 1.7 to 6.2 m (5.6 to 20 ft). Although the largest individuals were relatively small for a sauropod, bone histology (tissue-structure) studies showed that they were dwarf adults, not juveniles. The paper that reported this finding suggested that the animals lived on a large island around the Lower Saxony Basin and hence were an example of a dwarf island species.

      The newly discovered species Guanlong wucaii from the early Late Jurassic of the Junggar Basin in northwestern China was described as the oldest-known tyrannosauroid, with an age of approximately 160 million years. The larger of the two fossil specimens that were found stood about 1 m (3.3 ft) tall at the hips and was about 3 m long. The most unusual feature was a large fragile cranial crest that was among the most elaborate of any theropod. In another study involving tyrannosaurs, the size and age structures in the populations of four genera of tyrannosaurs determined from counts of annual rings in leg and foot bones of fossilized specimens showed that juvenile tyrannosaurs had a very high survival rate and that the mortality of the tyrannosaurs increased from midlife to their maximum age. The study indicated that the low juvenile mortality rate could explain the rarity of juvenile tyrannosaur fossils and that the survivorship patterns found were typical of some long-lived species of modern birds and mammals.

      Buitreraptor gonzalezorum was a newly described dromaeosaurid (a type of small theropod) from the Late Cretaceous Candeleros Formation of Río Negro province in Argentina. The specimen represented the earliest and most-complete member of the Maniraptora known from South America, and it provided newly described evidence for an independent Gondwana lineage of dromaeosaurs with an origin that predated the separation of Laurasia and Gondwana (the northern and southern continental landmasses).

      An analysis of coprolites (fossil dung) from India of a Late Cretaceous sauropod dinosaur showed the presence of at least five taxa of grasses in the animal's diet. Dated to be about 65 million years old, the coprolites were the earliest fossil record of the presence of grass and the first evidence that any dinosaurs ate grass.

      A newly described specimen of the most-famous transitional fossil, the primitive bird Archaeopteryx, not only was scientifically valuable but also was the source of controversy. The specimen was arguably the best Archaeopteryx in the world. In particular, an examination of the details of its foot showed for the first time that the bird had a hyperextensible second toe, like the enlarged claw in dromaeosaurs. The finding further strengthened the link between theropods and birds and suggested that Archaeopteryx might have lived on the ground rather than in trees. At issue was the fact that the specimen was privately owned and on exhibit in a small private museum in Wyoming rather than held as part of a world-class museum collection. Professional paleontologists were concerned that fossils in such collections could easily be lost to scientific study.

      Newly described three-dimensional specimens of Gansus yumenensis, an amphibious bird from the Early Cretaceous of northwestern China, showed that the bird had advanced features previously found only in Late Cretaceous and Cenozoic ornithuran birds. Phylogenetically this placed Gansus as the oldest-known member of the Ornithurae—the lineage of birds that includes all modern birds—and supported the idea that modern birds originated in aquatic or littoral (shore) environments.

      Feilongus youngi and Nurhachius ignaciobritoi were two newly described Early Cretaceous pterosaurs from northeastern China that paleontologists related to pterosaur groups previously unknown in China. Feilongus exhibited two crests on its head and a protruding upper jaw, whereas Nurhachius had very unusual teeth with compressed triangular crowns.

      A very unusual marine crocodile-like animal was discovered at the Jurassic-Cretaceous boundary in Patagonia (Arg.). The snout and lower jaw of the animal were very short and high and had only a few teeth present. This morphology deviated considerably from the long gracile skull with numerous teeth that was typical of crocodilians. The study that described the specimen suggested that the animal was adapted for eating small fish or mollusks.

      A general understanding concerning the evolution of snakes was that they underwent progressive loss of their limbs through a gradual decrease of their use. A newly described Late Cretaceous fossil snake from Patagonia, Najash rionegrina, had functional hind legs and a pelvic girdle supported by a sacrum, a triangular bone at the base of the vertebral column. Although this was not the first snake reported with hind limbs, it was the first with a well-developed sacrum, and it suggested that for a time snakes retained at least partially functional hind limbs after the loss of the forelimbs. A phylogenetic analysis showed that the specimen was the most primitive snake known and supported the hypothesis of a terrestrial rather than a marine origin for snakes.

      A new method of studying the DNA of mammoths improved the reconstruction of their gene sequence. The study indicated that the mammoth was more closely related to the modern Asian elephant than to the African elephant but that the divergence of the three species happened over a short period of time. A new phylogenetic study of the Pleistocene Irish elk, known for having the largest antlers of any living or extinct deer, concluded that the animal was not closely related to the living fallow deer, as had been assumed, but was actually related to the living red deer or wapiti (American elk).

William R. Hammer

▪ 2006

Introduction

Geneticists completed the mapping of the rice genome. Zoologists identified the Laotian Rock Rat and classified it within a new rodent family. The FDA approved BiDil as a drug for a specific racial group. Paleontologists found soft tissue preserved in dinosaur fossils. And scientists studied the snap of the Venus flytrap.

Zoology

      In 2005 zoological research explained how honeybees navigate from their hive to a food source. Honeybees had been the focus of behavioral studies for decades, and many researchers were especially fascinated by the implications of the “waggle dance” performed by honeybees on the vertical surface of the honeycomb within the hive when they return from a newly discovered food source. The function originally ascribed to the dance, now accepted by most zoologists, is to allow the returning bee to convey to other bees (the recruits) the direction and distance of the new food source from the hive. Some investigators, however, challenged this interpretation. They suggested that the recruits that attend the dance do not decode the motions of the dancing bee but merely pick up odours of the food source from particles still clinging to the bee. The recruits then search for the food by tracking down the source of these food odours borne on the wind. Joe R. Riley of Rothamsted Research in Harpenden, Hertfordshire, Eng., and colleagues tested the effectiveness of the waggle dance as a navigational guide. They placed tiny antennae that functioned as radar transponders on recruits that left the hive in search of the designated food source, an unscented artificial feeder 200 m (660 ft) east of the hive. They released some of the recruits at the hive and others at sites 200–250 m (660–820 ft) southwest of the hive. Using signals from the transponders, the scientists mapped the flight paths of the bees. Of the recruits released at the hive, most flew unerringly to the immediate vicinity of the feeder. A small number of these recruits succeeded in locating the feeder itself, but most were unable to do so, presumably because no scents or visual cues were available to them. These results not only provided very strong support for the hypothesis that the waggle dance communicates distance and direction but also showed that the target is ultimately located by cues that are related to natural food sources. The flight paths taken by the recruits released from the locations away from the hive provided even stronger support for the hypothesis; these bees did not fly toward the feeder but instead flew in the same direction and for the same distance as the bees released at the hive. The radar tracks also demonstrated that most of the recruits compensated accurately for lateral drift caused by the wind, even though they were flying to destinations that they had never visited.

      Birds have always been noted for sexual dichromatism (differences in colouring between males and females), with males characteristically being the more brightly coloured. Sexual dichromatism is particularly dramatic in tropical parrots. In most species of tropical parrots, the males have bright plumage and the females are much less colourful. Robert Heinsohn of the Australian National University, Canberra, and colleagues reported on an eight-year study of an Australian parrot (Eclectus roratus) in which the opposite is true—the red-and-blue females are more brightly coloured than the green males. The females and males of E. roratus are so distinctly different in appearance that in the original descriptions of the birds, they were classified as separate species. In all other bird species in which females are more colourful than males (a characteristic referred to as reversed sexual dichromatism), there is also a sex-role reversal; that is, females compete with each other for male mates, and males care for the eggs and young. Despite the disparate colour patterns of the sexes of E. roratus, however, males compete for mates and females tend the nests while males feed them. The investigators attributed independently operating selection pressures related to ecology and behaviours of the female and male parrots to explain how they could evolve to have reverse sexual dichromatism without sex-role reversal. The females live most of the year in tree hollows where they also nest. They forage near the hollows, to which they can quickly retreat from aerial predators. The females are therefore freed from the need for camouflage, but there are relatively few tree hollows in which the females can nest, and the conspicuous display of a female helps ward off other females from its nesting place. In contrast, males have been selected for green plumage, which makes them less conspicuous to predators against the leaves in the tree canopy yet more visible against tree trunks, where they compete for female mates.

 A newly identified species of rodent from Southeast Asia was described by Paulina D. Jenkins of the Natural History Museum, London, and colleagues. It was so distinctive that the scientists placed it in a new family—the first new family of mammals to be described by scientists in more than three decades. The Laotian rock rat (Laonastes aenigmamus), known as the Kha-nyou in food markets in the Khammouan province of Laos, where the scientists first found a specimen, reaches approximately 0.3 m (1 ft) in length and most closely resembles a squirrel or rat in general appearance. The skull and other bone structures, however, are atypical of those of other rodents. DNA analysis confirmed the genetic individuality of the species and showed that its closest relatives are rodents from Africa and South America rather than Asia.

      The rediscovery in an Arkansas forest of the ivory-billed woodpecker (Campephilus principalis), believed to be extinct in the United States since the mid-1950s, was reported by John W. Fitzpatrick of Cornell University, Ithaca, N.Y., and colleagues who included ornithologists and conservationists. They confirmed the presence of at least one male ivory-billed woodpecker in the Big Woods area in eastern Arkansas. The sightings were first made in 2004 but were not disclosed until 2005. A video of a brief visual encounter and recordings of tree-drumming sounds characteristic of ivory-billed woodpeckers gave further evidence that the species still existed.

      Discoveries were also made in the global-distribution patterns of well-studied groups of animals, as reported by M.S. Min of Seoul National University and colleagues. They described the first plethodontid (lungless) salamander known from Asia. The salamander, Karsenia koreana, was given the common name Korean crevice salamander. With the exception of six species from the Mediterranean region, all members of the family were known only from the Western Hemisphere. The family Plethodontidae comprises more than 377 of the 550 species of salamanders. Characteristic of plethodontids, the new species has nasolabial grooves but no lungs or pterygoid bone. The species differs from those of other genera in the bone structures of its feet and skull. The investigators determined that there was a high level of genetic divergence between Karsenia and other plethodontids. This finding, coupled with its geographic isolation in Asia, suggested that Karsenia was possibly separated from North American members of the family before the Tertiary, at least 65 million years ago.

      Julia A. Clarke of North Carolina State University and colleagues challenged a long-held conviction among many paleontologists that modern birds arose as a distinct phylogenetic lineage after the extinction of nonavian dinosaurs at the end of the Cretaceous Period (about 65 million years ago). The lack of convincing evidence of true bird fossils prior to the Tertiary has suggested that birds did not coexist with dinosaurs. The investigators described a new species of bird, Vegavis iaai, from Antarctica, that was associated with sediments dated to be from about 66 million to 68 million years ago. The researchers placed the specimen in the avian group Anseriformes (waterfowl) and suggested that the specimen was closely related to the Anatidae (ducks and geese). Their conclusions, based on where they located the new species in the bird evolutionary tree, indicated that avian relatives not only of ducks but also of other modern birds lived in the Cretaceous contemporaneously with nonavian dinosaurs.

      An explanation for how changes in climatic conditions could cause concurrent changes in the sizes of spatially distributed populations of a species was given by Isabella M. Cattadori and Peter J. Hudson of Pennsylvania State University and Daniel T. Haydon of the University of Glasgow, Scot., based on more than a century of records of red grouse (Lagopus lagopus scoticus) in northern England. The investigators tested competing hypotheses to explain the concurrent decreases and increases of grouse populations in each of the five distinct regions they investigated. One hypothesis, the climate hypothesis, was that fluctuations in grouse populations were caused directly by the effects of the climate on the breeding success of the grouse and on the survival of grouse chicks. Alternatively, the climate-parasite hypothesis held that the climate affected the interaction between the grouse and the parasitic nematode Trichostrongylus tenuis, which reduces fecundity in the grouse and is known to affect the abundance of the bird populations. Using elaborate modeling and detailed weather data for each region, the researchers verified that environmental conditions favourable for the spread of the parasitic infection among grouse led to widespread declines in grouse populations, whereas unfavourable years for the parasitic infection resulted in increases in grouse survival. The findings were seen not only to be applicable to the management of grouse populations but also to be indicative of how regional changes in climate could result in local changes in parasite burdens that lead to concurrent changes in the size of host populations.

J. Whitfield Gibbons

Botany

      A major milestone in plant science reported in 2005 was the completion of the mapping of the complete sequence of the rice genome. This achievement was expected to pave the way for making critical improvements in rice, the staple food for more than one-half of the world's population. The genome project took six years and involved 32 research groups from more than 10 countries. Although the function of many of the rice genes remained unknown, about 70% of them mirror those in Arabidopsis thaliana (thale cress), the only other completely sequenced plant genome. By teasing out the roles of the newly sequenced genes, researchers hoped to identify beneficial genes much more quickly and accurately and to develop strains of rice with the most advantageous combinations. They believed that the genetic modification and traditional plant breeding of rice would gain from a more complete understanding of the genome. Breeding of other major cereal crops also stood to benefit, because all of the major cereal crops—including rice—descended from a common, grasslike ancestor. “The rice genome is the Rosetta Stone of all the bigger grass genomes,” said Joachim Messing of Rutgers University, Piscataway, N.J., one of the research leaders. After analyzing the completed genome, the scientists found that rice has more genes than humans do—37,544 genes compared with the estimated 25,000 genes of the human genome.

      Botanists were astonished by the remarkable discovery of plants whose reproduction seemed to shatter the laws of inheritance of all living things, as first described by Gregor Mendel in the mid-1800s. The plants were mutant specimens of Arabidopsis thaliana with closed, deformed flowers in which some flower parts were fused together. Each plant had two copies of a mutant, defective gene. When two such plants are cross-pollinated, the expectation under the Mendelian laws of inheritance is that all the progeny of the two plants will produce deformed flowers. Instead, the botanists observed that 10% of the progeny had normal flowers. Genetic analysis determined that the normal offspring had somehow replaced the defective gene in their DNA. Robert Pruitt, whose team at Purdue University, West Lafayette, Ind., made the extraordinary discovery, believed that the normal offspring somehow acquired genetic information from an earlier generation than their parents'. The team had not determined the source of the genetic instructions for repairing the defective gene, but one possibility was RNA, molecules that are used by cells for the manufacture of proteins and that can also be passed directly from parent to offspring. What would trigger the plant to revert to healthier ancestral genes also remained a mystery, but it might be related to stress or to the severity of a mutation. “This means that inheritance can happen more flexibly than we thought in the past,” said Pruitt. “If the inheritance mechanism we found in the research plant Arabidopsis exists in animals, too, it's possible that it will be an avenue for gene therapy to treat or cure diseases in both plants and animals.”

      Advances continued to be made in 2005 in the genetic modification of plants. Chinese scientists led by Jingxue Wang of the Agri-Biotechnology Research Centre of Shanxi Province inserted two animal genes—one from a scorpion and one from a moth—into rape plants (the source of rapeseed oil, or canola) to make them poisonous to insects that feed on them. The researchers said that the use of two foreign genes instead of one would reduce the likelihood that insect pests would develop resistance to the genetically introduced toxins. Researchers at the University of Victoria, B.C., succeeded in inserting a modified frog gene into potato plants to give the plants resistance against a range of microbial diseases. The gene was taken from Phyllomedusa bicolor, a poisonous frog of South American rainforests, and it produces dermaseptin B1, a skin toxin that helps protect the frog against fungi and bacteria that thrive in the hot and humid conditions. The scientists found that the toxin also inhibits the growth of some of the fungi and bacteria that cause plant diseases.

      Scientists at Max Planck Institute of Molecular Plant Physiology, Golm, Ger., reported a role for hemoglobins in plants. Hemoglobins are proteins best known for their function of carrying oxygen in human blood, but they are also found in high concentrations in the root nodules of legume plants. The nodules are home to symbiotic bacteria, called rhizobia, that take nitrogen from the air and turn it into ammonia, a nitrogen compound that plants can use readily. The nitrogen supplied by the bacteria is vital for plant growth in nitrogen-poor soils. (In return, the bacteria obtain food and shelter from the plant.) Thomas Ott and his team of scientists at the institute found that the plant hemoglobin carries away oxygen from the nodules and thereby helps protect nitrogenase, an oxygen-sensitive enzyme found in the nodules, from damage. Nitrogenase is needed by the bacteria for nitrogen fixation.

      Scientists have long been astonished by the speed at which the Venus flytrap snaps shut—its two leaf lobes close together to entrap prey in a mere tenth of a second. The movement was believed to involve the pumping of water into or out of motor cells within the plant, but the release of water in these tissues is about 10 times too slow to explain the speed of the trap. A study by Yoël Forterre and colleagues at the University of Cambridge used high-speed video to record the snapping-shut motion of the Venus flytrap. Fluorescent reference dots painted onto the leaf lobes helped reveal how the lobes suddenly snapped and buckled. As the plant lies in wait for an insect, the leaf is curved outward. When an insect wanders into the trap and trips over a trigger hair on the leaf, the plant pumps water into the motor cells along the outside of the leaf. This action alters the curvature of the leaf until it flips rapidly from a convex to concave shape, similar to the way a bowed plastic lid will spring inward and outward. It is likely that other uncommon fast motions in plants—such as the explosive propulsion of seeds by the squirting cucumber—depend on similar mechanisms.

Paul Simons

Molecular Biology and Genetics

The Genetics of Race.

      The human genome, like every other naturally occurring genome, is a rainbow of variation. Indeed, there is not one human genome; there are as many distinct human genomes as there are distinctly conceived individuals on Earth. To be sure, the differences are minute—single-base substitutions, small additions, deletions, or rearrangements that involve only a tiny fraction of the more than three billion base pairs of DNA sequence that make up a haploid human genome. Nonetheless, it is these differences, working in concert with environmental factors, that make people who and what they are—that make them unique.

      DNA sequence variations are passed from parents to children in the normal Mendelian fashion; parents carry two independent copies, or alleles, of every gene, and from each pair one is passed to each child, with random distribution. Humans also share relationships that extend beyond immediate family boundaries. As successive waves of human emigration out of Africa populated the continents of the globe, groups became isolated from one another by distance, by physical barriers such as mountains, deserts, or oceans, and by social factors such as language, religion, and culture. This separation, coupled with differing founder groups and differing selective pressures, resulted in detectable and heritable genetic differences between distinct human populations. Some of these genetic differences are visible in terms of physical appearance and give rise to the commonly held notion of race. Other genetic differences are not visible but instead are evident in differing carrier frequencies for specific disease genes; for example, thalassemia mutations are most common in peoples of Mediterranean and Southeast or East Asian descent, cystic fibrosis mutations are most common in peoples of northern European descent, and Tay-Sachs disease mutations are most common in peoples of Eastern European Jewish or French Canadian descent. Beyond differences in disease frequency, distinct human populations can also show varying degrees of disease severity. For example, although many different populations in Africa suffer from a high prevalence of sickle cell anemia, some tend to be more mildly affected than others because they continue to produce fetal hemoglobin, which blocks or limits aggregation of the mutant “sickle cell” hemoglobin protein. From these observations it is reasonable to conclude that different human groups might also have strikingly disparate response rates to specific disease treatments.

      In June 2005 the U.S. Food and Drug Administration (FDA) approved the use of BiDil, the first medication targeted to a specific racial group. BiDil, a product of NitroMed, Inc., in Lexington, Mass., is prescribed to prevent heart failure and is a combination of isosorbide dinitrate (a medication used to treat angina) and hydralazine (a medication used to lower blood pressure). Originally tested on a racially mixed population, BiDil appeared unimpressive. When a reanalysis of the study data took self-declared race into account, however, a striking outcome emerged— African American patients responded much better to the drug than did their Caucasian counterparts. In the original analysis, this response was masked owing to the preponderance of Caucasian patients in the study. In a subsequent study of more than 1,000 self-declared African American patients with congestive heart failure, BiDil reduced deaths by 43%, a result so dramatic the trial was stopped early, in 2004. Largely on the basis of these results, the FDA approved the sale of BiDil with its racially designated target population.

      The implications of the BiDil study were both simple and complex. If a medication worked well in some patients but not in others, the best medical practice clearly was to target the medication to those patients most likely to benefit. Ignoring factors, such as race, that might influence patient response would be negligent. Nevertheless, as pointed out by Francis Collins, director of the National Human Genome Research Institute in Bethesda, Md., self-declared race was a “biologically inaccurate and socially dangerous” surrogate for the more specific genetic and environmental factors that underlay the different responses that different patients had to any given treatment. The challenge was to identify and characterize those factors so that every patient could be assessed as an individual rather than as a member of a preestablished group and could thereby be treated with whatever medications were most likely to provide personal benefit. Classifying patients strictly by race assumes that all members of a race are identical, which clearly they are not. Further, racial classification discounts the existence of mixed-race individuals, who make up a significant and growing segment of most societies.

      Another concern raised by the BiDil example stemmed from the design of the follow-up study on which the new FDA approval was granted. Although the initial, smaller study suggested a racial disparity in drug response, the follow-up study lacked a racial control group—only one group was studied. By itself, therefore, the study could not claim differential drug efficacy in different racial groups, and marketing BiDil as a racially targeted therapy potentially limited access of non-African Americans to a treatment from which they also might benefit. Clearly, resolving this issue would require further studies involving a large number of patients from many different racial groups.

How Auxin Works.

      Indole 3-acetic acid, or auxin, is a plant hormone that helps plants to grow their shoots upward and roots downward and to flower and bear fruit. The process by which auxin works was not determined until 2005, some 70 years after the hormone was first identified in plants. (Since that time other auxins have been discovered, but it became common practice to use the term to refer specifically to indole 3-acetic acid, the most important one.) In May two groups working independently, one headed by Mark Estelle from Indiana University and the other headed by Ottoline Leyser from the University of York, Eng., reported that auxin binds to a protein complex called SCF^TIR1 and that, once bound, the complex acts to target a specific set of proteins, called Aux/IAAs, for degradation. Since Aux/IAA proteins normally repress the transcription of growth-related genes, auxin effectively induces transcription and thereby promotes cell growth.

      The discovery that auxin binds directly to SCF^TIR1 and results in the degradation of a transcriptional repressor was striking for at least two reasons. First, this mechanism of action is distinct from those of other hormone receptors that had been studied either in plants or in animals. Most hormone receptors influence gene expression by entering the nucleus in response to hormone binding or through a complex cascade of signaling enzymes. Second, SCF^TIR1 is an F-box ubiquitin protein ligase. Like other such molecules, it tags specific proteins for degradation by attaching a small protein marker called ubiquitin to them. Given that plants express about 700 different F-box proteins, the new findings suggested that at least some of these other F-box proteins might serve similar functions, perhaps mediating responses to other plant hormones. Indeed, the group headed by Estelle further reported that SCF^TIR1 is highly related to the F-box proteins AFB1, AFB2, and AFB3, each of which also functions as an auxin receptor, ostensibly triggering the degradation of different Aux/IAA targets. By controlling which F-box auxin receptors and which Aux/IAA proteins are expressed in specific cells and tissues, the plant could facilitate the many diverse physiological responses attributed to auxin.

      Much remained unknown about the newly discovered process. For example, it was unclear how auxin interacts with SCF^TIR1 and how binding this small ligand alters the activity of SCF^TIR1 with respect to Aux/IAAs. Also, the F-box proteins might represent only one of many auxin-receptor-and-response pathways. Finally, and perhaps most important, if indole 3-acetic acid could modulate the function of SCF^TIR1, were other ubiquitin protein ligases in plants and perhaps also in animals similarly subject to regulation by small-molecule ligands?

Judith L. Fridovich-Keil

Paleontology

 Perhaps the most astonishing event in paleontology in 2005 was the discovery of blood vessels and other soft tissues inside the femur (thighbone) of a Tyrannosaurus rex from a remote field site in the Hell Creek Formation in Montana. This amazing find fueled discussion as to whether protein or DNA might have survived within the tissues, which were about 70 million years old. Experts on fossil preservation were skeptical, saying DNA could not be preserved for so long a time. In one study of the tissues inside the bone, researchers identified a way of determining the gender of a dinosaur for the first time. Some of the tissue they found was similar to specialized bone tissue that forms in female birds during ovulation. In female birds the tissue, pitted with channels through which blood vessels run, serves as a source of calcium to produce egg shells. Based on the presence of such tissue in the T. rex bone the researchers concluded that the specimen was a female.

 In the summer of 2001, a team from the Burpee Museum of Natural History in Rockford, Ill., that was conducting fieldwork in the Hell Creek Formation discovered what it later determined to be the world's most complete juvenile tyrannosaurid. In 2005 the museum unveiled the mounted skeleton, named “Jane,” and cosponsored (with Northern Illinois University) a major symposium at which most of the world's experts on tyrannosaurs presented papers. The primary point of debate at the symposium was the status of Nanotyrannus, a genus of small tyrannosaurs. Although some paleontologists contended that it was a valid taxon, others argued that the specimens that represented it were juveniles of T. rex. The Burpee Museum specimen lay at the heart of the controversy, since the museum and some of the presenters claimed that it was a juvenile (11-year-old) T. rex, whereas other presenters contended that it was a new specimen of Nanotyrannus.

      Until recently most specimens of deinonychosaurs, the theropods most closely related to birds, had been found only in North America and Asia. In 2005 the specimen of a new species was described that provided the first noncontroversial evidence of the existence of deinonychosaurs in the Southern Hemisphere. The specimen, Neuquenraptor argentinus, was found in Patagonia (Argentina). Another new species of dinosaur, Falcarius utahensis, was described from the Early Cretaceous Cedar Mountain Formation in Utah. It represented the earliest-known therizinosauroid dinosaur from North America. Previously, this enigmatic group of theropods had been known mainly from China. F. utahensis lay at the base of the therizinosauroid clade and therefore documented the earliest stage in the transition from carnivorous to herbivorous traits that took place within the evolution of this group.

      The discovery of an oviraptorosaurian pelvis and a pair of eggs with shells within the pelvic cavity helped answer questions about the reproductive biology of oviraptorosaurians and other theropods collectively known as maniraptoran dinosaurs. The specimen was from the Upper Cretaceous Nanxiong Formation near Ganzhou, China. The relatively large size of the eggs, their placement within the pelvic cavity, and the anatomy of the pelvis led the authors to conclude that maniraptoran dinosaurs, like crocodilians, had two functional oviducts but that, like birds (which have only one oviduct), each oviduct produced only one egg at a time.

      Other dinosaur research during the year included a study of a new specimen of the theropod Majungatholus atopus. Openings within the vertebral column of the specimen were compared with similar structures for pulmonary air sacs that are found in living birds. The study concluded that the dinosaur had both cervical (neck) and abdominal air-sac systems, which implied, in turn, that it and other nonavian theropods had an avianlike pulmonary system. The finding supported other evidence that dinosaurs had relatively high metabolic rates.

      The discovery of embryos of the primitive dinosaur Massospondylus carinatus, which was believed to be related to the sauropods, provided insights into its developmental growth. Although adult members of this species had forelimbs significantly shorter than their hind limbs, the embryos had large forelimbs relative to body size. As the animal grew, therefore, the hind limbs must have grown rapidly in relation to the forelimbs. The authors suggested that the four-legged posture that evolved in sauropods might have been the result of a reduction in this disparate growth.

      Although fossil bird embryos had been previously reported in the scientific literature, no specimen had been found with its feathers preserved. A newly discovered embryo from the Early Cretaceous of Liaoning, China, had a hardened, nearly complete skeleton with sheets of feathers. The embryo was in the final stages of development prior to hatching and represented some type of early precocial bird (a bird capable of independent survival immediately after birth). A rare partial skeleton of a new bird, Vegavis iaai, from the Cretaceous of Antarctica, was the first Cretaceous bird that could be definitely placed within the lineages of living birds, which suggested, therefore, that living-bird lineages emerged prior to the Cretaceous/Tertiary mass-extinction event. (See Zoology.)

      Other excavations of the Early Cretaceous deposits of Liaoning yielded a new species of mammal, Repenomamus giganticus, that was the largest known from the Mesozoic—larger than some small dinosaurs. The body of the specimen was more than 1 m (3.3 ft) long, including the tail, and was estimated to have weighed up to 14 kg (30 lb). A specimen of a related smaller species, R. robustus, from the same deposit was found with the skeleton of a juvenile Psittacosaurus near where its stomach would have been. This finding provided the first evidence that some primitive mammals fed on dinosaurs.

      Taxonomists had long suggested that the nearest living relatives of the hippopotamus were the pigs and the peccaries. A recent study argued instead that hippos are the only surviving members of anthracotheres, a group that shared ancestry with the cetaceans (whales, dolphins, and porpoises). This view supported genetic studies that had shown that the whales were the nearest living relatives of the hippopotamus. New North American fossil material from Paleocene to Eocene apheliscine “condylarths” (early primitive ungulates) showed that a close relationship exists between this group and the extant African Macroscelidea (elephant shrews). This finding countered the idea that placental diversification was driven by the breakup of Gondwana.

      Research on the oldest known monotreme, Teinolophos trusleri from the Early Cretaceous, indicated that the complex structures of the mammalian inner ear evolved independently in the evolutionary lines of monotremes and therians (placentals and marsupials). Another study showed that Fruitafossor windscheffeli, a recently described Late Jurassic mammal from the Morrison Formation of Colorado, had highly specialized teeth similar to those of some placental mammals and very different from the generalized dentitions of most Jurassic mammals. This specimen also exhibited forelimb features that were specialized for digging.

      A new study of Late Permian terrestrial vertebrate faunas from the Karoo Basin of South Africa claimed that the fossil record showed a gradual extinction in the Late Permian followed by an increased rate of extinction at the Permian-Triassic boundary (particularly of small herbivorous reptiles called dicynodonts). This paper argued that the fossil record did not support theories in which the impact of an asteroid or meteorite caused the Permian-Triassic mass extinction and that the extinction event was protracted, lasting tens or hundreds of thousands of years. Another study of Late Permian fossils—of temnospondyl amphibians from the Moradi Formation of Niger—revealed characteristics of faunas that lived in the dry climate that prevailed at low latitudes at that time. These specimens were found to be surprisingly different from the much better-known Late Permian fauna of higher latitudes and thereby indicated that Late Permian faunas were less homogeneous than had previously been theorized.

William R. Hammer

▪ 2005

Introduction

Zoology

      During 2004 advances in zoological research of birds and insects increased scientists' understanding of the complexity of biological systems involving brood parasitism, aggression, and thermoregulation. Studies of fish and bats revealed information about the role that ecology and single phenotypic traits (observable properties) could play in the evolutionary divergence that might lead to the formation of species. Through an examination of the fossilized skull of Archaeopteryx, insights were gained into the way flight evolved in the earliest birds. In the field of conservation, two endangered West Indian insectivorous mammals were found to represent the only remaining species of an evolutionary divergence that occurred during the Cretaceous Period. DNA analyses played a prominent role in much of this work.

      Brown-headed cowbirds (Molothrus ater) lay their eggs in the nests of birds of different species—a behaviour that is called brood parasitism. The unsuspecting foster parents raise the baby cowbirds as their own. Offspring of some brood parasite species kill host young to ensure for themselves greater resources from attending parents. Likewise, it might appear to be in the baby cowbird's best interests for survival to kill the host birds' offspring, but baby cowbirds seldom do so. Rebecca M. Kilner and Joah R. Madden of the University of Cambridge and Mark E. Hauber of the University of Auckland, N.Z., studied this behaviour with an experiment in which single cowbird eggs were placed in each of 20 nests of the Eastern phoebe (Sayornis phoebe). Once a cowbird egg hatched, the researchers removed the remaining eggs from the nest. In 10 of the nests, they left the cowbird as the only bird in the nest. In the other 10 nests, the researchers introduced two newly hatched phoebes. Therefore, adult phoebes in 10 of the nests were left with a single baby bird (a cowbird) to tend, and in the other 10 nests, the parents were left with three baby birds. Using body weight as a measure of how effectively the baby cowbirds acquired food, the investigators found that cowbirds with two nest mates gained weight more rapidly than cowbirds alone in a nest. By filming the birds in their nests, the researchers discovered that parent birds with three baby birds brought food about 2¹/₂ times more often than those in nests with a single bird. A cowbird in a nest with two phoebes typically took more than half the food the parents brought, so it fared better than the lone cowbirds even though the lone cowbirds got all of the food that was brought to their nests. The study demonstrated that a cowbird's apparent altruism toward baby birds of other species is simply a strategy to get more food.

      Female honeybees (Apis mellifera) regulate the temperature of their hives, maintaining it close to 35 °C (95 °F) by fanning their wings for cooling in hot weather and huddling to generate heat from their bodies in cold weather. Honeybees operate as a single superorganism to regulate the temperature inside a hive as the outside temperature rises or falls. Julia C. Jones and colleagues of the University of Sydney, Australia, combined behaviour observations and DNA analyses to demonstrate that the temperature in a hive is more stable and better controlled when the bees are the offspring produced by the mating of the queen with a number of drones rather than with only a single drone. The researchers conducted experiments on pairs of hives having an equal number of bees. One hive had worker bees of mixed genetic parentage (offspring of a single queen and multiple drones), whereas the other housed bees of uniform genetic heritage (offspring of a single queen and a single drone). Worker bees in both hives ultimately maintained an average temperature of 35 °C. In the hive with bees of a mixed genetic makeup, the temperature remained relatively constant, regardless of the outside temperature. In contrast, the temperature in the hive with bees of uniform genetic makeup varied greatly and took longer to regulate than in the genetically diverse hive. The researchers then used DNA tests to confirm the existence of a relationship between genetics and the behaviour of bees of a genetically mixed hive. The tests showed that all the bees that started fanning at a given temperature were more likely to have the same father than those that began fanning at some other temperature. These results suggested that the threshold temperature at which an individual bee begins participating in thermostatic regulation in the hive is genetically based. The bees in a genetically diverse hive are able to keep the temperature more stable because they respond to a broader range of temperatures, some bees beginning the cooling or warming process sooner than others.

      Markus Knaden and Rüdiger Wehner of the University of Zürich, Switz., studied aggression in Saharan desert ants (Cataglyphis fortis), which become combative upon encountering ants from colonies other than their own. Desert ants will travel more than 100 m (1 m = 3.3 ft) to gather resources, and as the ant moves away from its nest, its level of aggression decreases. The greater belligerence of the ants in the vicinity of their nest might serve a protective role in guarding the nest of a colony, but the way in which the ants determine their proximity to the nest was unknown. The researchers trained ants to visit a feeding area 20 m from their homes, a distance at which the ants have reduced aggressiveness toward other ants. Ants from four different colonies were captured at the feeding area and then were marked with coloured dots for identification and transported to a distant site. Upon being released at the distant site, the ants immediately began crawling toward their respective nests. Some ants were allowed to travel 20 m toward their nest; others were allowed to travel only a quarter that distance. The investigators then captured the ants again and took them to a laboratory to test their level of aggression. Each ant was paired in a box with an ant from a different colony, and their behaviour was videotaped. Ants that had traveled the 20 m toward their nest were significantly more likely to attack than those that had traveled only the shorter distance. The experiment suggested that the aggressiveness of the Saharan desert ant is based on its perception of the proximity to its home and that the ant does not use sight, smell, or landmarks in determining its location. Instead, some yet-to-be-understood internal means of navigation allows the Saharan desert ant to know how far it has traveled from home.

      Tigga Kingston of Boston University and Stephen J. Rossiter of Queen Mary, University of London, showed that the echolocation used by three distinct sizes (morphs) of large-eared hoseshoe bats (Rhinolophus philippinenesis) of Indonesia is accomplished at the same basic frequency of sound but with harmonically distinctive echolocation calls. The different harmonics allow each morph to use echolocation to detect its own suitable prey. The researchers suggested that natural selection for prey-related shifts in echolocation harmonics can lead to related shifts in the sounds used for communication within morphs during mating. These shifts would enhance evolutionary divergence by means of assortative mating (selective mating between individuals in a population) and subsequent reproductive isolation. The investigators showed through DNA analyses that the three morphs have indeed become genetically diverse, while remaining sympatric (occupying the same geographic area).

      Studies with three-spined sticklebacks (Gasterosteus aculeatus) by Jeffrey S. McKinnon of the University of Wisconsin at Whitewater and colleagues provided further evidence that evolutionary divergence and reproductive isolation can be caused by only one or a few ecologically significant traits. Sticklebacks make up a species complex that includes two ecotypes—stream-dwelling populations and anadromous populations (populations that live in the ocean and migrate to fresh water to breed). Both types are found across the Northern Hemisphere and are found together, but typically only minor genetic exchange occurs between them. The researchers collected samples of both ecotypes from a variety of locations and maintained them in the laboratory. Anadromous sticklebacks typically grow to a larger size than stream-dwelling sticklebacks, but the investigators controlled the growth of the fish to produce females with a range of body sizes in both types. During experiments the primary factor influencing mating compatibility between females and normal-sized males was similarity in body size, although similar-sized pairs of the same ecotype were slightly more compatible reproductively than similar-sized pairs of different ecotypes. Colour patterns and genetic similarities were not significant factors.

       Archaeopteryx, which lived in the Late Jurassic Period, is the epitome of a transitional form on an evolutionary continuum: it possesses teeth characteristic of a reptile but also has feathers, which are characteristic of birds. Although a number of fossils of Archaeopteryx have been discovered and studied, the question remained whether the animal was able to fly. Patricio Domínguez Alonso and colleagues of Complutensian University, Madrid, examined Archaeopteryx fossils with computed tomography, a technique for obtaining cross-sectional images of a solid object by scanning it with X-rays. The investigators found unequivocal evidence of an enlarged forebrain and of optic and auditory systems typical of animals adapted for flight.

      Only two species of insectivorous mammals are extant in the West Indies. Both are extremely rare and endangered. One, Solenodon cubanus, is found in Cuba and the other, S. paradoxus, is found on Hispaniola. Alfred L. Roca, Gila Kahila Bar-Gal, and William J. Murphy of the Laboratory of Genomic Diversity, Frederick, Md., and colleagues used DNA gene sequencing to determine that the solenodons diverged from the insectivores, such as shrews, moles, and hedgehogs, during the Cretaceous Period 76 million years ago and that the two species diverged from each other around the time Cuba and Hispaniola split into separate islands, 25 million years ago. The continued existence of both species was being threatened by a variety of human-caused environmental changes, including deforestation and the introduction of predatory species such as dogs, cats, and mongooses. From the perspective of conservation, the findings accentuated the significance of the two species, since they represent a complete phylogenetic lineage that predates the appearance of many present-day orders of mammals.

J. Whitfield Gibbons

Botany

      Research into microRNAs—short strands of RNA that regulate gene expression—made significant progress in 2004. Hundreds of different microRNAs were believed to exist in every species of plant and animal, but the function of only a few had been understood. Researchers found that the microRNA called miR164 played a vital role in the development of flowers, leaves, and stems of Arabidopsis thaliana, a plant commonly used in genetics studies. The researchers created one mutant strain that produced excess miR164 and another that was not affected by it. In both mutant strains the leaves and flowers developed abnormally; in the strain that made excess miR164, the organs tended to fuse together, and in the strains that did not respond to it, the wrong number of petals or other organs formed.

      Another type of microRNA was found to act as part of a gene-switching mechanism dating back 400 million years to the very first land-based plants. Plant biologists at the University of California, Davis, found that the microRNA controlled a gene family called class III HD-Zip, which is required for the development of stems and leaves. The microRNA behaved in the same way in all the major groups of land plants that were studied. It was also the first microRNA shown to regulate genes in nonflowering plants such as mosses.

      Scientists at the John Innes Centre and Institute of Food Research in Norwich, Eng., reported the discovery of a gene that offered the hope of breeding food crops that have both an increased resistance to disease and properties that promote human health. The gene, HQT, was identified in tomato plants and produces chlorogenic acid (CGA), which functions as an antioxidant—that is, a substance that inhibits chemical reactions involving reactive forms of oxygen. By increasing the activity of HQT in tomato plants, the scientists raised the levels of CGA in tomato fruits, helping to protect them from bacterial disease. The antioxidant had also been shown to be beneficial in humans, especially in protecting against age-related disease.

      One way a plant controls the sprouting of branches, which affects the overall shape of the plant, was traced to a gene called MAX3. Researchers reported that Arabidopsis plants that bear an unusually high number of side shoots tended to have mutations in this gene. Auxin and cytokine hormones were already known to influence branching, but they also were known to have a wide range of other developmental effects. It was hoped that disruption of the MAX3 gene could be used to modify branching without these additional effects. Such modification could potentially offer benefits in plant breeding, including improvements in the appearance of ornamental plants and a reduction in branching in trees grown for timber.

      Progress in genetic modification produced some fascinating new plants. Aresa Biodetection, a Danish biotechnology company, developed a genetically modified variety of A. thaliana that could help detect land mines. Buried land mines typically emit a small amount of nitrogen dioxide gas, and the plant was modified so that within a few weeks' exposure of the roots to the gas, the leaves of the plant would change colour from green to red. The researchers manipulated the natural anthocyanin pigments in the plant leaves by first turning off the genes that produce the red version of the pigment and then inserting a gene that turns on the pigment-making apparatus when nitrogen dioxide is present.

      A previously unknown form of natural protection from disease was discovered in cocoa leaves. Biologists had been baffled by the vast variety of fungal species that live inside plant leaves and had assumed that many of the fungi were parasites. Scientists studying cocoa trees, however, found that some of the fungi inside the leaves of the cocoa tree are beneficial to the tree. The research involved growing cocoa seedlings under conditions that kept some of the leaves free from fungi and then introducing a fungal disease known as Phytophthora. Leaves devoid of fungi were three times as likely to die from the disease as the leaves that contained the fungi, and they lost twice as much leaf tissue. This finding could lead to an inexpensive and environmentally friendly way to protect cocoa trees and many other crops from the ravages of microbial diseases.

      Worrying indications were found of the effects on plants of the increasing levels of carbon dioxide (CO₂) in the atmosphere and how this in turn could have an impact on global climate. A team of botanists discovered that large fast-growing trees in a pristine part of the Amazon rainforest had been increasingly dominating their slower-growing neighbours over the past 20 years. The fast-growing trees might have gained the upper hand over other trees by being able to absorb more CO₂ to support photosynthesis and hence growth. This phenomenon could potentially reinforce the threat of increased CO₂ emissions on the global climate because the demise of the slower-growing trees might lead to a drop in the amount of CO₂ that the rainforest removes from the atmosphere. In comparison with fast-growing trees, slower-growing trees tend to absorb more carbon dioxide from the atmosphere because they have denser wood and a higher carbon content. The entire Amazon rainforest absorbed around 600 million metric tons of the gas per year (around 8% to 10% of that emitted in air pollution) and thereby helped hold in check its greenhouse effect on rising global temperatures.

      The rising level of CO₂ was decreasing the rate of photorespiration in plants. Photorespiration is a process in which plants turn sugars produced during photosynthesis back into carbon dioxide and water. The process had long baffled plant scientists because it uses up about 25% of the energy that a plant captures during photosynthesis. As photorespiration rates decreased, some biologists sought through genetic engineering to eliminate photorespiration altogether in crop plants to make them more productive. A team of University of California, Davis, researchers led by Arnold Bloom warned against such efforts, however, because they had determined that photorespiration enables plants to absorb nitrates from the soil and convert them into chemical compounds the plants need for their growth. Inhibiting photorespiration eventually starves the plant of nitrogen, weakening the plant. “This explains why many plants are unable to sustain rapid growth when there is a significant increase in atmospheric carbon dioxide,” said Professor Bloom. “As we anticipate a doubling of atmospheric carbon dioxide associated with global climate change by the end of this century, our results suggest that it would not be wise to decrease photorespiration in crop plants.”

      Scientists also found that changes in the amount of CO₂ in the atmosphere played a vital role in plant evolution. Between 340 million and 380 million years ago, when the amount of the gas in the atmosphere plunged, the size of plant leaves increased 25-fold, on average. Examination of two fossil species revealed that the average number of leaf pores, called stomata, on each leaf increased eight times over the same period. “This all suggests that the crash in carbon dioxide triggered the evolution of leaves,” said Colin Osborne at Sheffield (Eng.) University. When plants first appeared on land, the atmosphere was so rich in CO₂ they hardly needed leaves, but when the level of CO₂ plunged, the plants were left “suffocating” and evolved bigger leaves to absorb more of the gas.

Paul Simons

Molecular Biology and Genetics

The Brave New World of Bioplastics.

       Plastics are polymers—assemblies of like chemical subunits, called monomers, linked in the form of a chain. The properties of a plastic, like those of other polymers, are defined by the monomers it contains and by the number of links and cross-links in its structure. Cross-linking of the monomers increases a polymer's rigidity and thermal stability. As their name suggests, plastics can readily be molded into various shapes. Plastics such as polystyrene (polymerized styrene [CH₂=CH(C₆H₅)]), polyethylene (polymerized ethylene [CH₂=CH₂]), or polypropylene (polymerized propylene [CH₂=CH(CH₃)]) are molded into a wide variety of everyday and specialized products—eating utensils, coffee cups, synthetic fabrics, park benches, automobile parts, and surgical implants, to name but a few.

      The past 100 years have seen an explosion in the development and use of plastics, and their utility and importance have become so great that it is difficult to imagine modern life without them. Virtually all plastics are derived from petroleum, through chemical extraction and synthesis. Because petroleum-based plastics are generally not biodegradable, plastic refuse is very durable, and disposing of it can become a problem. Despite efforts to encourage and support recycling, landfills are becoming filled with plastic refuse, which also accumulates in the environment. An additional problem with petroleum-based plastics is that sources of petroleum are being used up; conservative sources estimate that at current rates of consumption, all known sources of petroleum on Earth will have been depleted before the turn of the next century. How can quality of life, with its dependence on plastics, be maintained in the long term, given that petroleum is a nonrenewable resource and that petroleum-derived plastic waste degrades the environment? The answer might be bioplastics.

      Bioplastics are polymers of monomers that are derived from or synthesized by microbes such as bacteria or by genetically modified plants. As is the case with petroleum-based plastics, the physical properties of bioplastics differ according to their monomer composition and macromolecular structure. Unlike traditional plastics, bioplastics are obtained from renewable resources, and, best of all, they are biodegradable.

      The first known bioplastic, poly(3-hydroxybutyrate), or PHB, was discovered in 1926 by the French researcher Maurice Lemoigne from his work with a bacterium called Bacillus megaterium. Unfortunately, the significance of the discovery was overlooked for many decades, in large part because petroleum was inexpensive and abundant. The petroleum crisis of the mid-1970s brought renewed interest in finding alternatives to petroleum-based products. The rise of molecular genetics and recombinant biotechnology after that time further spurred research, so that by late 2004 the structure, method of production, and application for numerous types of bioplastics had become established. Bioplastics that were either in use or under study included PHB and PHA [poly(3-hydroxyalkanoate)], both of which are synthesized within specialized microbes, and polylactic acid (PLA), which is polymerized from lactic acid monomers produced by microbial fermentation of plant-derived sugars and starches. Recent technological advances further improved the strength and thermal stability of bioplastics by permitting the incorporation of strong plant fibres. Although the commercial manufacture of bioplastics initially had low yields and was expensive, improvements in metabolic and genetic engineering produced microbial and plant strains that significantly improved yields and production capabilities while reducing overall costs.

      Bioplastics production was still insignificant in terms of the total world production of plastics in 2004, but Toyota Motor Corp. was using bioplastics (primarily for interiors) in some new vehicles, and Sony Corp. was using bioplastics in the casing of Walkman portable stereos. Technical improvements in the production of bioplastics and their application, together with an increase in oil prices and environmental awareness, were sure to expand the market share of bioplastics in the years to come.

Advances in Personalized Medicine.

      Personalized medicine continued to develop as an area of study in which biomedical researchers and health-care providers explored the genetic differences between individuals and investigated how to take these differences into account in order to provide health care tailored to each individual. One of the toughest challenges in providing proper medical care arises from the fact that a disease can affect different people in different ways. In two patients with the same disease, there can be large variations in the symptoms, severity, and progression of the disease, as well as in how well each patient responds to a specific form of treatment. Some of the variations have behavioral causes, such as whether the patient smokes, exercises regularly, or eats a healthy diet. Other variations however, appear to be intrinsic to the individual, and are likely genetic in origin.

      Relevant genetic differences between patients can include mutations that alter the structure of proteins that are targetted by a specific drug, rendering the patient either more or less susceptible to treatment by the drug. Genetic differences might also have an effect on the expression levels of numerous nontarget genes and proteins in the cell and thereby produce cellular environments with either a heightened or a muted sensitivity to a given drug. For example, there could be genetic differences that alter the efficiency with which the drug enters the cell or the efficiency with which the drug is metabolized and thereby either activated or inactivated. Many of the studies that were being conducted simply searched for correlations between patient outcome and specific mutations or expression profiles. (An expression profile for a cell or tissue does not identify mutations but rather describes the levels at which many different genes are expressed.) Understanding the mechanisms that lead to specific patient outcomes might be the ultimate goal, but the identification of correlations between outcome and mutations or expression profiles could itself be a powerful advance. For example, a recent collaborative study led by researchers from Massachusetts demonstrated that patients with lung cancer whose tumour cells carried specific mutations in their epidermal growth-factor receptor (EGFR) gene were more likely to respond to therapy with the drug gefitinib (Iressa), an EGFR kinase inhibitor, than were patients whose tumours did not carry the mutations. Another study, led by researchers from Oregon, involved expression profiling of the so-called GABAergic-system genes in patients with neuroblastoma. The expression profiles that the researchers obtained improved their ability to predict patient outcome beyond what was achieved with other prognostic indicators.

      Many diseases remained poorly understood, and identifying which genetic markers were relevant and identifying their influence on the severity of a disease and the disease's response to treatment could be determined only empirically. New studies that monitored large numbers of patient markers and compared this information with the treatment and outcome of certain diseases offered both physicians and patients a new tool to help them make the often difficult choices between different types of treatment. Sets of markers that were associated with the occurrence of breast cancer, cardiovascular disease, and other diseases were becoming better defined, and markers that indicated a patient's response to certain diseases and specific treatments were also becoming more apparent. Additional examples included markers that helped in predicting a patient's susceptibility to atherosclerosis and markers that were linked to how well a patient with prostate cancer responded to treatment with selenium.

      As simple correlations, these data enabled physicians to begin making choices among potential treatments. Already patients with specific forms of cancer, including breast cancer, prostate cancer, and lung cancer, have benefitted from the first forays of the medical profession into the world of personalized medicine. Perhaps more important, investigations into the mechanistic reasons different genetic or expression profiles in patients have different outcomes might enable the development of improved forms of treatment.

Judith L. Fridovich-Keil

Paleontology

      The year 2004 in paleontology began with a press conference in February sponsored by the U.S. National Science Foundation at which the discovery of two dinosaurs from Antarctica was announced. One dinosaur was a small Late Cretaceous theropod (a bipedal flesh-eating dinosaur); the other, an Early Jurassic sauropod (a plant-eating dinosaur with a long neck and tail). The theropod was collected on Ross Island, off the Antarctic Peninsula. The primitive sauropod was retrieved on Mt. Kirkpatrick near the site that in 1991 yielded Cryolophosaurus, the first theropod discovered in Antarctica. Mt. Kirkpatrick is only about 600 km (370 mi) from the South Pole.

      The 125-million-year-old (Early Cretaceous) Yixian Formation of Liaoning province, China—a feature known for its well-preserved specimens of feathered dinosaurs and birds—continued to be a source of notable discoveries. A cluster of 34 juvenile specimens of the ornithischian dinosaur Psittacosaurus was unearthed together with a single adult, which indicated that psittacosaurs provided parental care to their offspring. An amazing specimen showing an embryo of a pterosaur inside an egg was found, evidence that pterosaurs, like dinosaurs, were egg layers. Another discovery was a feathered dinosaur—a troodontid—with its head tucked under a forearm. The fossil, called Mei long (Chinese for “soundly sleeping dragon”), was 130 million years old and the earliest known specimen to exhibit such markedly birdlike behaviour. Yet another discovery from the region was Sinodelphys, which, with an age of 125 million years, was 50 million years older than the next oldest known marsupial. The oldest known placental mammals were also from the Yixian Formation, which suggested that both groups might have originated in Asia in the early part of the Cretaceous. In a related story published in 2004, Chinese paleontologists expressed concern that the treasure trove of magnificent fossils in the Liaoning deposits was being rapidly depleted by the illegal collection and sale of specimens. They claimed that weak laws together with a failure to enforce them were to blame for the situation, and they were working to help establish the governmental reforms necessary to stop the illegal trade of Chinese fossils.

      Other dinosaurs from China that were described included a number of specimens found in Mongolia: Pinacosaurus from the Late Cretaceous and a duckbilled dinosaur from the Early Cretaceous. A comparison of the Pinacosaurus specimens showed how the dinosaur grew and changed as it aged. The duckbill was the oldest known specimen from Asia; it raised the possibility that duckbilled dinosaurs might have originated there before spreading to other parts of Laurasia (the land mass that became Asia, Europe, and North America). Deposits dated to 55 million years ago (Early Eocene) of the Hengyang Basin in China recently yielded the oldest known euprimate, Teilhardina. Euprimates are animals with modern primate features.

      A find in northwestern China of a new crocodylomorph from the Middle Jurassic was identified as a sphenosuchian (a class of small, slender land-dwelling animals) that had several features typical of living alligators and crocodiles. It was speculated that this animal was the closest relative of the living crocodilians.

      A combination of recent advances in techniques for measuring annual growth rings in fossil bones and for estimating the body mass that would be supported by dinosaur bones of different sizes was used to calculate growth curves for Tyrannosaurus rex and three other theropods closely related to it. The study showed that T. rex reached skeletal maturity in about 20 years and lived for as long as 28 years. All four dinosaurs experienced a comparable period of rapid growth during adolescence, but the growth rate of T. rex—an average of 2.1 kg (4.6 lb) per day over four years—was several times faster than that of the other dinosaurs.

      A remarkable pterosaur specimen recently found in the Early Cretaceous Santana Formation of Brazil has the tooth of a spinosaurid theropod dinosaur embedded in one of the cervical vertebrae—an indication that spinosaurs might have been capable of catching flying prey. New views of the internal features of skulls of pterosaurs and fossil early bird skulls made possible by computed tomography scans helped in interpreting how these animals flew. A paper describing the large size of a pterosaur inner ear indicated that pterosaurs had well-developed balance organs, which would have given them agility during flight. A new study of the brain case of Archaeopteryx suggested that its brain was similar to that of modern birds. (See Life Sciences: Zoology.)

      A recent study of Neanderthal tooth enamel concluded that Neanderthals grew up and reached maturity much faster than Homo sapiens individuals. The authors suggested that this finding strongly supported the idea that H. neanderthalensis was a separate species rather than a subspecies of H. sapiens. (See Anthropology and Archaeology Sidebar. (Neanderthals-the Latest News ))

      A fossil lower jaw found in Belgium represented the first known Late Devonian tetrapod from Western Europe. The jaw was very similar to that of Ichthyostega, from the Late Devonian of Greenland, and confirmed a link between Greenland and Europe.

      Newly discovered well-preserved soft-bodied fossils of deuterostomes from the Lower Cambrian Chengjiang deposits near Kunming in southwestern China represented a new group of echinoderms (a group of marine animals). Named vetulocystids, these deuterostomes were a diverse superphylum that included the chordates, hemichordates, and echinoderms. The find shed some light on the origin of the echinoderms.

      A fossilized specimen of the newly described arthropod Marrella splendens from the 505-million-year-old (Cambrian) Burgess Shale of British Columbia showed the organism in the act of molting. Before 2004 the existence of molting in early arthropods had been only inferred from what was known about their living relatives. In other news related to arthropods, a fragmentary fossil found in Scotland's Old Red Sandstone deposits (396 million to 407 million years old) was called the world's oldest known true insect. The specimen had features common to winged insects, which suggested that the origin of wings might have occurred earlier than previously believed.

      A study dealing with the regeneration of arms in crinoids indicated that they suffered nonlethal attacks by predators. The analysis of Paleozoic crinoids showed an increase in the incidence of arm regeneration during the Silurian and the Devonian. The authors referred to the increase in the diversity of shell-crushing predators and the antipredatory morphologies crinoids and other prey species developed in response as the Middle Paleozoic Marine Revolution.

      Newly discovered soft-bodied fossils from Spaniard's Bay in eastern Newfoundland showed a greater level of preservation than previously described Ediacaran (Late Precambrian) fossils. The fossils were classified as rangeomorphs, a group that dominated what is called the Mistaken Point assemblage, which lived from 575 million to 560 million years ago. The study indicated that the rangeomorphs were not ancestral to any organisms known to have existed since the beginning of the Cambrian.

William R. Hammer

▪ 2004

Introduction

Zoology

      Primate research in 2003 provided new insight into the evolution of culture—the transmission of socially learned knowledge or tradition to succeeding generations. Humans once had been thought to be the only species in which differences ascribed to culture exist between populations. In 1999, however, observed differences in chimpanzee behaviour in different geographic regions were cited as evidence of culture. During 2003 Carel P. van Schaik of Duke University, Durham, N.C., and colleagues documented geographic variation in the behaviour of another nonhuman primate species, orangutans (Pongo pygmaeus), in Borneo and Sumatra. The investigators examined wild orangutan populations at six sites to determine if tool using and other specific behaviours were present in a population at one site but absent in all the others, findings that would support the position that cultural evolution had taken place. Population-specific behaviours that the investigators classified as “very likely cultural variants” included using leaves to wipe the face, poking into tree holes with a tool to obtain insects, using a leafy branch to scoop up water from a tree hole, and making characteristic spluttering sounds when bedding down for the night. The scientists also noted that dissimilarities in the behaviours increased with geographic distance between orangutan populations, which supported the interpretation that the behaviours were culturally based, and that the habitat of a population appeared not to influence whether a given behaviour was present or absent. Further, they suggested that cultures not only can be found currently among the great apes but also may have existed for 14 million years in this group of animals.

      As two or more species interact, they can evolve in response to each other, a process called coevolution. In 2000 Ethan J. Temeles and colleagues of Amherst (Mass.) College reported on the dynamics of such a relationship between purple-throated carib hummingbirds (Eulampis jugularis) on the island of St. Lucia in the Lesser Antilles and the plants on which the birds feed. The hummingbirds obtain nectar from two heliconia species, Heliconia caribaea and H. bihai, for which the birds are the only means of pollination. The investigators focused their study on the evolution of sexual dimorphism in the birds—i.e., the differences between males and females in body size or in the proportions and appearance of body parts. Male hummingbirds have larger bodies, longer wings, and shorter, straighter bills than females. They were found to dominate feeding at the more energy-rich plant, H. caribaea, which also bears shorter, less-curved floral structures that correspond to the males' bills. In contrast, females were found to feed at H. bihai, which bears longer, more curved floral structures corresponding to the females' bills. Temeles and his Amherst colleagues concluded that the differences between the sexes in bill size and shape have an ecological cause involving the birds' specialization on the two flower types. At the same time, they noted that in parts of St. Lucia where H. caribaea is rare or absent, H. bihai exists in two forms, one with many shorter, straighter flowers matching the bills and energy needs of males and the other with fewer longer, curved flowers matching the bills and energy needs of females. This prompted speculation that H. bihai had evolved in response to the birds' sexual dimorphism.

      In 2003 Temeles and W. John Kress of the Smithsonian Institution's National Museum of Natural History published a follow-up report on the relationships among carib hummingbirds and heliconias on Dominica, another Lesser Antillean island. In contrast to the situation on St. Lucia, H. caribaea was the more abundant plant species. Moreover, analogous to H. bihai on St. Lucia, H. caribaea was found to have evolved two flower forms, one matching the bills and energy needs of males and the other of females. Together the two studies demonstrated that differences in flower forms drive evolution of sexual dimorphism in the hummingbirds and that hummingbird dimorphisms and partitioning of resources between the sexes drive specialization between and within Heliconia species—all in support of the hypothesis that a coevolutionary association indeed exists.

      Two research teams independently came to complementary conclusions about the way reproductive success can hinge on a factor that influences a female bird's choice of a mate. The factor in question is a group of organic pigments, called carotenoids, that occur widely in plants and that are the basis for many of the yellow-to-red hues in both plants and animals. Birds and other animals cannot synthesize carotenoids but must obtain them from their diet. In some bird species they are responsible for a secondary sexual trait, the colour of the male's bill, which is used to advertise fitness and influence mate choice and competition between males. In many animal species carotenoids also have important roles in maintaining health. In birds they participate in immune responses, for example, to challenges from foreign invaders such as parasites.

      In one study Bruno Faivre of the University of Burgundy, Dijon, France, and colleagues conducted experiments with Old World blackbirds (Turdus merula), a species in which males with higher carotenoid levels have brighter orange bills and presumably greater mating success because they are more likely to be chosen by and to mate with healthier females. The investigators tested how carotenoids were allocated between sexual display and immune defenses. They discovered that bill colour faded in birds that had been injected with foreign blood cells to stress their immune system, evidence that the sexual signal of bill colour is indeed an indicator of the individual's health. In the second study Jonathan D. Blount of the University of Glasgow, Scot., and colleagues confirmed the phenomenon in experiments with zebra finches (Taeniopygia [or Poephila] guttata) in which each of 10 pairs of sibling males were fed either carotenoids or distilled water (the latter as controls). The bills of birds receiving the carotenoids turned significantly redder than those of the controls, and females spent significantly more time perched next to the males with brighter bills and thereby indicated a preference for them. A plant protein that provokes an immune response in birds then was injected into both the carotenoid-supplemented and the control males. The carotenoid-supplemented birds showed a much stronger immune response, which had already been documented to increase a bird's chances for survival. A significant finding of the studies was that secondary sexual traits used in mate-choice decisions by females can be true indicators of health and presumed fitness of males.

      Another study, in this case involving mammals, called attention to a different kind of factor that can affect mate choice and reproductive success. Joseph I. Hoffman and William Amos of the University of Cambridge and Ian L. Boyd of the Natural Environment Research Council, Cambridge, correlated details of breeding behaviour in Antarctic fur seals (Arctocephalus gazella) with the reproductive success of male seals to assess the importance of male competition and territorial defense on the breeding beach relative to alternative male strategies (e.g., aquatic mating before females reach the beach) and female choice of mates. Classically, in a mammalian breeding colony with male territoriality, the mating system is expected to be polygyny, in which one male mates with multiple females. Successful defense of a territory increases the chances of successful mating with any females living within the defended area. A successful territorial male thus has a higher probability of reproductive success than one having no territory.

      To confirm this expectation for Antarctic fur seals, the investigators determined paternity of seal pups by conducting genetic analyses on tissue samples from 1,800 individuals taken over a seven-breeding-season period from Bird Island, South Georgia. Of 415 males for which genetic identity could be determined, 22 (about 5%) successfully defended territories. Of 660 seal pups for which paternity could be determined, the 22 territorial males were the fathers of 59%. Although most males had only one successful reproductive season, those returning to the same breeding beach had increasing success in subsequent years. Especially interesting was the observation that the success of territorial males varied among females depending on their maternal status—females that arrived at the breeding beach and did not have pups were more likely to mate with males from other beaches that season. Although the research confirmed that polygyny was the norm in the species, the importance of maternal status in male mating success was unexpected.

J. Whitfield Gibbons

Botany

      British scientists in 2003 reported the results of a large study of the environmental effects of genetically modified (GM) crops. The farm-scale trials, which cost $8.5 million and lasted four years, were designed to test whether weeds and insects, such as butterflies, bees, and beetles, fared better in fields of conventional crops or of crops that had been genetically altered to be resistant to a herbicide for weed control. A major emphasis of the study was on the importance of crop weeds, which were well known to be of benefit to wildlife by providing cover and food for insects (as well as seeds for birds). The experiment found that fields of GM sugar beet and oilseed rape (canola) were worse for insects than fields of conventional varieties of the crops. GM corn (maize), on the other hand, was better for many types of insects than conventional corn. The study attributed the variation to a difference in the weed burdens of the crops. GM beet and rape were associated with fewer weeds than their non-GM equivalents, whereas GM corn actually had more weeds than conventional corn.

      It already had been determined that GM crops can crossbreed with wild plants through the spread of their pollen, but new work revealed that the dispersal of seeds carrying modified genetic material also can play an unexpected role in the long-distance spread of the genes. A team headed by Jean-Franƈois Arnaud of the University of Lille, France, found that seeds from hybrids of weed beets and GM sugar-beet crops had escaped to more than 1.5 km (about one mile) from the commercial fields in France where they had arisen. These results suggested that seeds carrying GM material may accidentally be spread by humans, most likely in soil caught on vehicle wheels or transported by other agricultural activities. Once the seeds have escaped, the plants can then cross-pollinate with nearby wild relatives and create new and possibly damaging hybrids with modified genes.

      Despite the concerns over safety, new and intriguing uses for GM plants were under investigation. The Defense Advanced Research Projects Agency, part of the U.S. Department of Defense, awarded a $2 million grant to plant biologist June Medford of Colorado State University for an ingenious plan to genetically engineer plants to detect a chemical or biological attack by changing colour.

      Big strides were made in understanding the master controls that plants use to organize their shape and development. A gene dubbed PHANTASTICA was found to control whether tomato plants develop their normal featherlike (pinnately compound) leaf arrangement or an umbrella-like (palmately compound) arrangement like clover. “It's a very surprising finding, that modifying one gene in the tomato alters the leaf from one form to another,” said Neelima Sinha of the University of California, Davis, who was involved in the research. The same genetic mechanism appeared to be shared by a wide group of flowering plants.

      In another breakthrough, for the first time in plants, tiny genetic components called microRNAs were found to switch off the expression of shape-regulating genes. MicroRNA molecules, which were first recognized in the early 1990s, are short strands of RNA that are transcribed from parts of an organism's genetic blueprint that once had been thought to be useless, or “junk,” DNA. Rather than being merely the intermediaries between DNA and protein, as are messenger RNA (mRNA) molecules, they have critical roles themselves in the regulation of gene expression. MicroDNAs work by recognizing and binding to specific mRNAs and bringing about their inactivation or destruction at the appropriate time. A team led by Detlef Weigel of the Max Planck Institute for Developmental Biology, Tübingen, Ger., and James Carrington of Oregon State University found overly high levels of one such microRNA in a mutant Arabidopsis thaliana plant (a favourite model organism of plant geneticists), which grew unusual crinkled and wrinkly leaves. The researchers showed that this microDNA regulates the expression of a set of genes (named TCP genes) that prevent excess cell division in the growing plant. Too much microDNA in the mutant plant allowed too many cells to proliferate in the leaves and caused the crinkling. By contrast, microDNA in normal plants appears at the right level, time, and place to create flat leaves. As more microRNAs were being discovered, their importance in plant growth and development was becoming clearer. This opened up entirely new and exciting possibilities for the use of these molecules as tools to manipulate the activities of plant genes, with potentially enormous scientific and economic benefits.

      With overtones of the movie Jurassic Park, the oldest plant DNA found to date was extracted from drilled cores of frozen soil in Siberia by a team led by Eske Willerslev of the University of Copenhagen. The DNA fragments, some from plants that lived as long as 400,000 years ago, were identified as belonging to at least 19 different plant families. This ability to recover specimens of ancient DNA directly from soil samples, which would obviate the need for identifiable fossils, could revolutionize studies that attempt to construct a genetic picture of past ecosystems. Because the extracted DNA was broken up into tiny pieces, however, there seemed little chance of resurrecting any of the species.

      The changing world climate was having wide-ranging effects on the productivity of plant life. From 1982 to 1999, climate change resulted in a 6% increase in plant growth over much of the globe, reported Ramakrishna Nemani of the University of Montana and colleagues after they analyzed climatic ground and satellite data. The largest increase occurred in tropical ecosystems and especially in the Amazon rainforests, which accounted for 42% of the global increase, owing mainly to less cloud cover and the resulting increase in sunlight in that region. As trees and other vegetation grow, they take carbon dioxide from the atmosphere and convert it to solid carbon compounds. It was not clear, however, whether or how the observed growth increase would affect the removal of carbon dioxide, a greenhouse gas widely cited as the major driving force behind global warming, and its storage in terrestrial ecosystems over the long term.

      The increasingly important role of botanic gardens in understanding and conserving plant life was recognized in July when Kew Gardens in London was made a World Heritage Site by UNESCO. In addition to being known internationally for its historic public gardens and buildings, Kew is a world famous scientific organization, renowned for its living and herbarium collections of plants, research facilities, and contribution on a major scale to conservation and biodiversity.

Paul Simons

Molecular Biology and Genetics

DNA at 50.

      “We wish to suggest a structure for the salt of deoxyribose nucleic acid (D.N.A.). This structure has novel features which are of considerable biological interest.”

      So began, in the April 25, 1953, issue of Nature, the deceptively modest description of DNA that would be hailed a half century later, in 2003, as one of the truly groundbreaking advances in science. In their one-page paper, James Watson and Francis Crick depicted the molecular repository of genetic information as “two helical chains each coiled round the same axis”—a now-iconic image known worldwide. Although these researchers clearly achieved their feat by standing on the shoulders of other giants, perhaps most notably Oswald Avery, Erwin Chargaff, Rosalind Franklin, Linus Pauling, and Maurice Wilkins, their seminal publication has often been cited as the birth of the modern era of molecular genetics. In keeping with that status, the golden anniversary year of the double helix was celebrated with much pomp and ceremony, including an official announcement in April by the Human Genome Project of the completion of its sequencing of the entire human genetic blueprint, or genome, whose rough draft had been announced two years earlier.

      It was especially fitting in 2003 to ask how far, in real terms, science and medicine have come and what challenges and opportunities lie ahead. Also appropriate were questions about investigators' current views on DNA structure and on the role of structure in defining DNA's biological functions. The answers to these questions are complex and, in most cases, only poorly understood.

      In terms of progress, the past five decades have witnessed nothing short of an explosion of new knowledge and new technology. Scientists have come to understand, on a molecular and biochemical level, not only many of the normal workings of living systems, both human and nonhuman, but also the basis of many diseases. Indeed, this new knowledge has revolutionized the ability to diagnose a variety of conditions and has begun to offer novel therapies that previously were unimaginable. Finally, scientists have taken the first steps toward understanding not only the expression and function of individual genes within the genomes of humans and other species but also the anatomy and regulation of the genomes themselves. Thanks to the public availability of the more than 100 genomes, ranging from bacterial to human, that had been sequenced as of 2003, researchers have detected patterns in both the unique and the repeated elements of these genomes that offer tantalizing clues to the evolution of humans and many other species.

      Regarding the true structure and function of DNA, appreciation has grown that Watson and Crick's famed right-handed double helical structure is but the tip of the iceberg. Researchers in the field have come to recognize that DNA in living cells is not static in form but continuously moving and changing as it assumes different shapes and associates with different proteins, other macromolecules, or both. For example, in 2001 a research team led by Keji Zhao of the U.S. National Heart, Lung, and Blood Institute, Bethesda, Md., found evidence that part of the regulatory sequence of an immune system gene must transition from its more familiar right-handed form into Z-DNA, a left-handed helical conformation identified in 1979 by Alexander Rich of the Massachusetts Institute of Technology, in order for the gene to be activated. In 2002 Stephen Neidle of the Institute of Cancer Research, London, reported that single-stranded DNA sequences called telomeres, found at the ends of linear chromosomes such as those in humans, can weave themselves into a complex four-stranded loop structure known as a G-quadruplex. Other G-quadruplex forms of DNA were proposed to mediate the regulation of genes, including genes involved in cancer inducement (oncogenes), elsewhere in the genome.

      Beyond basic structure, both DNA itself and the proteins with which it associates can be chemically modified—for example, by the addition or removal of simple methyl (CH₃) or acetyl (COCH₃) groups. These changes can alter both the structure and the function of DNA. Indeed, some researchers have concluded that the structure, state of modification, and macromolecular associations of DNA may be as important to its function as its sequence of bases.

      Although human understanding of DNA may be marking a golden anniversary, those regions of the human genome that have been studied in detail demonstrate a complexity and interdependence that is nothing short of humbling, and clearly the current level of understanding for even these systems is superficial. Perhaps even more humbling is that the vast majority of the human genome has yet to be studied, and despite the declaration of completion in April, many gaps and uncertainties remain in the available human genome sequence database. If the 1953 paper by Watson and Crick was a birth, the status of molecular genetics in 2003 might appropriately be described as a first toddling step.

Killing the Messenger.

      If genes encode the building blocks of life, the controlled expression of those genes must define the shape and function that the blocks can assume. Gene expression is clearly a highly regulated affair in humans and other living systems, and changes in this regulation underlie both normal processes—such as tissue differentiation, development, and adaptation—and many abnormal conditions, including numerous cancers. A variety of mechanisms are known to mediate gene regulation, and they can operate at almost any of the many steps that must occur for a gene to give rise to a finished protein product. Some of these steps are transcription of the sequence of bases in DNA into the corresponding base sequence in single-stranded messenger RNA (mRNA), processing and stabilization of the mRNA transcript, transport of the mRNA into the cell's cytoplasm, translation of the mRNA into a linear chain of amino acids, processing and folding of the chain into a three-dimensional protein molecule, and binding of additional required atoms or molecular groups called cofactors.

      In 1978, a novel mechanism of gene suppression was discovered that involved the activity of short single-stranded RNA or DNA pieces (oligonucleotides) whose sequence is complementary to a specific part of a target mRNA transcript. These bits of sequence, termed antisense oligonucleotides (more specifically, antisense RNA and antisense DNA), appeared to interfere with the manufacture of the gene product at either of two steps: they blocked translation of the target message, or they marked the message for destruction by an enzyme. In both cases they did their work by binding to the mRNA transcript, forming a short stretch of double-stranded RNA similar to the DNA duplex in the double helix.

      Later, a second form of oligonucleotide-mediated gene suppression was identified that involves the use of double-stranded RNA sequences. It was called RNA-mediated interference (RNAi), a term coined by Andrew Fire, Craig C. Mello, and colleagues at the Carnegie Institution of Washington (D.C.) and the University of Massachusetts Medical School. These researchers pioneered the field of RNAi in 1998 when they reported that the introduction of minuscule quantities of specific double-stranded RNA sequences into the nematode Caenorhabditis elegans (a favourite laboratory animal in molecular genetics research) could effectively silence the expression of a target gene not only in the injected animals but also in their progeny. RNAi subsequently was demonstrated to work in a broad variety of species and cell types. Like antisense oligonucleotides, RNAi also was found to be a naturally occurring method of gene regulation.

      Researchers believed that the mechanism of RNAi gene suppression starts with the activity of a specific naturally occurring RNA-cleavage enzyme (RNase) dubbed Dicer. The enzyme recognizes the anomalous double-stranded RNA molecules and cuts them into short pieces that are each about 22 nucleotides long. The fragments, often referred to as siRNA (for short, or small, interfering RNA), are then unwound into their separate strands. One strand associates with a set of specific proteins to form an RNA-induced silencing complex (RISC). Because the RNA portion of the RISC remains exposed near the surface of the complex, it is able to bind with its complementary base sequence in the target mRNA transcript. Once this binding has taken place, an enzyme known as Slicer (which may be part of the RISC complex) recognizes the assembly and cuts the RISC-tagged mRNA in two. The RISC then releases the destroyed mRNA pieces and moves on, ready to bind other complementary targets. In this manner the siRNA-containing RISC acts as an efficient catalyst for the destruction of specific mRNAs in the cell.

      By 2003 RNAi already had evolved not only into a useful laboratory tool but also into a promising approach for treating medical conditions in humans, including cancer, neurodegenerative diseases, and viral infections. In each medical application the design involved suppression of the unwanted expression of a gene, with the targets ranging from oncogenes to viral genes from HIV. Although numerous technical hurdles remained, the progress at this point appeared swift and promising.

Judith L. Fridovich-Keil

Paleontology

      Among the more intriguing stories in paleontology during 2003 was the discovery of the dinosaur Microraptor gui, a small dromaeosaur from the Early Cretaceous Jiufotang Formation of Liaoning, China. Xing Xu of the Chinese Academy of Sciences and colleagues reported that the 77-cm (2.5-ft)-long animal, which lived between 124 million and 144 million years ago, had fully modern, asymmetrical feathers on all four limbs. Dromaeosaurs belong to the dinosaur subgroup called theropods, which were bipeds (with hind limbs adapted for locomotion) and flesh eaters (ranging from species as small as chickens to the huge Tyrannosaurus). Beginning in the late 1990s with the discovery of the first fossils of feathered dinosaurs, it became widely accepted that birds evolved from small light-boned theropod dinosaurs and that feathers originated in nonavian theropods. The question of how flight itself evolved, however, was not settled and continued to be debated. The arguments centred on two hypotheses—the arboreal theory, which suggested that flight arose in tree-dwelling animals through an intermediate gliding stage, and the competing cursorial idea, which suggested that flight evolved in fast-running ground-dwelling animals. If the four feathered limbs on M. gui were used for gliding, as the authors proposed, it would strengthen the arboreal theory for the origin of flight.

      Over the years many types of evidence have been applied to determine the feeding habits of theropod dinosaurs. A report by Raymond Rogers of Macalester College, St. Paul, Minn., and co-workers on the large Late Cretaceous theropod Majungatholus atopus from the Maevarano Formation of Madagascar described heavily tooth-marked fossil bones in support of the idea that the animal defleshed other dinosaur carcasses as it fed. Majungatholus, which was as much as 9 m (30 ft) from nose to tail, lived about 70 million years ago. The investigators found well-gnawed bones both of plant-eating sauropods and of Majungatholus itself bearing marks that matched the characteristics of the latter animal's teeth, which indicated that the dinosaur was a cannibal.

      Paul Sereno of the University of Chicago, Jeff Wilson of the University of Michigan, and colleagues reported that while sifting through fossil remains collected years earlier from deposits along the Narmada River in western India, they found fossil bones belonging to a new species of Late Cretaceous dinosaur. Rajasaurus narmadensis was a 9-m (30-ft)-long dinosaur of the theropod family Abelisauridae that lived about 67 million years ago. Abelisaurs are Cretaceous theropods with short, high skulls. They were also known from South America and Africa, which like India were part of the ancient supercontinent Gondwanaland.

      The caves and deposits at Sterkfontein near Johannesburg, S.Af., are among the richest fossil hominid sites in the world. A report by Timothy Partridge and co-workers from the University of the Witwatersrand, Johannesburg, and Purdue University, West Lafayette, Ind., described recently discovered hominid specimens—possibly of Australopithecus—from Jacovec Cavern and elsewhere at Sterkfontein and contended that the specimens are a surprising four million years old. Previous age estimates for the hundreds of hominids from Sterkfontein ranged from 1.5 million to 3.5 million years. The more ancient age values came from a recently developed technique, called cosmogenic burial dating, that was used on the buried sediments associated with the fossils. When sediment is on the surface, its minerals are bombarded by cosmic rays from space. This process continually produces unstable isotopes of beryllium and aluminum that have fairly long half-lives—about a million years. After the sediments are buried, the bombardment stops and the radioactive isotopes decay without further replenishment. By measuring the quantity of the isotopes that remain in a sample, scientists can determine the age at which the sediment and its content of once-living remains were buried.

      Since the 1970s the origin of modern primates (euprimates) has been a subject of considerable debate, and a variety of scenarios have been offered to explain how and why their evolution occurred. Jonathan Bloch and Doug Boyer of the University of Michigan presented evidence from a well-preserved 56-million-year-old specimen of Carpolestes simpsoni from the Clarks Fork Basin of Wyoming that could resolve this debate. Previous phylogenetic analyses had concluded that the carpolestids are closely related to the euprimates. From their examination of the skull and foot bones of the skeleton, the most complete carpolestid found to date, the investigators inferred that this 30-cm (one-foot)-long animal—and, hence, that the ancestor of modern primates—lacked forward-facing eyes and convergent vision but had an opposable big toe and that it was a grasper adapted for feeding in terminal tree branches. This is in opposition to other hypotheses that suggested the ancestor of the euprimates was either a specialized leaper or a visually directed predator.

      The very earliest tetrapods (vertebrates with limbs) date back to the Late Devonian, about 370 million to 354 million years ago. Until 2003 the nine genera described from that age were known only from North America, Europe, and Greenland, apart from a single fragmentary specimen found in Australia. Min Zhu and colleagues from the Chinese Academy of Sciences and the Natural History Museum, London, reported discovery of the first Late Devonian tetrapod fossil from Asia. Their identification of an incomplete left mandible from nonmarine sediments of the Ningxia Hui region of northwestern China indicated that tetrapods became quite widely dispersed in a relatively short time.

      A study by Moya Smith of King's College, London, and Zerina Johanson of the Australian Museum, Sydney, concluded that teeth evolved more than once in primitive fish. It formerly had been assumed that teeth evolved only once, in a fish ancestral to all vertebrates with jaws, the gnathostomes. In examining specimens of members of the Arthrodira, an advanced group of extinct predatory jawed fish called placoderms, the investigators found teeth made of dentine. Previously all placoderms had been thought to lack true teeth. If, as speculated, the arthrodires derived from toothless placoderms that were not ancestral to other fish groups, then teeth must have evolved independently in the two lineages.

      Samuel Zschokke of the University of Basel, Switz., described an unusual specimen of Early Cretaceous fossil amber from Lebanon in which could be seen an individual thread of viscid (sticky) silk from a spider web. This specimen demonstrated that both the spider superfamily Araneoidea and the use of viscid silk in aerial webs date back at least 130 million years. This silk thread still bore dozens of the glue droplets that typify this type of arachnoid silk.

      The gymnosperm ginkgo tree (Ginkgo biloba) is in some ways a living fossil, having existed at least since the Middle Jurassic Period 170 million years ago. Previously, there had been a gap of 100 million years in the ginkgo fossil record. In 2003, however, Zhiyan Zhou of the Chinese Academy of Sciences and Shaolin Zheng of the Chinese Ministry of National Land and Resources described a fossil from the Early Cretaceous Yixian Formation of China that fit near the middle of the gap, with an age of 121 million years. The specimen was found to have reproductive structures different from the Jurassic fossils but similar to the modern ginkgo, which showed that the morphology of this ancient tree had changed little over the past 100 million years.

William R. Hammer

▪ 2003

Introduction

Zoology

      Insects, the most abundant and diverse group of animals on Earth, were a major focus of research in 2002. An understanding of their evolutionary relationships is based on fossil records dating back more than 390 million years; nevertheless, the first 60 million years of insect evolution derived from paleontological data has remained poorly understood. To examine very early evolutionary relationships between five insect orders, Michael W. Gaunt and Michael A. Miles of the London School of Hygiene and Tropical Medicine developed a molecular clock based on selected amino acid and DNA data from the proteins and genes of existing insects to trace their origins back to approximately 430 million years ago. A molecular clock dates evolutionary divergence by determining the rate of DNA or amino acid mutations from a known evolutionary time, or calibration point, such as a major group of fossils. In a very slowly evolving gene, for example, a change of a single amino acid in the gene's protein product may occur on average every four million years.

      From their molecular clock Gaunt and Miles concluded that insects and fairy shrimps (order Anostraca) were derived from a common ancestor about 430 million years ago, during the transition from the Ordovician to the Silurian Period. Thus, insects emerged as a separate line at the same time that the earliest land plants appeared. The investigators also found that a major group of bloodsucking insects, the triatomines in the order Hemiptera (true bugs), became isolated in South America around 95 million years ago during the breakup of the supercontinent Gondwanaland. All of the findings were consistent with, and augment, earlier interpretations based on the fossil record. Such molecular dating also provided time points for additional studies of more recent evolutionary divergences, such as insect families and genera.

      Krill are tiny planktonic crustaceans that are a major prey item for birds, fish, and several whale species. During the year Andrew S. Brierley of the University of St. Andrews, Scot., and colleagues reported the results of a study in which echo sounding from a battery-powered robot submarine was used to survey the distribution and abundance of Antarctic krill (Euphausia superba) beneath sea ice and open water. The researchers determined that krill densities were significantly higher under sea ice than in the open ocean. The underwater vehicle continuously recorded underice densities of krill for as far inward as 27 km (17 mi) from the ice edge, the highest densities being between 1 and 13 km from the ice edge. The underice habitat serves as protection from predators; it is also a favourable habitat for krill because they feed on algae in the melt zone of the ice, where primary productivity is high. The findings helped to explain why krill-eating whales often congregate along the edges of sea ice and to determine how krill distribution and abundance patterns may be affected by anticipated climate changes that could alter ice patterns in the Antarctic.

      Does the glow that some bird feathers give off under ultraviolet light have a biological function or merely represent a by-product of pigment structure? Kathryn E. Arnold of the University of Glasgow, Scot., Ian P.F. Owens of Imperial College at Silwood Park, Eng., and N. Justin Marshall of the University of Queensland, Australia, gained insight into this question after conducting tests on the common shell parakeet, or budgerigar (Melopsittacus undulatus), to determine if its fluorescent head plumage is used as a signal. Both sexes have fluorescent yellow plumage on parts of the head that is used for display during courtship. The investigators applied sunblock to key areas of the heads of birds to reduce the amount of ultraviolet light reaching the feathers and stimulating fluorescence. They also treated the heads of a control group of birds with petroleum jelly alone, which does not reduce fluorescence. In subsequent mate-choice trials, both male and female parakeets showed a sexual preference for members of the opposite sex exhibiting strong fluorescence. Neither sex showed a social preference for members of the same sex whether fluorescence was normal or artificially subdued. The investigators suggested that the biochemical pathways that produce fluorescence may be so energetically costly that brightly fluorescent plumage would serve as a true indicator of an individual bird's good health and overall quality to the opposite sex.

      Analyses of isotopic ratios figured in two independent studies on New World migrant songbirds. In one, Dustin R. Rubenstein of Dartmouth College, Hanover, N.H., and colleagues used ratios of naturally occurring stable isotopes of carbon and hydrogen in feathers of black-throated blue warblers (Dendroica caerulescens) to determine the degree to which birds from different breeding populations in continental North America mix in their Caribbean wintering quarters. The isotope ratios in the feathers become fixed at molting, which in this case was at or near the breeding site, and they reflect the diet of the birds at the time. Thus, the ratios can be used to indicate the breeding origins of birds whose feathers are analyzed. The researchers found that birds wintering on western Caribbean islands migrate from northern areas of North America, whereas those on eastern islands are from more southern regions. Such studies can help assess how the loss of wintering habitat affects the size of breeding populations. For example, observed declines in southern breeding populations of black-throated blue warblers could be explained by severe deforestation in Haiti, on the island of Hispaniola, where the southern populations spend the winter.

      In an extension of the previous study, Gary R. Graves of the Smithsonian Institution, Washington, D.C., and Christopher S. Romanek and Alejandro Rodriguez Navarro of the Savannah River Ecology Laboratory, Aiken, S.C., used patterns in the ratios of stable carbon and nitrogen isotopes in the warblers' feathers to study their preference for breeding territory in southern Appalachian Mountains. The investigators found that, on their return in spring from their wintering grounds, yearling males in their first breeding season showed no preference for the altitude of their breeding territory, whereas adult males were strongly inclined to seek altitudes they had occupied the previous year.

      Owing to negative public attitudes about snakes, limited research funding, and the secretive nature of the animals themselves, the conservation status and population trends of most snake species are poorly known. Robert N. Reed and Richard Shine of the University of Sydney, Australia, examined Australian snakes to address the question of why some species decline rapidly when disturbed by human activity whereas others readily exploit disturbed habitats. One purpose of the study was to identify ways to predict the vulnerability of a species. The investigators examined more than 18,000 specimens of snakes of the cobra family (Elapidae) in museums to identify common traits among threatened and nonthreatened species. Most traits that typically correlate with endangerment, such as large body size, low number of offspring, and specialization for particular habitats or prey, were judged to be inapplicable to Australian snakes. Instead, threatened species were characterized by two primary traits related to foraging behaviour and mating systems. Threatened species were generally ambush predators, rather than wide-ranging active foragers, and they did not engage in male-male combat for females. A plausible explanation for the first relationship is that ambush predators do not move long distances in search of prey; consequently, they may be more dramatically affected when habitat disturbance reduces the density of prey. The explanation for the second relationship may be that, because females grow appreciably larger in species without male-male combat, they may be more obvious to humans and therefore more likely to be killed. Once humans alter the habitats of these species, the added impact of the loss of the reproductively important large females may result in rapid population declines. Understanding how specific biological traits may make some species more susceptible to human-caused changes could help identify potentially vulnerable species not currently protected.

      Determining the actual number of living species within a region or taxonomic group continued to be a challenging task in efforts to characterize biodiversity. An unsettled question was how closely the number of known species in a phylum represents the actual number in existence. Mollusks in the world's oceans have the highest-known diversity of any animal group, a diversity that is especially high in the tropical Indo-Pacific region. Philippe Bouchet of the National Museum of Natural History, Paris, and colleagues conducted an intensive survey of mollusk species within a 30,000-ha (74,000-ac) site on the west coast of New Caledonia, collecting more than 127,000 specimens of 2,738 species of mollusks—numbers that exceeded any previous surveys. Rare species, represented by single specimens, made up 20% of the species collected. When the data were projected beyond the actual captures by means of a species accumulation curve, the estimated total species ranged from 3,358 to 3,971. The results suggested that current estimates of global biodiversity of mollusks were greatly undercalculated.

J. Whitfield Gibbons

Botany

      The potential dangers of genetically modified (GM) plants continued to be debated in 2002. New research suggested that it would be impossible to avoid interbreeding between GM crops and neighbouring plants, despite the efforts of many governments to impose safety limits around fields of GM plants. Mary Rieger of the University of Adelaide, Australia, monitored the spread of genes from canola (oilseed rape) that had been bred for herbicide resistance and found that the pollen could reach up to three kilometres (almost two miles) away and fertilize small numbers of nonresistant plants.

      For the first time, it was shown that weedy relatives can be strengthened considerably by genes passed from nearby GM crops. Researchers found that a gene engineered into sunflower crops to repel moth and butterfly larvae also migrated into closely related weeds and made them more pest-resistant and, surprisingly, more productive. “Weeds are already hardy plants; the addition of transgenes [i.e., artificially inserted genes] could just make them tougher,” said Allison Snow, one of the investigators at Ohio State University involved in the study.

      The idea that GM crops can provide a powerful weapon against pests received a setback when it was discovered that potato plants that had been genetically engineered to resist sap-sucking insects turned out to be vulnerable to other kinds of pests. Nicholas Birch and his team at the Scottish Crop Research Institute near Dundee examined plants that had been modified to produce lectins, which sap suckers find distasteful. They found that the plants also had lower levels of glycoalkaloids, which repel many other insects.

      The debate over the safety of GM crops grew more heated when the science journal Nature took the highly unusual step of criticizing in an editorial note (April 11, 2002, issue) a report that it had published the previous November about the leakage of foreign genes from GM corn (maize) into traditional corn crops in Mexico. The note accompanied scientific challenges to the paper that focused primarily on what happened to the genes once they had invaded the native corn. Nevertheless, the original researchers, David Quist and Ignacio Chapela of the University of California, Berkeley, stood by their contention that transgenes had entered traditional strains of corn in Mexico, a development that was accepted as likely by their critics. In addition, a survey of native corn samples in Mexico revealed that as many as 25% in some regions contained GM corn, despite a four-year-old moratorium on planting GM crops in Mexico.

      That foreign genes are not always needed to modify a plant genetically was demonstrated by Peter Horton and colleagues at the University of Sheffield, Eng. The researchers developed an ingenious technique that allowed them to make extra copies of a plant gene involved in the production of xanthophyll, a substance that protects plants from intense heat and light, and then reinsert them into the same plant. The result was a plant in which the pool of substances that participate in xanthophyll production are increased, which thus enables the plant to withstand a far harsher climate.

      A powerful new herbicide was discovered when scientists identified the biochemical weapon unleashed by spotted knapweed (Centaurea maculosa), an aggressive weed that had spread over large areas of the northwestern U.S. Jorge Vivanco of Colorado State University found that the plant's roots secrete catechin into the soil, killing most other plants in the immediate vicinity, apart from grasses. Scientists hoped to exploit catechin as a powerful natural weed killer that leaves grasses and cereal crops, such as wheat and rice, unharmed.

      Carnivorous pitcher plants have unusual tubular leaves shaped like urns or small pitchers that collect rainwater in their bases. Insects that walk around the pitcher mouth tend to slip and fall into the pitcher, where they drown and are broken down by digestive enzymes. During the year a carnivorous pitcher plant, Nepenthes albomarginata, was reported to use a unique trick to lure termites to its traps. The pitcher rim grows hairs that mimic a favourite food of the termites; once one termite has fed on the hairs, it calls on others to join in, many of which then end up being caught. This degree of specialization on a particular prey was unprecedented for a carnivorous plant.

      The oldest seed ever observed to sprout into a fully grown plant was reported by a team headed by Jane Shen-Miller of the University of California, Los Angeles, which succeeded in germinating a 500-year-old lotus seed. Interestingly, the lotus plant showed abnormal growth, which was attributed to prolonged exposure to low-level radiation in the soil in which it had been buried—possibly the world's longest-running radiation experiment. In another experiment on seed longevity, the seeds of two common plants, moth mullein (Verbascum blattaria) and common mallow (Malva rotundifolia), kept in a bottle of soil since 1879 were also found to be viable, the longest-running test of seed dormancy in soil.

      In contrast, a global survey of seeds stored in seed banks revealed that much of the plant material was deteriorating and needed replanting to stay viable, a laborious and costly process at a time when many seed banks were suffering budget cuts and staff shortages. In August the UN Food and Agriculture Organization sanctioned a new international fund, the Global Conservation Trust, with the aim of raising $260 million to help rescue these stocks. Seed banks around the world held some two million varieties of crop plants, an invaluable repository of plant genes vital for agricultural breeding.

      At least 22% of the world's plant species could be facing extinction, almost double the rate that had been assumed previously. Peter Jorgensen of the Missouri Botanic Garden, St. Louis, and Nigel Pitman of Duke University, Durham, N.C., based their assessment on the numbers of plant species endemic to each country, which they used as a rough guide to the number threatened. This approach gave a better estimate of endangered species in the tropics, where most of the world's plants grow.

      A new national park on the Kitulo Plateau in the southern highlands of Tanzania was established to protect scores of terrestrial orchid species, many of them unique to the region and under threat of extinction from being harvested for their edible tubers. This was the first protected area in tropical Africa set aside primarily to preserve its plant life.

      Scientists were heartened when a new and unusual conifer tree was discovered in Vietnam. The mature tree is highly distinctive in bearing two different types of leaves, needles and scale leaves, and it formed a new genus, Xanthocyparis. This was only the second new conifer species to be found in the past 50 years.

Paul Simons

Molecular Biology

Stem Cell Research.

      Once of interest mainly to developmental biologists, stem cells stood definitely at centre stage in 2002 in a debate of international proportions involving scientists, healthcare professionals, politicians, theologians, and many others. At stake was the future of a new and potentially very powerful technology that could one day offer treatment, if not cure, for many serious medical conditions such as diabetes, stroke, spinal cord injury, and neurodegenerative disorders such as Parkinson's disease.

      At the core of the debate lay the fact that this technology was not entirely artificial—it involved the use of specialized cells called stem cells that are, at least in some cases, of human fetal origin. Whether it was just for any society to use fetal stem cells for biomedical application in living adults or children was clearly a complex question; it was, in essence, the abortion debate reincarnated with a biomedical twist. Nevertheless, not all stem cells are of fetal origin, and new research suggested that with some modification stem cells derived from nonfetal sources, such as adult donors, could prove to be as useful as, or even more useful than, previously studied fetal cell lines.

      The term stem cell is applied to any living cell that retains the ability not only to replicate itself indefinitely but also to give rise to distinct differentiated cell types. Some stem cells are already somewhat specialized—in addition to replenishing themselves, they can also give rise to only one differentiated cell type or, at most, a small number of related types. These cells typically are referred to in terms of the differentiated tissue they represent—for example, myogenic (muscle) stem cells or hematopoietic (blood) stem cells. In contrast, other stem cells can give rise to a variety of distinct cell types; these are typically called multipotent or pluripotent cells. Finally, some stem cells remain competent to give rise to every possible cell type; these are called totipotent cells.

      Although specialized stem cells have been known for many years to exist in the accessible tissues (e.g., blood or bone marrow) of living adults and children, multipotent stem cells historically have been derived only from adult cancers or from embryonic or fetal cells. (In this context, embryonic refers to the earliest stages of prenatal development; fetal refers to the later stages.) Indeed, until recently only three different types of multipotent mammalian stem cell lines had been isolated: embryonal carcinoma cells, which are embryoniclike cells derived from testicular tumours in adult males; embryonic stem cells, derived from preimplantation embryos (embryos not yet implanted in the lining of the uterus); and embryonic germ cells, derived from primordial germ cells of postimplantation embryos. During 2002, researchers reported that they had derived additional multipotent stem cells from adult bone marrow, offering hope not only to ethicists opposed to the use of fetal cells but also to the biomedical community at large, because using such cells derived from patients themselves might circumvent the problems of host-graft rejection so often seen with cells donated by a second individual. Theoretically at least, multipotent stem cells harvested from a patient could be used to grow any replacement tissue needed by that individual, from new spinal cord neurons to a new heart. Furthermore, if those stem cells could be genetically modified before they were induced to differentiate, then a long list of genetic disorders previously considered incurable or treatable only with high-risk therapies would become reasonable targets for application.

      Studies of hematopoietic stem cells (HSCs) from both mice and humans revealed some important statistics about the potential of these cells for proliferation and differentiation and about the success of their subsequent engraftment into a host. In brief, all of these properties vary with the age of the donor, with the youngest cells faring best. For example, HSCs from fetal mouse liver have a greater proliferation potential than do their counterparts harvested from the bone marrow of either younger or older postnatal donors. Furthermore, the proportion of “more specialized” HSCs that can give rise to only red or white cells, but not to both, goes up with age. Finally, stem cells derived from human umbilical-cord blood engraft 10–50 times better than do stem cells derived from adult bone marrow. Although none of these observations precludes the successful use of adult-derived stem cells, each represents a technical hurdle to be overcome if these cells are to become a reliable clinical tool.

      Stem cells derived from adult tissues had been believed to be competent only to differentiate into additional cells of the tissue of origin. Thus, adult-derived hematopoietic stem cells could give rise only to blood cells, not to liver or nerve cells. Given that many genetic or degenerative diseases affect tissues (e.g., the brain) that cannot easily be accessed for stem cell harvesting, this limitation of stem cell potential represented a significant problem. In 2002 several reports suggested that stem cells derived from adult bone marrow can, albeit by some as yet poorly understood process, become other types of cells, including skeletal-muscle, cardiac-muscle, lung, skin, liver, and even neuronal cells.

      In one major study, Catherine Verfaillie of the University of Minnesota's Stem Cell Institute and colleagues identified a rare cell type within adult human bone-marrow mesenchymal stem cell cultures that could be expanded through more than 80 population doublings and also differentiated in culture into many distinct cell types. Switching to a mouse model to enable further manipulation, the researchers identified similar cells from mouse bone marrow. These cells were cultured and manipulated in the laboratory and then injected back into early blastocyst mouse embryos and followed. Although they were derived originally from adult bone marrow, the descendents of these cells turned up in the injected host embryos in a multitude of different tissue types, including blood and the epithelia of the liver, lung, and gut. Given that these cells, called MAPCs, for multipotent adult progenitor cells, were capable of extended if not indefinite culture in the laboratory and could differentiate and engraft into a multitude of different tissue types in the recipient, they represented a nearly ideal source for therapy of inherited or degenerative diseases. Whether this success in mouse embryos could be duplicated in adult human hosts remained to be determined.

Applications and Issues of Stem Cell Technology.

      The potential medical applications of human stem cells, especially if they are host-derived, were enormous. For example, for a patient with spinal cord injury, rare multipotent stem cells could be harvested from a sample of bone marrow, expanded in culture, and then returned to the site of the injury to engraft and differentiate into new neurons. For a patient with diabetes, multipotent stem cells could be returned to the appropriate location in the pancreas to engraft and differentiate into insulin-secreting beta cells. Indeed, given that diabetes is an autoimmune disease and that the new beta cells could eventually become depleted as did their predecessors, some of the extracted stem cells could be frozen and the engraftment procedure repeated on an as-needed basis. For a patient with a recessive genetic disorder such as cystic fibrosis (CF), multipotent stem cells could be harvested from bone marrow, genetically engineered in culture to express functional CFTR, the protein defective in CF, and then expanded in culture and returned to the patient's airway epithelium (lungs) and pancreas, the two major organs affected by CF. Such examples represented just the tip of the iceberg.

      As with any powerful new technology, myriad political, social, and ethical issues surrounded stem cell research. Perhaps the most obvious ones dealt with human embryo- or fetal-derived stem cells, owing to ethical or religious concerns. To date, different communities and countries had addressed these concerns in their own way, each attempting to balance the desire for new clinical treatments with the desire to preserve and protect all forms of human life. For example, by late 2000 authorities in Great Britain had allowed for the laboratory creation and use of human embryos up to 14 days old, subject to a government license and strict guidelines. Similar standards had been enacted in Singapore as of 2002. In contrast, Pres. George W. Bush in 2001 decided to restrict the use of federal funds for embryonic stem cell research in the U.S. to work with embryonic cell lines that already existed. The question of how embryonic stem cells may be derived, and how their use will be funded and regulated in different countries, remained unclear. Nonetheless, the great promise of stem cell technology was certain to keep it a topic of hot discussion for years to come.

Judith L. Fridovich-Keil

The Origin of Organelles.

      Cells of eukaryotic organisms—that is, humans and other animals, plants, fungi, and protists—contain membrane-enclosed structures called organelles in which certain specialized activities take place. Mitochondria and chloroplasts, two kinds of organelles that are intimately involved in cellular energy production, possess their own DNA, which encodes a fraction of their own proteins. Mitochondria and chloroplasts also contain the machinery needed to transcribe that DNA into RNA and to translate the RNA into the corresponding proteins. This retained autonomy of protein synthesis, as well as many other similarities between these organelles and free-living prokaryotes—single-celled organisms, such as bacteria, that lack a nuclear membrane and many other components of eukaryotic cells—has led to the view that mitochondria and chloroplasts are descendants of symbiotic prokaryotes that took up residence within primitive eukaryotic cells. During the year this well-accepted hypothesis gained support and insight from two reports that contributed additional details about the mechanism by which these organelles divide. One, by Janet Shaw of the University of Utah and Jodi Nunnari of the University of California, focused on budding yeast; the other, by Shin-ichi Arimura and Nobuhiro Tsutsumi of the University of Tokyo, focused on the green plant Arabidopsis.

      In prokaryotic cells the binary division that follows replication of the DNA occurs by the pinching of the mother cell into two daughter cells. The contractile protein that causes this pinching is called FtsZ. During division FtsZ assembles into a ring around the equator of the cell; the ring then draws chemical energy from the hydrolysis of the energy-rich molecule guanosine triphosphate (GTP) to power constriction. The chloroplasts of green plants also use FtsZ to carry out binary division. Experimentally inhibiting the production of FtsZ inhibits this division, which ultimately results in the presence of one or only a few giant chloroplasts per cell. In the mitochondria of algae, which are eukaryotic protists, FtsZ is also the motor of binary division and has been observed to assemble into a ring at the site of pinching.

      On the other hand, the mitochondria of two other eukaryotes, yeasts and nematodes (roundworms), have been found not to use FtsZ. In its place they use another protein related to a class of proteins called dynamins, which also use the energy of GTP hydrolysis to drive constriction. Likewise, the mitochondria of higher plants such as Arabidopsis have been shown to employ the dynamin-related protein. One can thus envision that in primitive mitochondria, division was carried out by FtsZ, as is still the case in bacteria, but at some point in the coevolution of mitochondria and their host eukaryotic cells, the job of constriction was taken over by the dynamin-related protein.

      One possible scenario of how this could have happened is based on a postulated intermediate stage of mitochondrial evolution in which both FtsZ and the dynamin-related protein functioned together. Consistent with this hypothesis, FtsZ has been found to form a constricting ring on the inner surface of the inner membrane of gram-negative bacteria, the chloroplasts of green plants, and the mitochondria of red algae. In contrast, the dynamin-like protein forms a similar ring, but on the outer surface of the inner membrane, in green-plant mitochondria. From this evidence one can visualize a transition organism in which both proteins acted together, one on the inner surface of the inner membrane and the other on the outer surface. The existence of such redundancy could then have allowed the loss of FtsZ from the mitochondria in higher plants without loss of constriction function. There may exist as-yet-undiscovered organisms in which mitochondrial division depends on both FtsZ and the dynamin-related protein acting in concert, and their identification would strongly support the evolutionary scenario described above.

Intracellular Rail Transport.

      A substance made in one part of a cell may be quickly needed in another part of the cell, or it may have to be sent through the cell to be secreted for use elsewhere in the body. In the case of large cells, simple diffusion is far too slow to meet these intracellular-transport requirements. An example is a motor neuron that must transmit signals to a muscle fibre in the lower leg. That neuron has a projecting extension, the axon, that may be more than a metre (3.3 feet) long, yet the nucleus that contains the DNA encoding all the proteins made in that neuron is at one end. How are the proteins, made in the vicinity of the nucleus, moved efficiently to the rest of the cell?

      Microscopy reveals an array of thin fibres aligned in the axon and, in addition, numerous membrane-enclosed vesicles, or organelles, attached to and moving along those fibres, much like railroad cars moving along a track. The fibres are called microtubules. Each is a hollow bundle of 13 strands that are composed of a protein called tubulin. Various organelles, some of which may be filled with proteins or neurotransmitters, move along the microtubule tracks, some in one direction and others in the opposite direction. The tiny “locomotive engines” carrying out this movement are proteins called kinesins and dyneins. Kinesins travel in one direction and dyneins in the other. Directed movement requires energy, which the proteins obtain from the hydrolysis of the energy-currency molecule of the cell, adenosine triphosphate (ATP). During the year, David Hackney of Carnegie Mellon University, Pittsburgh, Pa., reported new details regarding the interaction of kinesins and microtubules.

      To comprehend the scale involved, it is helpful to know that a microtubule is only 25 billionths of a metre (25 nm [nanometres], or about a millionth of an inch) in diameter. Kinesin is 80 nm long, and it moves along the microtubule in steps of 8 nm, using the energy of one ATP molecule per step. The rate of this movement is about 640 nm per second. Hence, the kinesin protein makes 80 steps per second while pulling along its burden. Because there are several kinds of organelles requiring transport and because each must be recognized by, and bound to, its own specific kinesin or dynein, it is not surprising that there are multiple kinesins and dyneins. The kinesin molecule has two globular head groups, which bind to microtubules, and a stalklike tail. It is possible that kinesin pulls itself along the microtubule in hand-over-hand fashion, using its head groups, while the tail remains tethered to the vesicle being transported. The details of that mechanism were among the many unanswered mysteries about intracellular transport to be addressed by future research.

Irwin Fridovich

Paleontology

      Highlighting the year 2002 in paleontology were several spectacular fossil discoveries reported from China, including that of a nonavian theropod dinosaur covered with primitive feathers. According to a number of paleontologists, of the previous finds of the past few years that had been first described as representing feathered dinosaurs, at least some, such as Caudipteryx and Protarchaeopteryx, were actually flightless birds. During the year, however, a specimen that was clearly a dromaeosaur—one of a family of nonavian theropods thought to share a common ancestry with birds—from the Early Cretaceous Jiufotang Formation. The investigators who described the approximately 120-million-year-old specimen claimed that the discovery finally proved that modern feathers evolved in theropod dinosaurs prior to the emergence of birds and flight.

      Sinovenator changii, a second Chinese dinosaur described during the year, represented a very early basal troodontid, a primitive member of another nonavian theropod family believed to share an ancestor with birds. From the Cretaceous Yixian Formation and more than 130 million years old, the fossil has several features found in both dromaeosaurs and birds that are not typical of the more advanced troodontids. No feathers were identified, although they may have been present on the animal but not preserved in the fossil. The study concluded that several principal avian structures had developed earlier than previously thought but were then lost in some later theropod lineages.

      An Early Cretaceous fossil from Liaoning province, China, Jeholornis prima, was described as a turkey-sized bird that lived between 110 million and 125 million years ago. Its remains included several dozen well-preserved seeds in the stomach, the first direct evidence of seed eating in a bird. Unlike other Cretaceous birds, the animal retained a long skeletal tail resembling the tail of dromaeosaurid theropods. This specimen provided yet another strong link between birds and nonavian theropod dinosaurs. Eomaia scansoria, a small animal found in the same deposits in Liaoning province, was established to lie at the very base of the lineage leading to placental mammals. The find pushed the origin of the placentals back to 125 million years ago.

      Yet another discovery from the Liaoning area was hailed as one of the most significant flowering-plant fossils ever found. The well-preserved 125-million-year-old specimen of Archaefructus sinensis suggested that the ancestors of the modern flowering plants, or angiosperms, may have been aquatic weedy plants. The closest-known living relatives of the flowering plants, the gymnosperms, are all woody plants. Hence, before this discovery paleobotanists had generally agreed that the angiosperms arose from woody plants similar to the magnolia tree.

      Among other Chinese fossils was an important new invertebrate animal species from the Early Cambrian Chengjiang Lagerstätte near Kunming. Didazoon haoae represented an entirely new phylum of metazoans (multicellular animals), the phylum Vetulicolia. The specimen has a series of gill slits, which suggested that this new group illustrates an early stage in the diversification of the deuterostomes, one of the major animal divisions. Other deuterostome groups are the chordates (which includes the vertebrates), hemichordates, and echinoderms. Also reported was a Devonian Chinese fossil fish, Styloichthys changae, that has features linking the lungfish to tetrapods (four-legged vertebrates).

      Biomechanical studies to determine the traveling speeds of ichthyosaurs in water and large theropod dinosaurs on land were concluded during the year. The ichthyosaur study, carried out by scientists from the Royal Ontario Museum, Toronto, estimated that the aquatic reptiles swam at speeds similar to those of large modern fish such as the tuna. A U.S.-based project used estimates of extensor muscle mass to measure maximum running speeds for some dinosaurs. They concluded that huge bipedal theropods such as Tyrannosaurus rex were not capable of running very fast.

      Paleontological discoveries from other parts of the world during the year included a new candidate for the oldest land-walking tetrapod, from Scotland. With an estimated age of 344 million–354 million years, Pederpes finneyae filled in what previously had been a 20-million-year gap in the early evolution of the land vertebrates. A new species of placental mammal from the Bissekty Formation of Uzbekistan pushed the known fossil record of the primitive zalambdalestid mammal group back 10 million years. With a mid-Cretaceous age of 90 million years, this animal became one of the oldest-known placental mammals. A new statistical analysis of the primate fossil record arrived at a postulated age of 81.5 million years for the oldest common ancestor of the primate order. The study also estimated that no more than 7% of all primate species that ever existed were known from the fossil record.

      In central Colorado a highly diverse fossil leaf site from the early Tertiary Period was dated to an age only 1.4 million years younger than the Cretaceous-Tertiary extinction event 65 million years ago. The existence of such a high-diversity tropical rainforest had been unexpected because most other known Paleocene plant assemblages showed a very low variety of species, particularly those that were close to the extinction event.

      A second analysis of the wormlike Silurian invertebrate Acaenoplax hayae concluded that this animal, from 425-million-year-old deposits in England, does not represent the oldest-known aplacophoran mollusk, as had been reported in early 2001. The new study contended that the fossil exhibits more characteristics of polychaetes than of mollusks and hence should be placed in the class of marine worms, Polychaeta. A reanalysis of what had been described in 1998 as the world's oldest-known worm tracks, at 1.1 billion years old, suggested that the groove-shaped structures are actually 1.6 billion years old. Because this age predates that of the earliest accepted trace fossil of a metazoan by almost a billion years, the study also questioned whether the structures are actually fossils. The authors argued that one would not expect a billion years to pass without similar fossils being preserved.

      Microscopic traces in approximately 3.5-billion-year-old Australian chert that were reported in 1993 to be of bacterial origin and to represent the oldest-known fossils also came under question. A paper published in March contended that the squiggle-shaped structures were formed chemically in an ancient hot spring and hence were not fossils. Another project, which examined fossils of single-celled eukaryotic algae from Roper Group rocks in northern Australia, found structural evidence that complex processes already were present in these cells; consequently, eukaryotes must have evolved much earlier than the 1.5-billion-year age of the specimens. This added support to previous studies suggesting that eukaryotes originated at a much earlier time than the age of the oldest preserved cells. On the basis of molecular and geochemical evidence, the researchers estimated that eukaryotic cells first appeared between 2.5 billion and 2.7 billion years ago, in the late Archean Eon.

William R. Hammer

▪ 2002

Introduction

Zoology

      Zoological researchers in 2001 continued to assess the effectiveness of the protection mechanisms that animals use against predators. Such knowledge was fundamental to the understanding of certain aspects of population dynamics—the ways in which the size and composition of a population change over time and the factors that influence those changes. A basic principle of Batesian mimicry is that an edible prey species, the mimic, is afforded some level of protection from predators when it closely resembles a venomous or distasteful species, the model. By definition such protection should be less effective or absent in regions where the mimic, but not the model, is present. David W. Pfennig and William R. Harcombe of the University of North Carolina at Chapel Hill and Karin S. Pfennig of the University of Texas at Austin conducted field experiments to test whether the close likeness between nonvenomous king snakes and venomous coral snakes should be regarded as a case of Batesian mimicry.

      The investigators used three-coloured snake-shaped replicas made of plasticine (a nontoxic modeling substance) that duplicated the conspicuous red, black, and yellow-to-white ringed pattern of either scarlet king snakes (Lampropeltis triangulum elapsoides) or Sonoran mountain king snakes (L. pyromelana). The patterns of these nonvenomous snakes are similar to the ringed patterns of the eastern coral snake (Micrurus fulvius) and western, or Arizona, coral snake (Micruroides euryxanthus), respectively. Regional predators avoid the venomous coral snakes, which are presumed to be the models that the king snakes mimic. The investigators placed their king snake replicas at a series of eastern sites in North Carolina and South Carolina and at western sites in Arizona in tests to determine whether predators would avoid them. Two other kinds of snake replicas, one plain brown and one with conspicuous longitudinal stripes, were placed within 2 m (6.6 ft) of the ringed replicas as controls. Tests were conducted using 480 replicas at 16 eastern sites where scarlet king snakes occurred—at 8 sites where they occupied the same range as coral snakes and at 8 sites where they were outside the coral snake's range. In Arizona 720 replicas were used at 14 sites where king snakes and coral snakes occurred together and at 10 sites where only king snakes occurred. The replicas were removed from the eastern sites after four weeks and from the western sites after two weeks.

      Studies of bite and gouge marks left in the replicas revealed that predators had attacked the king snake replicas significantly more often at sites outside the coral snakes' ranges than at sites where king snakes and coral snakes coexisted. Furthermore, outside the coral snakes' ranges, the three types of replicas were attacked indiscriminately, whereas in the coral snakes' ranges, king snake replicas were attacked significantly less often than the others. The results supported the premise of Batesian mimicry that the benefit of being a mimic depends on the model's being present in the same area.

      A universal challenge in investigations of the demography and population dynamics of animals has been to determine how different factors influence the sometimes large fluctuations in population size that are observed in some species over time. Distinctive regulators of population size include those that depend on population density (the population size in a given area) and those, resulting from environmental effects, that are independent of density. To determine the relative influence of various factors on population dynamics, Tim Coulson of the Zoological Society of London and colleagues studied the effects of age, sex, density, and winter weather over more than a decade on fluctuations in the population of Soay sheep (Ovis aries) on the island of Hirta, northwest of Scotland. One objective was to quantify how interactions between different variables affected the population fluctuations. Since the 1950s the Soay sheep population has varied in size from fewer than 600 individuals to more than 2,000, with the proportion of various sex and age categories (e.g., numbers of male and female lambs, prime adults, and old adults) varying independently of population density.

      The severity of winter weather can differentially affect survivorship in the various age and sex categories; through such interaction with the population demography, it can increase or decrease the total size of the population. For example, during periods of high population density, lambs and old females were observed to fare worse at survival than female yearlings and prime adults. On the other hand, in response to winter weather, survival rates for lambs and males were negatively affected by bad weather throughout the winter, whereas those for yearlings and prime adult females were negatively affected primarily by heavier rainfall at the end of winter. Thus, the dynamics of a population of large mammals can take dramatically different courses depending on differences in the age structure and sex-ratio pattern of the population and in the weather conditions that it experiences. The investigators concluded that management and conservation models that rely on predictions of population size must incorporate the effects of demographic variations and their interaction with climate.

      Although all species respond to natural threats to individuals through either evolution or extinction, the current decline in biodiversity observed in terrestrial, freshwater, and marine habitats as a consequence of human activities was causing alarm on a global scale. Understanding the natural processes that regulate species diversity and abundance was seen as an important step toward developing conservation and management approaches that address the human-based causes of species loss. David R. Bellwood and Terry P. Hughes of James Cook University, Townsville, Australia, studied the distribution patterns of species of fish and corals in Indo-Pacific waters to determine what factors influence the variation in species composition in coral reefs, which are highly diverse habitats. Of four major variables that were examined because of their potential to explain such variation, the available area of shallow-water habitat was found to be the most influential. Two other variables, latitude and longitude, were of minor significance in explaining the species diversity of fish and corals. The fourth variable, reef type, was found to be of little importance. The investigators concluded that suitable habitat had to be protected on a regional scale if the diversity of coral reef assemblages was to remain intact in the Indo-Australian archipelago—a principle that presumably would be applicable globally.

      Setting aside protected areas for wildlife is traditionally accepted as a primary conservation strategy to guard against the detrimental effects of human activity. Jianguo Liu of Michigan State University and colleagues, however, challenged the effectiveness of the approach at the Wolong Nature Reserve for giant pandas (Ailuropoda melanoleuca) in Sichuan province, China. The investigators used Landsat and declassified spy-satellite data collected before and after establishment (1975) of the 200,000-ha (500,000-ac) reserve to assess the rates of change in giant panda habitat. In addition to forest cover, the slope of the terrain and elevation are also important habitat variables that affect pandas. Two key observations were made in the analysis of habitat changes from 1965 to 1997. First, panda habitat within the reserve continued to decrease in quality and quantity and became even more fragmented after 1975. Second, the habitat areas most severely affected were those deemed to be of high quality for pandas. The direct cause of the ecological degradation was attributed to the presence and rapid increase in numbers of humans living within the reserve, most of whom were minority ethnic groups that were exempt from China's one-child-per-family policy. A variety of socioeconomic activities, including tourism, collection of wood for fuel, and road construction all contributed to habitat loss. One conclusion of the study was that the development of effective conservation policies for protected lands required the integration of ecological principles with human demography, behaviour, and socioeconomics.

      Further insights into the ancestry of humans were provided during the year through the use of molecular techniques. Li Jin of Fudan University, Shanghai, and the University of Texas at Houston and colleagues sampled genetic material from 163 populations of living humans in 13 geographic regions in Asia, ranging from India to Siberia, to test competing hypotheses of the origin of modern humans. The so-called out-of-Africa hypothesis maintained that the ancestors of present-day humans originated in Africa approximately 100,000 years ago and totally replaced all other hominids, such as Neanderthals (an early form of Homo sapiens), during their dispersal to other regions. The alternative view was that some genetic mixing occurred between dispersing Africans and other hominids, such as Peking man and Java man (H. erectus), in Asia. The investigators tested for genetic markers on the Y chromosome of more than 12,000 human males to determine if all carried one of three chromosome polymorphisms (distinct genetic variations) characteristic of an African origin. Without exception, all of the individuals sampled had genetic markers of African derivation; this supported the hypothesis that hominids dispersing onto the Asian continent from Africa displaced all other hominids already present. (See also Anthropology and Archaeology : Physical Anthropology.)

J. Whitfield Gibbons

Botany

      Among milestones in plant science that became widely acknowledged in 2001 was the mapping of the genomes of two plants. The entire DNA blueprint of thale cress (Arabidopsis thaliana), a weed related to cabbage and mustard and long a favourite laboratory organism in plant research, was published at the end of 2000. The sequencing of its more than 115 million pairs of chemical bases, the molecular building blocks of DNA and the basis of the genetic code, was the culmination of a six-year, $70 million project involving 300 scientists worldwide. The achievement, the first for a plant genome, promised new types of genetically modified crop plants and a better understanding of the process of evolution.

      Early in 2001 scientists announced completion of the sequence of the larger genome of the rice plant (Oryza sativa), which comprises 430 million base pairs representing some 50,000 genes. It was anticipated that, as the functions of many of these genes were worked out, the information could lead to significant improvements in the major cereal crops, which are all closely related to rice. Unraveling of the rice genome by the Swiss agribusiness firm Syngenta International AG and the American firm Myriad Genetics, Inc., beat the efforts of a publicly funded international team to map the same genome. The two companies indicated that they would make the rice genome data publicly available through collaboration agreements.

      Research into the evolution of plants also made great progress during the year. The Deep Green project, which had been established to investigate the ancestry of green land plants, was completed after five years' work involving more than 200 scientists worldwide. Data were integrated from morphology, biochemistry, and fossil sources to construct the most complete “tree of life” for any group of living things on Earth. Results indicated that green algae and land plants form the plant kingdom, clearly separated from other algae such as red algae, brown algae, diatoms, and dinoflagellates. It was also revealed that the first plants to grow on land were members of a class of freshwater green algae called Charophyceae; this overturned the idea that seawater algae spearheaded the land-plant invasion. Another surprise from the project was that the Charophyceae are the ancestors of all green land plants now alive. Although some other plant groups established themselves on land, they later died out for reasons not yet understood. In the animal kingdom, by contrast, many different groups made the jump from water to land successfully.

      Another mystery of plant evolution came one step closer to being understood—the sudden appearance of flowering plants around 130 million years ago. Biologists at the University of California, San Diego, led by Martin Yanofsky, converted leaves of Arabidopsis plants into petals by activating five different genes involved in the formation of flower organs lying dormant in leaves. This achievement indicated that flowers evolved from modified leaves. It also could lead to the development of interesting genetically modified plants, such as ornamental flowering varieties that have colourful petals growing along their stems.

      One of the most significant genetic-engineering breakthroughs of the year was the creation of a tomato plant that thrives in salty water, even seawater. A gene for salt tolerance, discovered in Arabidopsis in the late 1990s by plant biologists at the University of California, Davis, and the University of Toronto, was introduced into tomato plants. The gene protects the plants from salt damage by coding for a protein that pumps salt into sealed compartments inside leaf cells; the tomato fruit produced was claimed to have no salty taste. Salty water blights 40% of the world's irrigated land, and engineering salt tolerance into crops could exploit huge tracts of this poisoned land. In addition, the modified tomato plants soaked up so much salt that they could be used to help clean up salty water supplies. Field trials were needed to make sure that the gene would not cross to other plants to create salt-tolerant weeds.

      The possibility of unintentional transfer of genes from genetically modified crops was spotlighted in November when researchers from the University of California, Berkeley, reported detection of transgenic DNA in native maize (corn) from remote regions of Mexico, despite a ban in that country on planting genetically modified maize since 1998. Native maize and other ancestors of crop plants were considered a vital genetic resource for crop-breeding programs, and their contamination with foreign genes could threaten global food security.

      Many plant roots form partnerships with fungi that live in the soil. Typically the fungi supply phosphorus from the soil to the plant in exchange for carbohydrates and other nutrients. John Klironomos and Miranda Hart of the University of Guelph, Ont., revealed that the roots of the eastern white pine (Pinus strobus) support a carnivorous fungus that kills and devours small insects in the soil. Radioactively labeled nitrogen was used to track nutrients as they were absorbed from the animal bodies into the fungus, which then passed them into the tree. Klironomos speculated that, because similar root fungi are nearly ubiquitous among trees, the relationship could be very common and that scientists might have to rethink their ideas about how woodland ecosystems work.

      In a rare example of research on flower movements, Michael Bynum of the University of Wyoming and William Smith of Wake Forest University, Winston-Salem, N.C., made a fascinating study of the flowers of the Arctic gentian (Gentiana algida) at several field sites in Wyoming. The plant blooms in August, the peak month for thunderstorms in the region. As rain approaches, the flower pinches its petal tube shut and reopens after the rain has passed. The investigators determined that these movements protect the pollen and nectar from being ruined by rain, which in turn helps both pollination and seed set. The movements are a response to the drop in air temperature that often precedes a storm.

      The importance of biodiversity was highlighted by scientists at the Imperial College of Science, Technology and Medicine, London, and the École Normale Supérieure, Paris. They found that communities of plants are more productive when they consist of “teams” of different species that specialize in roles that are complementary to one another. It may mean that some plants, and therefore their entire ecosystems, grow more poorly when team members become extinct. This finding gave support to conservationists campaigning to preserve natural environments in their totality.

      Scientists were heartened when a tree species, Trochetia parviflora, thought to have gone extinct in 1863 on the island of Mauritius, was rediscovered. Vincent Florens and Jean-Claude Svathian of the Mauritius Herbarium recognized the tree from old herbarium specimens. The scientists collected cuttings and seeds to try to propagate the species in hopes of boosting the remaining wild population.

Paul Simons

Molecular Biology

A Bucket Brigade for Copper.

      Soluble copper—copper in its +2 oxidation state [written Cu(II)]—is an effective and nonspecific catalyst of oxidation. As such, it can facilitate the oxidation of many biologically essential molecules such as ascorbic acid, glutathione, and polyunsaturated lipids and thereby prevent them from participating in vital reactions. For this reason, free Cu(II) cannot be tolerated by living cells and is considered a poison. Nevertheless, Cu(II) is found at the active sites of several enzymes and is essential for their catalytic functions. One such enzyme present in cells is superoxide dismutase, a protein that contains both Cu(II) and zinc at its active site. The enzyme catalyzes the elimination of the dangerously reactive superoxide radical that is produced as a by-product of normal respiration and thus serves as a defense against oxygen toxicity. Without such defense, aerobic life would not be sustainable.

      One intriguing question that life scientists had posed about superoxide dismutase was how Cu(II) is delivered to the enzyme during its synthesis in the cell without harming the cell. In 1997 a team of researchers from three U.S. universities reported that they had discovered part of the answer. Working with the yeast Saccharomyces cerevisiae, they found a protein that serves to deliver Cu(II) specifically to the active site of newly synthesized superoxide dismutase. They named it copper chaperone for superoxide dismutase (CCS). In 2001 the structure of CCS bound to superoxide dismutase was determined by X-ray crystallography, and that structure illuminated how CCS works. CCS contains two distinct structural parts, or domains. One domain has the same structure as a protein called Atx1, which was known to pick up Cu(II) ions from a transmembrane Cu(II) transporter, a protein embedded in the cell membrane that brings Cu(II) into the cell. The second domain of CCS is strikingly similar in structure to one-half of the mature form of superoxide dismutase, which is a homodimer, a molecule made of two identical subunits (monomers).

      The scenario that emerged from the most recent findings resembles a bucket brigade for passing Cu(II) ions from the outside of the cell to the superoxide dismutase without “spilling” them—i.e., without ever allowing the Cu(II) the freedom to catalyze unwanted oxidations. First, a Cu(II) ion outside the cell is bound by the transmembrane transporter, which moves it to the inside of the cell. Next, using its Atx1-like domain, CCS picks up the Cu(II) from the transporter and diffuses with it to a newly synthesized superoxide dismutase monomer, to which, using its superoxide dismutase domain, CCS then transiently binds and delivers the Cu(II). After receiving the Cu(II), the superoxide dismutase monomer binds with a second monomer (dimerizes) to form the stable and active mature enzyme.

      An analog of the yeast CCS was found in human cells, an indication that this type of Cu(II) delivery system is likely widespread in living species. If CCS is needed for delivery of Cu(II) to superoxide dismutase in the cell, then a mutant organism that lacks functional CCS should also lack superoxide dismutase activity. This was demonstrated to be the case in mice. Importantly, the mutant mice were normal with respect to the activities of other Cu(II)-containing enzymes. This was evidence for the existence of additional copper chaperones for delivering Cu(II) to other copper-containing enzymes. In 2001 these other chaperones were under investigation.

Progress in DNA Vaccines.

       Immunization of humans and other animals is traditionally accomplished by injecting a heat-killed bacterium or virus, or a component protein of it. The administered proteins are recognized as foreign by cells of the immune system, which respond by producing antibodies that circulate in the blood plasma and bind to the foreign proteins with great specificity and affinity. If the protein is from the surface of a bacterium or virus, the elicited antibody binds to and inactivates the bacterium or virus. To partially circumvent this defense, pathogens frequently mutate their surface proteins so that antibodies elicited against one variety cannot recognize and bind to a future variety.

      Another aspect of immunity involves the generation of specialized white blood cells, called T cells, that can recognize foreign proteins, or parts of them, displayed on the surface of cells and then kill those cells. Many virus-infected cells, within which virus proteins are being made for the assembly of new virus particles, will display fragments of those proteins on their surface. The killing of these infected cells by T cells sensitized to the foreign proteins serves to abort the infection. A problem with conventional heat-killed or protein-based vaccines, however, is that they do not cause foreign proteins to display on cell surfaces and thus do not elicit sensitized T cells.

      A relatively new approach to vaccines has focused on preparations of foreign DNA rather than foreign protein. The DNA is in the form of small circular molecules, called plasmids, that can be taken into cells and cause the cells to produce those proteins encoded by the plasmid. One might anticipate that a plasmid encoding a foreign protein or protein fragment would elicit both specific soluble antibodies and sensitized T cells and thus give rise to effective and long-lasting immunity.

      This approach was first tested in chickens, mice, and other animals, where it was found to work spectacularly well even against pathogens hard to target by traditional means. Unfortunately, when tested in humans, DNA vaccines proved disappointing in that much higher doses of the plasmid DNA were found to be required than had been anticipated from the prior studies with animals. These high doses would make the DNA vaccines prohibitively expensive.

      Because an essential step in the immune response occurs in lymphoid tissue, it seemed possible that the response to DNA vaccines could be increased if they were administered directly into lymph nodes, rather than into the skin or muscle tissue. During the year Thomas M. Kündig of Zürich (Switz.) University Hospital and his associates tried this approach and reported a 100–1,000-fold gain in response after injecting the vaccine into the peripheral lymph nodes of mice. If tests in humans proved successful, medical science could see the development of DNA vaccines vastly superior to the classic vaccines.

An Antimicrobial Peptide.

      All multicellular organisms have evolved a constellation of natural defenses to ward off infection from myriad disease-causing agents. One such defense is the production of peptides (molecules structurally like proteins but smaller) with antimicrobial properties. Families of cationic, cystine-rich antimicrobial peptides are found in plants (thionins and plant defensins), insects (heliomycin, thanatin, and insect defensins), mollusks (mytilin and myticin), and mammals (protegrins and alpha and beta defensins).

      During the year Tomas Ganz and co-workers at the Harbor-UCLA Medical Center, Torrance, Calif., reported the isolation of a defensin-type antimicrobial peptide from human urine and named it hepcidin because it is made in the liver. It is apparently common to vertebrates because the DNA sequence coding for hepcidin was identified in pigs, rats, and flounder. Hepcidin is antifungal as well as antibacterial, and it also inhibits the germination of fungal spores. In keeping with a defensive function, the synthesis of hepcidin in the liver is stimulated by specific molecules, called lipopolysaccharides, present on the surface of bacteria. The peptide could someday prove useful as an antibiotic or antifungal agent.

Irwin Fridovich

Genetics of the Senses.

      Humans, like all other species on Earth, have myriad systems for acquiring information about their surroundings. For humans and other mammals, these systems include the senses of sight, hearing, touch, smell, and taste. Humans and many other animals also have a sense of balance, which enables them to move and orient their bodies with reference to Earth's gravitational field. Some species, although not necessarily humans, even have a sense of direction based on the presence of tiny magnetic deposits in their bodies, which allows them to sense Earth's magnetic field.

      Although many species share a given sense, the exact range of that sense can vary between species according to need. For example, whereas the typical frequency range of human hearing is between 20 Hz and 20,000 Hz, dogs can hear sounds at much higher frequencies, and whales and elephants can hear sounds at much lower frequencies. Similarly, human vision responds to colours of light that range from 400 nm (nanometres; violet) to 700 nm (red), the so-called visible wavelengths. Bees and other pollinating insects, by contrast, can see colours into the ultraviolet range.

      Recent research by a number of teams has begun to reveal the genetics underlying the human senses, helping to explain how these complex biological systems work and enabling better diagnosis and intervention for those with genetic impairments of these systems. The results and implications of this effort were summarized in several papers published during the year. Some highlights regarding hearing and vision are discussed below, as well as legal and ethical dilemmas that have surfaced as a consequence.

Congenital Hearing Loss.

      Optimal human hearing requires not only proper structure and function of the outer, middle, and inner ears but also proper reception and interpretation of the electrical signals sent along the auditory nerve to the brain. Compromise at any level, due to gene defects or other causes, can result in impaired hearing. At least one in 10 adults aged 65 years or older experiences significant hearing loss, and about one in every 1,000 infants demonstrates profound congenital hearing loss.

      Most hearing loss, especially among older adults, is not considered genetic in origin but is typically the result of accumulated damage from trauma or infection. On the other hand, a majority of the cases of isolated hearing loss—hearing loss unaccompanied by other symptoms (such as blindness)—seen in young infants are genetic. Recent studies show that hearing loss in these infants is the result of mutations in one or more of an extraordinary number of different genes. Identification of the relevant genes and mutations has given powerful insight into the broad range of gene products that must function together to achieve normal hearing. They include intracellular motor proteins, ion channels and pumps, transcription factors that regulate the expression of other genes, and extracellular matrix proteins that help to form the tectorial membrane of the inner ear. More will likely be identified in the years to come. Perhaps the mutated genes seen most often in these patients, however, are those that code for the connexins. Connexins are gap junction proteins—proteins spanning the cell membrane that control the passage of small molecules directly from the interior of one cell to that of another. These gap junction proteins contribute to the communication between supporting, nonsensory cells of the inner ear. Mutations in the gene CX26, which codes for the protein connexin 26, account for almost half of all cases of isolated congenital deafness in Caucasian populations.

      In 2001 knowledge of the identities and functions of these genes and their products was leading to improved early diagnosis, which in turn was offering improved options for intervention, including cochlear implants. Early diagnosis followed by prompt intervention is important because the auditory regions of the brains of infants born with profound hearing loss will not develop properly unless hearing is restored quickly. Partly in recognition of this urgency, congenital hearing loss joined the list of other, mostly metabolic, impairments for which newborn screening procedures were mandated in some U.S. states and other parts of the world.

Colour Blindness.

      Like hearing, human vision involves the function and interaction of a multitude of gene products that together make up the sensing organ—the eye—as well as the proper transmission, reception, and interpretation of the electrical signals sent by the eye to the appropriate regions of the brain. Also like hearing, visual impairment is extremely common and complex, involving the interplay of genetic and environmental influences, including normal processes of aging. The underlying cause of late adult-onset farsightedness, for example, is generally considered to be a natural loss of flexibility of the lens with age. Similarly, late adult-onset cataracts are believed to result from natural progressive processes that alter the chemical properties of the lens, causing it to cloud.

      In contrast, hereditary loss of vision generally appears much earlier in life (childhood to early adulthood) and can be either accompanied by other symptoms (syndromic) or isolated. Examples range from albinism, a syndrome that includes severe visual impairment, to such isolated conditions as congenital glaucoma, progressive retinitis pigmentosa, and myopia (nearsightedness). Perhaps one of the best understood of the isolated hereditary causes of vision loss is colour blindness, a fairly common congenital inability to see or distinguish specific colours.

      Black-and-white vision is mediated by rhodopsin, a protein located in specialized cells, called rods, in the retina at the back of each eye. Three independent but related proteins, expressed individually in the cone cells of the retina, are responsible for normal human trichromatic colour vision. The gene coding for the protein most sensitive to blue light is located on chromosome 7, whereas the genes coding for the red-sensitive and green-sensitive proteins are both located on the X chromosome. This physical proximity, coupled with a close resemblance in the sequences of the red and green genes, results in a high frequency of unequal recombination events (regrouping of maternal and paternal genes during the formation of sex cells) involving these genes. This, in turn, can lead to either deletion or duplication of one or both genes on the resulting chromosomes. Because the chromosome involved is the X, females who inherit a deleted gene on one X chromosome will most likely carry a compensating normal copy on their other X chromosome, and so they will not experience visual impairment. In contrast, males, who carry only one X (and one Y) chromosome, will have no compensating copy, and so they will experience a form of colour blindness corresponding to the specific gene deletion inherited—either red or green. Indeed, in some studies close to 8% of all males demonstrated some form of colour blindness, generally characterized as red-green colour confusion.

      Although treatments for colour blindness were still lacking, studies to elucidate its genetic basis were leading to improved diagnosis and prognosis. In addition, the results of those studies were helping scientists and physicians gain a better understanding of the normal functioning of the human eye and thus of other, in some cases more debilitating, forms of visual impairment.

Legal and Ethical Issues.

      The recent gains in understanding the genetic basis of sensory impairments in humans have raised moral and legal questions, in large part because of the possibilities presented for prenatal diagnosis of these impairments and for their early diagnosis and intervention after birth. Issues that must be considered include, for example, whether the option of terminating a pregnancy should be offered to “hearing” parents of a child who will be born deaf or to deaf parents of a child who will be born “hearing.” Similarly, mandated newborn screening for profound hearing impairment, with the clear intent to encourage early intervention, has been taken by some members of the deaf community as a threat to the continued existence of the well-established deaf language and culture. Clearly, these issues emerge from differing opinions of what is a disease and what is simply a trait. In 2001 such dilemmas over sensory impairment remained but the tip of the iceberg with regard to human genetics. How individuals and societies handled these specific questions would set a precedent for the many similar problems that lay ahead.

Judith L. Fridovich-Keil

Paleontology

      The most primitive plants in the fossil record lack both seeds and leaves. In 2001 a study of the main lineages of living land plants, which considered both genetic and morphological features, concluded that horsetails and ferns belong to a monophyletic group (group with a single common ancestor) that includes the closest living relatives to modern seed-bearing plants. This finding refuted the prevailing view that horsetails and ferns, both of which are spore-bearing plants, represent transitional evolutionary steps from which seed plants eventually were derived. A second plant study suggested that the origin of leaves was caused by falling levels of carbon dioxide in the atmosphere during the Devonian Period (417 million to 354 million years ago), which drove plants to evolve structures for gas exchange and photosynthesis that were more efficient than simple green stems.

      For many years paleontologists had assumed that the complex tribosphenic molars typical of marsupial and placental mammals had a single origin in some fossil mammal group from one of the northern continents. An American-Polish joint study published during the year suggested that fossil tribosphenic mammals recently discovered from Gondwana, the ancient landmass that included all of the present southern continents and India, might have had a separate origin from their northern counterparts more than 100 million years ago. The investigators in the study speculated that this southern lineage gave rise to the monotremes, represented today by the egg-laying platypus and echidna, while the northern lineage produced all other modern mammals, both marsupials and placentals.

      Madagascar continued to yield fossils of unusual new Cretaceous dinosaurs. One report described a large sauropod belonging to the Titanosauria, the only group of sauropods that survived until the end of the Cretaceous Period, 65 million years ago. While most titanosaur material was very fragmentary, the new Madagascar genus, Rapetosaurus, was the most complete titanosaur skeleton found to date. A phylogenetic study that included this specimen confirmed that titanosaurs were closely related to brachiosaurs. Another new titanosaur, Paralititan, was discovered in Egypt's Bahariya Formation of the Late Cretaceous (99 million to 65 million years ago). With a length of nearly 1.7 m (5.6 ft), its humerus (upper forelimb bone) was the largest of any known Cretaceous sauropod. Paralititan was the first tetrapod reported from the Bahariya Formation since 1935.

      A second new dinosaur from Madagascar, Masiakasaurus, was shown to belong to the unusual group of Gondwana theropods called abelisauroids. Measuring 1.8 m (5.9 ft) long, it was unique in being the only theropod known with heterodont dentition (different teeth specialized for different functions). Whereas other theropods (and most other dinosaurs) were homodont (all teeth similar in shape), Masiakasaurus had distinctly differently shaped teeth in the lower jaw.

      A report by Chinese and American paleontologists revisited the controversial topic of the origin of feathers. Previous studies had suggested that some filamentous covering structures observed in the fossils of several theropod dinosaurs represent primitive feathers. Other analyses, however, had disputed the relationship between these structures and feathers. Moreover, two genera that had been described as having true feathers (Caudipteryx and Protarchaeopteryx) may have been flightless birds rather than theropods. The new study examined the filamentous covering structures in fossils of Sinornithosaurus, a basal dromaeosaurid dinosaur first described in 1999. The investigators concluded that the structures are composed of multiple filaments and show types of branching structure that are unique to bird feathers. If confirmed, this finding would substantiate the theropod origin of bird feathers.

      A recent reexamination of skulls of the ornithomimids Gallimimus and Ornithomimus showed that these toothless dinosaurs may have had keratinized beaks—i.e., beaks made of the fibrous protein keratin, the chemical basis of horny tissue. These animals appear to have had a comblike keratinized plate in the mouth, somewhat similar to the jaw comb of a duck. The researchers involved in the study speculated that the animals may have used their beaks to filter small invertebrates from the water and sediment—certainly a new idea about the diets of toothless theropods.

      A study combining medical scanning techniques and engineering analytic methodology analyzed the bite of the well-known theropod dinosaur Allosaurus of the Late Jurassic (159 million to 144 million years ago). While conventional wisdom assigned Allosaurus a very powerful bite, this research concluded that its bite force was quite low, similar to that of smaller living carnivorous mammals such as wolves and leopards and a sixth of the bite force calculated for Tyrannosaurus. The investigators suggested that, because of its comparatively weak bite, Allosaurus had to be more discriminating in how and where it attacked its prey.

      Late Cretaceous sediments in Mongolia yielded Apsaravis, a new fossil bird. According to a description published during the year, the specimen was one of the best ever found of a group of birds known as ornithurines. The new genus appeared to fill an important gap in avian evolution. Apsaravis also indicated that the Mesozoic relatives of modern birds were not restricted to nearshore and marine environments, as previously speculated, and it provided new insight into the evolution of flight following its appearance.

      A mass of juvenile bird bones reported from the Early Cretaceous of Spain (144 million to 99 million years ago) appeared to represent a regurgitated pellet. The mass contained bones from four individuals and provided the first positive evidence that Mesozoic birds were prey for other animals. Whereas the predator may have been an early mammal, lizard, crocodile, pterosaur, theropod dinosaur, or another bird, mammals and crocodiles typically do not regurgitate bones in pellets, and the lizards and birds found at the same site appeared too small to have been the predators. That means that either a nonavian theropod dinosaur or a pterosaur most likely produced the pellet.

      A group of Canadian invertebrate paleontologists recently reported finding the world's largest recorded trilobite in rocks of Late Ordovician age (458 million to 443 million years ago) near Manitoba. At more than 70 cm (28 in) long, the specimen was 70% larger than the previous record-sized trilobite. Another invertebrate study addressed the origin of modern corals. The primary reef-forming corals belong to the order Scleractinia, the stony corals. It had been postulated that the scleractinians may have evolved from a group of Paleozoic corals called rugosids (rugose corals). One problem with the theory was that the rugosids disappear from the fossil record at the end of the Permian Period (248 million years ago), whereas the scleractinians do not appear until some 14 million years later. The new study suggested that skeletons in corals may be ephemeral—that is, they are produced only when ocean chemistry is favourable for the precipitation of calcium carbonate. This idea would account for the gap in fossil evidence for the origin of modern corals from Paleozoic forms if conditions in the Early Triassic were unfavourable for skeleton formation.

      The Herefordshire Lagerstätte is a Silurian deposit in England of about 425 million years in age that has yielded marine invertebrate fossils in exceptional three-dimensional detail. A wormlike mollusk from this collection was described during the year as a plated aplacophoran. The Aplacophora, along with the chitons, are considered to be the most primitive living mollusks, but up until this discovery they had been unknown from the fossil record.

William R. Hammer

▪ 2001

Introduction

Zoology

      Research on animals in 2000 ranged from tiny hummingbirds to the giant extinct moas, delved into the evolutionary responses of prey to predators, and focused on dolphins to gain insight into the development of language. Issues in conservation biology continued to dominate concerns about wildlife on a global scale.

      Dolphins were considered to be among the most intelligent of nonhuman animals because of their large brains, their advanced social behaviour, and their ability—observed in captivity—to communicate with each other by whistling. A study by Vincent M. Janik of the University of St. Andrews, Scot., provided evidence that dolphin whistles are used for social communication between individuals in the wild. Captive dolphins previously had been documented to repeat underwater whistles immediately upon first hearing them and to develop individualized signature whistles with distinctive frequency patterns. To test the effectiveness of whistle communication in wild dolphins while avoiding observer effects, Janik analyzed recordings of hydrophones that had been placed in an area inhabited by groups of bottle-nosed dolphins (Tursiops truncatus) in the Kessock Channel of Moray Firth, Scotland. More than 1,700 whistles were recorded in instances when an average of 10 dolphins were in the vicinity. Of the total number of recorded whistles, Janik was able to pinpoint the exact location for 991 of them by means of direction-finding techniques. From an analysis of whistle timing and location, he was able to identify cases in which whistles had received responding whistles from dolphins in other locations. Whistle matching, in which one dolphin responded immediately by repeating the signature whistle of another dolphin, was documented in 39 instances. The average distance between animals was 179 m (587 ft), and the maximum signature whistle and response observed between two animals was 579 m (2,000 ft). Imitating the vocalizations of other individuals was considered a key step in the evolution of language among humans. Because, other than humans, no land mammals were known to imitate sounds, study of the communication mechanisms and vocal interactions between dolphins offered a valuable opportunity to gain new perspectives on the origin of language and vocal learning.

      The presence of predators was known to be responsible for the evolution of certain traits, including a variety of behavioral and morphological mechanisms of defense, in prey organisms. Ann V. Hedrick of the University of California, Davis, conducted experiments on field crickets (Gryllus integer) to test the assumption of evolutionary models that the fitness advantage conferred to males that exhibit conspicuous female-attracting behaviour is offset by their greater risk of predation. Male crickets attract female mates by calling, and the length of time that different individuals call is genetically inherited. Female field crickets generally select male mates with the longest calling times. Predators of field crickets, which include mice, birds, lizards, and toads, are able to locate calling crickets by their sound, as are parasitoid flies that deposit their larvae on crickets. Consequently, individual crickets having longer periods of calling are more likely to attract female crickets but also are at greater risk of being located by predators and parasitoids. In experiments designed to compare how male crickets having different call lengths responded to predators, Hedrick used two measures of predator avoidance—the length of time before a male left a protected shelter after exposure to a predator and the length of time before it began to call again. The study demonstrated that males exhibiting the most conspicuous and effective behaviour to attract mates also were the most cautious in their response to predatory threats. This result contradicted the assumption that the males that are most ostentatious and thus most alluring to females necessarily suffer the greatest cost from predation.

      Rick A. Relyea and Earl E. Werner of the University of Michigan at Ann Arbor observed a morphological response of prey to the presence of predators that suggested an adaptive process. The predators in this case were larval dragonflies of the genus Anax, and the prey were tadpoles of four species of frogs of the genus Rana. In each of the prey species of frogs, the investigators observed an ability to alter morphological development that depended on whether dragonfly predators were present. To assess morphological change in individual tadpoles, they conducted computerized image analysis of tail-fin, body, and tail-muscle measurements. For each of the four species of frogs, tadpoles reared with dragonfly predators showed significant morphological differences from those raised without predators present, which indicated a potential for plasticity in body shape for each species. (By contrast, the normal rates of growth and development of the tadpoles of each species were not affected by predator presence.) Many of the changes noted by the investigators, such as the development of deeper tail fins that increase swimming speed, previously had been shown to be effective antipredator mechanisms that could possibly have adaptive value.

      Among the various ways that morphological differences within a species can be expressed is sexual dimorphism, in which members of the two sexes vary in body size or proportions and appearances of body parts. Charles Darwin gave three possible explanations for the evolution of sexual dimorphism. Two are sexual selection (selection for traits that improve mating success) and fecundity selection (selection for traits that increase reproductive output); in each case a reproductive advantage accrues to a particular sex. Examples of both are apparent in many species. Ethan J. Temeles and colleagues of Amherst (Mass.) College provided evidence in purple-throated carib hummingbirds (Eulampis jugularis) for Darwin's third, rarely documented explanation of sexual dimorphism—ecological causation. The carib hummingbirds on the island of St. Lucia in the West Indies pollinate two plant species, Heliconia caribaea and H. bihai, from which they obtain nectar. Male hummingbirds are larger and have longer wings than females, but the bills of females are more than 30% longer and are curved downward at twice the angle of the males' bills. In a census of foraging hummingbirds, all of 15 males were observed to feed on H. caribaea, compared with only 7 of 18 females, presumably owing to males' defending their territories. Instead, females fed primarily on H. bihai. The two flower species differ in floral structure, with the bills and feeding times of males being more compatible with H. caribaea and those of females with H. bihai. The investigators concluded that the evolution of bill dimorphism had been driven by responses to specialization for the different flower types. In a comparison of wing length to bill length among purple-throated carib hummingbirds and several of their close relatives, no reliable pattern was apparent that might have been expected from phylogenetic similarity, which suggested that sexual dimorphism in bill length had been influenced by behaviour or ecology of the species.

      Zoological conservation continued as an important issue for a variety of animal groups and species. R.N. Holdaway of Palaecol Research, Christchurch, N.Z., and Christopher Jacomb of the Canterbury Museum, Christchurch, examined information relating to the extinction of 11 species of moas, the enormous flightless birds formerly indigenous to New Zealand. The study provided the disquieting revelation that the elimination of all species had probably been completed within a century from the time of arrival in New Zealand of the Polynesian ancestors of the Maori, possibly as late as the 13th century. The investigators used human colonization rates and the human exploitation of birds, habitat loss, and numbers of birds initially present to develop simulation models to estimate the rates of decline. In order not to underestimate the time necessary for extinction to have occurred, the most conservative figures were used for each variable. Even when only 100 original colonists and a large original population of 160,000 moas were assumed and when the environmental impact of habitat loss was discounted, none of the models yielded a span of more than 160 years between the arrival of the Polynesians and the extinction of the birds. From a conservation perspective, it was significant that a small number of original colonists exploiting a long-lived animal species with a low reproductive rate could cause adult mortality rates high enough to render extinct a major portion of a region's fauna in a relatively short time.

      The applicability of the moa study to modern species was reinforced with the report by investigators from the University of Georgia's Savannah River Ecology Laboratory, Aiken, S.C., that documented the decline of representatives of all major groups of reptiles on all continents within the past century. As had been reported previously for amphibians, many reptile populations were unquestionably declining in size and abundance on a global scale. When coupled with the problems experienced by amphibians, the evidence suggested that a worldwide crisis was in progress. The causes of declines for both reptiles and amphibians were known with certainty in many instances and were suspect in many more, the six most commonly identified threats being habitat loss and degradation, introduced invasive species, environmental pollution, disease and parasitism, unsustainable commercialization, and global climate change. The study emphasized that the decline and disappearance of populations of reptiles and amphibians or, in some instances, of entire species can occur with little awareness even by biologists. The threats to these animals, as well as to other wildlife, had to be viewed as a serious worldwide situation not only by scientists but also by the general public and government policy makers.

J. Whitfield Gibbons

Botany

      The potential dangers of genetically modified (GM) plants continued to be debated in 2000. The issue grew increasingly heated in both Western Europe and the U.S. as concerns were expressed about the effects of the plants on the environment and human health. Policy makers in Europe set new restrictions on how far away from conventional crops GM crops undergoing field trials had to be grown to prevent transfer of GM plant pollen, but these limits were later shown to be highly suspect as to their effectiveness. Results of the first large-scale study of the flow of genetic material from GM oilseed rape to its wild relatives suggested that hybridization between crops and weeds is rare but that it does occur. Alarm was also raised over the accidental planting of GM oilseed rape on several farms in Europe. That the problem of inadvertent mixing could be widespread was suggested by results of a random sampling conducted by a company that screened for GM material. Genetic ID of Fairfield, Iowa, found that more than half the samples of conventional seed taken from American distributors contained some GM seeds.

      Another controversy continued to brew over so-called GM terminator seeds. These seeds can give rise to only one generation of plants; the next-generation seeds are sterile. Poor farmers in the less-developed world saw this technology as a serious economic threat because they relied on saving some seeds from their crop for the next year's planting. In August the U.S. Department of Agriculture (USDA) announced that it would sanction the terminator technology, albeit with conditions to guard against environmental damage—for example, from cross-pollination with conventional crops, which might then become sterile. (The USDA was a joint patent holder of terminator technology, but it also regulated the engineered seeds to ensure that they were safe enough to be field-tested and sold commercially.) Biotechnology protesters vehemently opposed the decision.

      Despite significant biological and ethical concerns, the potential benefits of GM crops remained tantalizing. During the year a gene that helps determine the size of fruit in tomato plants was identified by Anne Frary, Steven Tanksley, and colleagues of Cornell University, Ithaca, N.Y.—the first time that a gene for a quantitative trait such as height or weight had been found in plants. Because related genes exist in many other plant species, the discovery could lead to the genetic engineering of giant fruit, vegetables, or grain and the development of small wild plants into new, larger crops.

      GM crops also had considerable potential to be tailored into products having therapeutic and health benefits. In September Charles Arntzen of Cornell University reported that his team had genetically engineered a vaccine into tomatoes and bananas that could wipe out hepatitis B and thus potentially save hundreds of thousands of lives each year. The edible vaccine awaited a license from the USDA to allow the plants to be grown commercially. (See Agriculture: Special Report.)

      The excitement surrounding GM research had a tendency to overshadow significant conventional plant-breeding work. A team at the John Innes Centre, Norwich, Eng., announced in May at an Institute of Food Research seminar in London that it had bred a “superbroccoli” by crossing ordinary broccoli with a wild relative that contains 10 times as much sulforaphane, a compound that helps neutralize cancer-causing substances in the human digestive tract. USDA researcher David Garvin and colleagues also pinpointed the gene in a strain of barley that allows it to tolerate high levels of aluminum in the soil. Aluminum toxicity blights half the world's arable land, and the discovery opened up the possibility of breeding aluminum tolerance into other crops and thus exploiting huge barren tracts.

      Fascinating insights were gained into the ways that plants fight off insect attacks. Whereas plants suffering damage by insects were known to release airborne chemicals to attract natural predators of the pests, lima bean plants under attack by mites also switch on the defenses of neighbouring plants to attract predators. A team led by Gen-ichiro Arimura of the Bio-oriented Technology Research Advancement Institution in Tokyo found that three volatile terpenoids released by the besieged plants turn on the defense genes of their neighbours. These chemicals potentially could be used in new “natural” forms of crop protection. Plants also were found to use astonishing defenses against insect eggs laid on the plants. A new class of compounds called bruchins was discovered in pea weevils during their egg-laying activity on pea plants. The chemicals switch on a gene in the plants that causes them to surround the weevil's eggs with small tumourlike growths, which impede the larvae after they hatch.

      For the first time, the explosive fertilization of a flower was observed. After a pollen grain lands on a flower's stigma, it germinates, sending a growing pollen tube down the style. When the pollen tube enters the flower's embryo sac, it thrusts between the two sterile, synergid cells located on either side of the egg, ruptures its tip, and releases its gametes. Tetsuya Higashiyama of the University of Tokyo and colleagues recorded the pollen tube exploding, discharging its contents at a flow rate some 50 times higher than the cytoplasmic flow observed in the tube prior to discharge, and instantly pulverizing one of the synergid cells.

      Assumptions about how trees respond to global warming and elevated atmospheric carbon dioxide were proving more complex than first thought. A study of tree growth in Alaska revealed that higher mean temperatures in the past century had caused drought and stress. This finding upset calculations that the northern forests would absorb some of the additional carbon dioxide being blamed for the rise in world temperatures. The British government's Hadley Centre for Climate Prediction and Research near London warned that global warming could wipe out a third of the Amazon rain forest by the end of the 21st century owing to rising temperatures and drought.

      Efforts to conserve plant species from extinction relied increasingly on storing seeds in seed banks, but disturbing evidence uncovered some alarming shortcomings with these banks. According to Stefano Padulosi of the International Plant Genetic Resources Institute in Rome, of 5,300 species of food plants collected worldwide, more than half had only a single sample left in a seed bank, even though each species may have hundreds or thousands of varieties. Many collections were being destroyed by seed banks short of money, especially in less-developed countries. Many of the stored seeds were also losing their viability. Either the seeds needed to be sown every few years and fresh seed collected, or they had to be stored in deep-freeze facilities. Most seed banks, however, did not have freezers.

Paul Simons

Molecular Biology

      Among the landmarks of human achievement, a major milestone was reported in 2000—the completion of a rough draft of the sequence of the human nuclear genome. This tome consists of more than three billion characters, arranged as linear sets of carefully ordered nucleic acid bases. The accomplishment was of profound significance and promised revolutionary advances not only in biology and medicine but also in the way humans perceive themselves. (See Special Report.)

Atherosclerosis as an Inflammatory Disease.

      Atherosclerosis is an insidious vascular disease in which lesions, called plaques, form inside arteries and gradually occlude them. The plaques are composed of variable proportions of smooth muscle, collagen, platelets, and lipids. The high lipid content of plaques, principally cholesterol, as well as the correlation of high levels of cholesterol in the blood plasma with the incidence of atherosclerosis, indicated that lowering blood levels of cholesterol should be beneficial in preventing or limiting plaque formation. Indeed, it was amply demonstrated that lowering circulating cholesterol, by means of drugs that inhibit cholesterol synthesis in the body or by lowered dietary intake of lipids and cholesterol, decreases the incidence and severity of atherosclerosis.

      As the principal cause of heart attacks, strokes, and circulatory insufficiencies, atherosclerosis remained under active investigation. Work in the late 1990s led to the view that plaque formation may be a response to chronic inflammation of the innermost arterial layer. This inflammation could be initiated by microbial or viral infection, or it could be due to damage to the artery's fragile endothelial lining caused by turbulent blood flow. Whatever the cause, it was becoming clear that treatment aimed at diminishing inflammation could be a new and effective means of treating atherosclerosis.

      Mice that are genetically prone to atherosclerosis and that are fed a diet rich in cholesterol develop the disease. These animals have proved useful in studies aimed at revealing the role of inflammation. Results obtained with such mice during the year demonstrated that decreasing inflammation—either by crippling a gene whose protein product plays a key role in inflammation or by inactivating that product with a specific antibody—reduces the formation and development of atherosclerotic plaques and also increases the structural stability of existing plaques by raising their content of collagen. Unstable plaques in large arteries can break down under the constant pounding of blood. Fragments released from such plaques are swept along with the blood flow until they lodge in and occlude a smaller artery. If this occlusion happens in the brain, it causes a stroke; if in the heart, a heart attack. The increase in understanding the causes of atherosclerosis to encompass the role of inflammation promised to lead to new and more effective methods of treatment and prevention.

Cryptochrome Resets the Biological Clock.

      Circadian rhythms are patterns of biological activity and rest attuned to the 24-hour day, and they are seen in virtually all animals and plants. These rhythms are controlled by biological clocks that are not perfect timekeepers—in the prolonged absence of external clues, they tend to drift and need to be reset. What serves to reset many biological clocks is light—specifically, blue light. That is why jet lag, or the lack of concordance of an individual's biological clock with the new environs, can be helped by exposure to sunlight. The ability of light to set the clock presupposes a pigment to absorb that light and to respond to it by some chemical change.

      In studies carried out in the past few years, the clock-setting pigment, a protein called cryptochrome, was found in the eyes of humans, in the brains of fruit flies, in plants, and even in unicellular cyanobacteria (blue-green algae). In mutant organisms with specific defects in cryptochrome, blue light fails to reset the circadian rhythms, which drift with respect to the 24-hour day. As reported in 2000 in a review of cryptochrome research by Aziz Sancar of the University of North Carolina School of Medicine, Chapel Hill, the amino-acid sequence of cryptochrome was found to have a close structural similarity to the protein photolyase. The two proteins also share the same two light-harnessing, pigmented prosthetic groups (nonprotein portions of the molecule)—one called a flavin and the other a pterin. It is the protein portion of each molecule, however, that dictates the particular use that will be made of the absorbed light energy. In the case of photolyase, the energy is used to reverse a specific kind of damage done to DNA by exposure to ultraviolet light. In the case of cryptochrome, the energy of blue light is somehow used to signal the nervous system to reset the biological clock. Just how that signal operates remained to be determined.

Evolution of a Defense Molecule in Plants.

      Plants, being subject to attack by disease organisms and insects and other herbivores, have evolved a large array of defenses. Indeed, it was estimated that about 15% of plant genes code for products dedicated to defense. Chitinase is one of those defensive proteins. Its specific target is chitin, a structural polysaccharide (complex sugar) made of subunits derived from glucose. Chitin is abundant in the cell walls of fungi and in the exoskeletons of insects, crustaceans, and other arthropods. The chitinase produced by plants defends against disease-causing fungi by breaking the chemical bonds that join the subunits of chitin. The fungi, in turn, have evolved countermeasures in the form of proteins that inhibit chitinase, and the plants have responded by modifying chitinase in a way that diminishes its susceptibility to inhibition by the fungal proteins.

      This scenario of the coevolution of plant defenses and countermeasures had led to the expectation that chitinase must have evolved at a high rate. This was affirmed by the work of J.G. Bishop of the Max Planck Institute for Chemical Ecology, Jena, Ger., and colleagues, who compared the chitinases of related species of plants and documented changes in their amino-acid sequences. The finding added to an appreciation of how selection pressure by predators or disease agents drives the coevolution of prey or host species.

Irwin Fridovich

Paleontology

      Studies in paleontology during 2000 offered intriguing new information on topics ranging from the origins of fish and feathers to long-term evolutionary patterns in marine communities. Until that time, little was known about the origin of vertebrates. While the Cambrian Period marked the beginning of an explosive evolutionary radiation among the major groups of invertebrates with hard parts, fish were absent from this first phase of rapid diversification of multicelled animals. Recently described finds from the Early Cambrian Chengjiang beds of China, however, included delicate small fossils that revealed vertebrate-like skulls, gills, and muscles. These specimens pushed the known origin of vertebrates back by as much as 50 million years; previously, the oldest known fish were from the Late Cambrian.

      Bony fish were the most diverse group (greatest number of species) of vertebrates, yet few fossils have been found that offer details of their origin and subsequent split into the ray-finned and lobe-finned clades. (A clade is a single lineage composed of a common ancestor and all of its descendants.) A recent paper published by a paleontologist from the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, however, extended the fossil record of bony fishes back to the boundary of the Silurian and Devonian periods, about 408 million years ago. Unlike previous early bony-fish fossils, the Chinese specimen included a mixture of lobe-finned and ray-finned features and thus could provide insight into the origin of bony fish from more primitive types of fish. A second study on early bony fish described the most primitive braincase ever found of a ray-finned fish. This 400 million-year-old specimen from southeastern Australia exhibited primitive features previously unknown from any bony-fish fossils, including an opening for a cartilaginous eyestalk.

      A controversial paper published in June again raised the issue of feathered reptiles and the origin of birds. After having been housed in a Russian research institute for decades, a 220 million-year-old fossil of a reptile named Longisquama insignis was reevaluated by a group of Russian and American scientists and determined to have had featherlike appendages. The scientists further suggested that the mouse-sized creature could have been an ancestor of modern birds. Because Longisquama was not a dinosaur and may not even have been an archosaur (a larger group that includes some primitive reptiles as well as dinosaurs, crocodiles, and pterosaurs), this suggestion conflicted with the prevailing idea supported by most paleontologists that birds evolved from theropods (carnivorous dinosaurs, including Tyrannosaurus). Critics of the study claimed that the structures described may not have been feathers at all but could instead have been indicative of large membranous scales. They also argued that even if the structures were feathers or featherlike, other birdlike features were not present in this primitive reptile. A complete analysis that included all of the important derived features of birds continued to place them with the theropod dinosaurs.

      A third specimen of the much-debated feathered theropod dinosaur, Caudipteryx, was described during the year by a scientist at the Institute of Vertebrate Paleontology and Paleoanthropology. Although the specimen lacked a head, the report indicated that it had well-preserved feather impressions and that the skeleton was much better preserved than those of the two earlier specimens described in 1998. The study claimed that although the specimen exhibited some new bird features that were generally not found in theropods, such as an appendage on the foot for perching, it also had 16 dinosaur-like characteristics previously unknown in Caudipteryx. Though many seemed convinced that these findings added new strength to the view that dinosaurs and birds are related, others questioned whether this animal had true avian feathers.

      A study of 12 articulated ornithomimid dinosaur skeletons discovered in 1997 in the Cretaceous Ulansuhai Formation in China revealed some startling new information about the diet of ornithomimids. Although ornithomimids were toothless, they clearly were theropod dinosaurs; consequently, it was long assumed that they were probably toothless carnivores, feeding on small prey much like modern carnivorous birds. These new skeletons, however, had preserved masses of gastroliths inside the rib cage of each animal. Gastroliths are commonly known as “stomach stones,” and they have also been found in the rib cages of many of the large sauropod dinosaurs, such as Apatosaurus. The presence of gastroliths suggested that ornithomimids, like the sauropods, were herbivorous rather than carnivorous. The gastroliths function as a grinding mechanism in the stomach to aid in the digestion of coarse plant material. Many modern herbivorous birds use fine-grained gravel or grit in a similar fashion to grind up plant material in the stomach.

      Investigators described an oviraptorosaur from Mongolia with a pygostyle, which suggested that this small theropod dinosaur may have had a tail fan of elongate feathers. The pygostyle, comprising several fused vertebrae at the end of the tail, is typically found only in birds. Because other features, however, place oviraptorosaurs at some distance from the origin of birds, the investigators suggested that this structure originated independently in the two groups.

      A recent study by a researcher at the National Geological Museum of China and others described Jeholodens jenkinsi, the most complete skeleton of a triconodont ever found. Members of the order Triconodonta were among the earliest (Late Triassic Epoch, 230 million to 208 million years ago) mammals known from the fossil record. They represent a clade much more primitive than even the modern egg-laying monotremes. Jeholodens exhibited predominantly primitive characteristics, but the structure of its shoulder was somewhat similar to that of more advanced mammal groups. This feature of Jeholodens apparently evolved independently of the advanced shoulder of modern mammals.

      A primitive, limbed fossil snake from the Middle East was reported during the year in the journal Science. This 95 million-year-old fossil found in carbonate deposits near Jerusalem preserved portions of the hind limb, including the tibia, fibula, metatarsals, and phalanges. This species, Haasiophis terrasanctus, appears to be evolutionarily near the time when snakes evolved from their limbed predecessors. Loss of limbs is an event that may have occurred more than once during the evolution of snakes in the Late Cretaceous.

      The extinctions of large terrestrial mammals and birds during the Quaternary Period (1.6 million years ago to the present) have long been a subject of debate among paleontologists. Contrasting theories blame the extinction on either dramatic climatic change or hunting by primitive human groups. A new study of the fossil record of a large flightless bird from Australia provided new evidence that climate was not a factor in these extinctions. The disappearance of this bird occurred approximately 50,000 years ago, a time when humans first arrived in Australia but not a time of major climate change.

      Owing to the poor nature of the hominid fossil record, little has been known about the origin of bipedalism in hominids. A report published during the year on specimens of Australopithecus anamensis and A. afarensis indicated that those primitive hominids retained the specialized wrist structure and function associated with knuckle-walking primates. Because Australopithecus was clearly a hominid, this suggested that bipedalism evolved from knuckle-walking ancestors.

      The fossil record often shows that when a new superior group of organisms arises, older, more primitive groups cannot compete and are quickly driven to extinction. Several paleontologists, however, reported that evolutionary patterns in bryozoans suggest that survival of the fittest does not necessarily require extinction of the less fit. They contended that the older cyclostome bryozoans coexisted with the newer cheilostomes for tens of millions of years during the Mesozoic Era until the mass extinction at the end of the Cretaceous Period 66.4 million years ago dramatically reduced the diversity of both groups. It was only then that the more advanced cheilostomes were able to significantly surpass the diversity level of the cyclostomes, which did not rebound from the extinction.

      A recent analysis of long-term evolutionary patterns claimed that major changes in marine communities since the beginning of the Phanerozoic Eon about 540 million years ago correlated with increased diversity in terrestrial communities. This was one of the first studies to link diversity trends in terrestrial and marine organisms.

William R. Hammer

▪ 2000

Introduction

Zoology

      A basic goal of zoology is to explain the distribution and abundance of animals. During 1999, behavioral factors such as feeding and mate selection and environmental factors including temperature and pollution were shown to affect distribution and abundance in animals ranging from zooplankton to insects, amphibians, and seals.

      Over the years marine biologists have proposed several explanations to account for the geographic distribution and diversity of zooplankton in the world's oceans. Until 1999, however, none of the explanations had been quantitatively tested on a large scale. One widely held perception regarding zooplankton was that species diversity of one-celled microbes called planktic foraminifera decreases steadily from the warm tropical seas at the Equator toward the icy waters at each pole. Scott Rutherford and Steven D'Hondt of the University of Rhode Island and Warren Prell of Brown University, Providence, R.I., tested this assumption. They selected 1,252 samples of foraminifera and analyzed many environmental variables to determine which factors were most influential in determining distribution patterns of these animals. Their results showed that the notion of greatest diversity at the Equator was incorrect; planktic foraminifera were most diverse at middle latitudes. This held true in all oceans, along with the lowest diversity's being seen at the poles and intermediate diversity at the Equator. Analyses of ocean temperatures in the Atlantic revealed that almost 90% of the variation in diversity could be explained by temperature alone. Furthermore, the greater diversity at middle latitudes was found to be the result of that region's thicker thermocline—the layer of water separating the warm surface from the colder depths below. The thermocline's greater thickness allows for more ecological niches, which in turn results in a greater diversity of species.

      On a more localized scale, Perri K. Eason and Gary A. Cobbs of the University of Louisville, Ky., and Kristin G. Trinca of Northeast Louisiana University conducted field experiments with cicada-killer wasps (Sphecius speciosus) to confirm anecdotal reports that naturally occurring landmarks are used to define territorial boundaries. Adult male wasps emerge in late summer before the females, with the males setting up mating territories that they defend against other males. Emergent females generally mate immediately with an available territorial male. To test the importance of visual landmarks in territorial behaviour, the investigators caught, marked (with patterns of coloured dots), and released 62 male wasps into a flat, grassy lawn with no obvious landmarks. The researchers then laid 30 randomly placed 90-cm (3-ft) dowels on the lawn to serve as landmarks in the otherwise homogeneous habitat. The next morning the researchers found that the wasps had defined 42 territories within the study area, using the dowels as boundaries, and none of the wasps had crossed into another territory. Further observation showed that wasps defending territories marked by dowels on two sides but with no such boundary on the other two spent significantly more time defending the unbounded sides (19% to 3%). One conclusion offered by the investigators was that the use of natural landmarks to define territorial boundaries could have evolved because of the reduction in costs of territorial defense.

      Perceived declines of herpetofauna (reptiles and amphibians) worldwide have generated concern among conservation biologists for several years. Declines in population and in the number of species have been reported, and many of these declines had been inexplicable. Research in the past year provided insight into the variety of factors that can negatively affect animal populations, thus emphasizing the complexity of global ecology. Recent warming trends were implicated in herpetofaunal declines by the team of J. Alan Pounds and Michael P. L. Fogden of the University of Miami, Fla., and the Tropical Science Center, Costa Rica, and John H. Campbell of the Tropical Science Center, who used a global climate model to determine if events such as the disappearance of the Costa Rican golden toad (Bufo periglenes) during the late 1980s could be explained. The investigators concluded that population crashes observed in several species of frogs and other vertebrates in the region were linked to a reduction in the frequency of mists during the dry season, which in turn was correlated with ocean surface temperatures in the equatorial Pacific.

      A more specific, biological cause for frog deaths was determined by Karen R. Lipps of Southern Illinois University at Carbondale, who reported mass mortality of amphibians along streams in Panama. Frogs of several species were abundant when sampled in 1993–95, but by 1997 few frogs of any species could be found. The researcher necropsied 18 dead specimens and discovered that all were infected with a specific fungus associated with amphibian deaths in other parts of the world. Lipps hypothesized that this fungus—a chytridiomycete—could also be responsible for the declines of frogs in Costa Rica.

      In the United States a combination of field observations and laboratory experiments was used by two sets of investigators to establish that abnormal limb development in frogs can be caused by parasitic flatworms called trematodes. Stanley K. Sessions, R. Alan Franssen, and Vanessa L. Horner of Hartwick College, Oneonta, N.Y., analyzed deformities (extra legs) found in five species of frogs to determine if retinoids were responsible. Retinoids are potent teratogens, or inducers of deformities, that are similar to some pesticides, and retinoids had previously been implicated in reports of deformed amphibians. Analysis of the abnormal frogs, however, revealed that the deformities were related almost exclusively to infestations of a trematode (Ribeiroia), not to retinoids.

      A sample of 1,686 long-toed salamanders (Ambystoma macrodactylum) that also displayed limb deformities supported the conclusion of the frog research. Pieter T.J. Johnson and colleagues at Stanford University and James Cook University, North Queensland, Australia, observed abnormal limb development and low survivorship in Pacific tree frogs (Hyla regilla) experimentally exposed to concentrations of trematodes comparable to those found at field sites. The abnormal limb development was similar to that observed in frogs of the same species at field sites in California that harboured an aquatic snail (Planorbella tenuis). The snail is the primary host of the same trematode, and increases in both snail abundance and parasite infections had previously been shown to occur in response to some forms of pollution. Thus, amphibian deformities may not be caused directly by pollution but as a consequence of it via snails and trematodes.

      Although specific causes for declines can be identified in some cases, the intensity of the effects may result from lowered resistance due to other environmental stressors. Evidence of such a sublethal effect was provided by William A. Hopkins and Justin D. Congdon of the University of Georgia Savannah River Ecology Laboratory and Chistopher L. Rowe of the University of Puerto Rico. They compared trace element concentrations of toxic elements arsenic, cadmium, and selenium in two populations of banded water snakes (Nerodia fasciata). Snakes from a site polluted by coal-combustion wastes were compared with snakes of the same species from an unpolluted reference site. Snakes from the polluted habitat were found to have significantly higher levels of all three toxins in their livers than snakes from the unpolluted site. Concentrations of toxic elements at the polluted site were also dramatically higher than normal in tadpoles, a major prey of the snakes. One sublethal effect measured was that snakes from the polluted site had metabolic rates 32% higher than those from the unpolluted habitat. This indicates that a disproportionate amount of the snakes' energy was being allocated to maintaining their health rather than to reproduction, growth, and energy storage. The resulting lowered resistance would presumably make them more susceptible to other forms of physical, chemical, or biological hazards.

      In Antarctica, Randall W. Davis of Texas A&M University at Galveston and colleagues provided information on the underwater hunting behaviour of Weddell seals (Leptonychotes weddellii). Although extensive research had been conducted on the predation strategies used by carnivores on land, little comparable information was available for large marine carnivores. Weddell seals commonly dive to depths of 100–350 m (330–1,300 ft) for periods of up to 25 minutes. Consequently, where and how these seals find prey during the dive was unknown. The investigators placed data-collection equipment (video systems and data recorders for depth, speed, direction, and sound) on four adult seals to record their hunting behaviour beneath the Antarctic ice. The seals were found to stalk cod and other fish by diving beneath them to take advantage of backlighting from the surface ice and even blowing bubbles into ice crevices to flush out small fish (Pagothenia borchgrevinki). The study not only revealed previously unobserved behaviour in seals but underscored the research opportunities available through use of customized technologies.

J. Whitfield Gibbons

BOTANY

      The politics surrounding the genetic engineering of plants became rancorous in 1999. In many Western European countries, trials of genetically modified (GM) crops were destroyed by protesters concerned about the impact of the plants on the environment as well as on human health. For the first time, the European Union's scientific advisers recommended that a GM potato plant be withheld from commercial use because the group could not guarantee that the potato's marker gene, which provides resistance to an antibiotic, would not spread to other organisms. France withdrew consent for a GM corn (maize) plant pending a review of the dangers of antibiotic resistance in human health.

      In this volatile atmosphere, scientists at Cornell University, Ithaca, N.Y., made headline news when they revealed that in their experiments pollen from corn that had been genetically engineered to protect GM crops against insect pests also killed monarch butterfly caterpillars, which are harmless. The toxin used, called Bt, is produced naturally by a bacterium (Bacillus thuringiensis) and had been used for years as a biopesticide. The Bt in the experiment, however, was engineered into corn so that the plant itself produced the toxin. “This is a warning bell,” said one of the authors, Linda Rayor. “What is really new in this research is that we have shown that toxins can float in the wind.”

      Further worries over GM safety were raised by research suggesting that unrelated plants can, in exceptionally rare instances, exchange DNA by means of go-betweens such as fungi, viruses, or aphids. In late 1998 it was reported that Jeff Palmer and his team at Indiana University had discovered a “stowaway” gene segment in a number of unrelated plants. They suggested that the gene segment may have originated in fungi and subsequently been transported between plants by aphids or viruses.

Genetic Engineering: Putting Plants to Work.

      Despite the controversy, GM crop research advanced, with some of it actually promising to benefit the environment. GM tobacco plants were designed at the University of Cambridge to break down soil residues of the explosives TNT and nitroglycerin. A plant gene that allows plants to soak up toxic heavy metals from soils and store them in leaves was identified by researchers at the University of California, San Diego. The goal was to breed plants that could be harvested with the metals locked inside them and thus eliminate these pollutants from the environment.

      One plant's power to take up minerals could also be used for extracting gold from the ground. Ecologists in New Zealand reported late in 1998 that Indian mustard plants readily absorb gold dissolved in ammonium thiocyanate, a liquid commonly used in traditional mining to process gold ore. They hoped the method could one day be used commercially by harvesting the gold-loaded mustard plants, burning them in incinerators, and extracting gold from the ash.

      The ability of plants to concentrate and store minerals from the soil could also be used to ward off anemia in people who suffer diet-related iron deficiency. By adding a gene for an iron-storing protein to rice plants, Japanese scientists hoped to develop a GM rice that would be rich in iron.

      Scientists at CBD Technologies of Rehovot, Israel, claimed to have developed GM trees and other plants that grow up to 50% faster than usual. They inserted a bacterial gene, called the cellulose-binding domain, that affects the way that cellulose is manufactured and thereby results in faster and broader growth. The company expected the technique to be commercially available within five years.

      A new generation of “designer flowers” was already on the way, thanks to genetic engineering. An Australian company, Florigene of Melbourne, developed GM violet carnations for sale in late 1999, and they hoped to develop a black carnation in 2000. Meanwhile, geneticists at the Salk Institute for Biological Studies, La Jolla, Calif., discovered a gene, called LEAFY, that acts as a master switch for flower development, telling the plant when and where to make a flower. By altering reproductive organs, the gene also determines what the flower will look like.

      At a conference on floral scents at the University of Oxford, it was announced that different scents could be engineered into flowers. Philadelphia-based NovaFlora inserted a gene into roses that was to make them smell of lemons. The gene codes for an enzyme called limonene synthase, which citrus plants use to make scent molecules.

Physiology, Ecology, and Evolution.

      Plant physiologists continued to throw fascinating new light on the way plants combat disease. Belgian scientists detected “hot spots” on the leaves of plants infected with tobacco mosaic virus. These areas were about 0.4° C (0.72° F) warmer than their surroundings and corresponded to areas where the plant was killing its own cells with salicyclic acid, a hormone that prevents the invading virus from spreading. How the extra heat was created was not certain, but the effect appeared eight hours before any other symptom and could therefore be useful for early diagnosis of infection.

      Ecologists revealed that plants create a type of smog that had previously been thought to be man-made. According to German researchers, plants release toluene when they are suffering injury or lack of nutrients, and whole forests may be creating their own clouds of natural “pollution.”

      Australian scientists dated the origins of complex cells to 2.7 billion years ago, a billion years earlier than previously thought. In western Australia they discovered modified steroids within ancient shales. Steroid compounds are produced only by eukaryotes, organisms with complex cells containing a nucleus. The organism that produced the compounds may be the ancestor of all algae, fungi, plants, and animals. The discovery thus opened a new window on the earliest forms of life.

      The extinction of plant species continued to send shock waves through international conservation organizations as the World Conservation Union announced in 1999 that a quarter of the world's coniferous species were under threat. Many of these trees had changed little since the age of the dinosaurs, and some of the oldest living plants on Earth were conifers.

Paul Simons

Molecular Biology

Cellular Floodgates.

      The aphorism “Like dissolves like” is a useful guide to solubility. Accordingly, polar molecules such as sugar will dissolve in a polar solvent such as water, whereas nonpolar molecules, such as fats and oils, will dissolve in nonpolar solvents, such as benzene. Bipolar molecules, with one end polar and the other nonpolar, present a special case. When placed in water, these bipolar, or amphiphilic, molecules seek to expose the polar end to water while hiding the nonpolar end from it. Bipolar molecules accomplish this by aggregating in two layers, with the polar ends facing the water on both sides and the nonpolar ends facing each other in the middle. This two-layer arrangement forms spontaneously and is the basic structure of cellular membranes. Water should not be able to pass through such a membrane because it would be excluded from the hydrophobic core. Water commonly permeates—enters and leaves—cells, however. How does this happen?

      Real cell membranes permit the permeation of numerous substances, such as salts, nutrients, and hormones, in addition to water. Moreover, some of these substances are taken up against a concentration gradient, while the membrane continues to transmit signals in response to various molecules that bind to the outside of the membrane. This is achieved by proteins that are incorporated into the membrane. These proteins are themselves amphiphilic, having hydrophobic portions that insert into the nonpolar core of the membrane, as well as hydrophilic portions that extend into the water on both sides of the membrane. An analogy can be drawn between cellular membranes and brick walls with thick mortar seams. The membrane bilayer would act as the mortar seams, with the inserted proteins being the bricks. Whereas mortar is rigid, however, biological membranes are flexible, even semifluid, which allows the component molecules (the bricks) to drift freely within the membrane (the mortar).

      Study of water movement through membranes reveals that different types of cells differ greatly as to permeability, a phenomenon that cannot be explained on the basis of simple diffusion. Control over the rates of water movement through cell membranes is important to all cells, from bacterial to human. It is now known that a family of membrane-associated proteins called aquaporins controls the rate of water permeation. A single molecule of aquaporin 1 (molecular weight 28,000) allows three billion water molecules per second to pass through the membrane. Aquaporin 1 is amazingly specific for water; in addition to blocking transport of other small molecules, it even blocks protons. Knowledge of the aquaporins has provided explanations for both normal and pathological processes. For example, a person's kidneys filter almost 150 litres (about 40 gal) of liquid from the blood per day, with all but one litre or so being reabsorbed within the kidneys almost immediately. Aquaporin 2 is responsible for this massive reabsorption of water, but its activity is regulated by a hormone called vasopressin. Vasopressin causes the aquaporin to be delivered to the membranes of kidney duct cells responsible for reabsorbing water. Upon reaching the duct cell membrane, aquaporin 2 increases the flow of water into these cells. The small amount of fluid not reabsorbed is urine.

      Diabetes insipidus, a disease characterized by excessive urination, is caused by faulty reabsorption of water by the kidney duct cells. It can be brought on by subnormal amounts of aquaporin 2 or by mutations in the aquaporin gene. Lithium salts, which are widely used to treat bipolar disorder (manic depression), have the side effect of causing excessive urination (polyuria). The cause is now clear; lithium salts interfere with the production of aquaporin 2. Although vasopressin operates by regulating aquaporin's delivery to and from cell membranes, the cell can also control the concentrations of aquaporins by changing their rates of biosynthesis and degradation. Moreover, the activities of aquaporins can be modulated by slight chemical changes in the proteins themselves, giving cells, from the simplest to the most complex, a finely tuned and versatile system of controlling water transport.

Muscular Dystrophy: The NO Connection.

      Nitric oxide (NO), naturally produced from an amino acid by enzymes called NO synthases, serves as a signaling molecule within the body. One NO synthase in nerve cells produces NO that functions as a neurotransmitter. Another is found in certain white blood cells, and the NO that it produces helps these cells to kill invading microorganisms and virus-infected cells. The NO synthase in blood-vessel endothelial cells is responsive to the rate of blood flow, and the NO made by this enzyme causes relaxation of the vessel walls. The resultant vasodilation (increase in the diameter of the vessel) lowers blood pressure.

      New evidence suggests that there is also an NO synthase in skeletal muscle cells. The NO made by this enzyme is extremely important in increasing blood flow to the working muscles so that the vital functions of waste removal and delivery of oxygen and nutrients can be met. Without the vasodilation caused by NO, muscle contraction would actually decrease blood flow to the muscle.

      Muscular dystrophies of both the Duchenne and Becker varieties are linked to defects in a membrane-associated protein called dystrophin. NO synthase binds to a protein called syntropin that in turn binds to dystrophin. In this way the NO synthase is localized to the membranes of the muscle fibres—a position optimal for the delivery of NO to the surrounding blood vessels. In the Duchenne and Becker muscular dystrophies, the defective dystrophin fails to bind the syntropin-NO synthase complex, and the NO synthase remains within the cell rather than migrating to the muscle fibre membrane. The blood vessels fail to dilate; the muscles do not get the increased blood flow they need; and the muscles suffer damage.

Plants “See” Red.

      Not only is light a source of energy for plants, but the quality and quantity of light also provide growth signals—when seeds should germinate and when mature plants should blossom. One of the proteins that allows plants to “see” the light and to respond appropriately is phytochrome. This pigmented protein can exist in two forms, each of which can be converted to the other by light of specific wavelengths. It now appears that one of these forms of phytochrome modulates the activities of other proteins. Red light converts the inactive form of phytochrome to the active form. Far red light—longer wavelengths of red light—can convert the active phytochrome back to its inactive form. The phytochrome thus acts very much as a light-activated two-position switch, allowing the plant to sense the ratio of red to far red light and control its physiology appropriately.

How Plants Send an SOS.

      Plants have a very clever defense against the insects that eat them—they synthesize and secrete large amounts of volatile compounds that attract enemies (either predators or parasites) of the eater. Moreover, plants can distinguish herbivory (plant eating) from simple mechanical damage and can even tell one herbivorous insect from another, which keeps the plant from responding to a potentially beneficial herbivore (such as a seed-dispersing mammal) and allows for the attraction of only those species that prey on the insect damaging the plant. These volatile calls for help are produced by the plants in response to specific compounds, called elicitors, produced by the herbivorous insects. Sometimes an elicitor is a compound made entirely by the insect, and sometimes it is something that the insect obtained from the plant and then modified. Either way, the predators and parasites attracted to the site significantly decrease the life span and reproductive potential of the herbivore and thus provide the plant with a delayed, but effective, defense.

Irwin Fridovich

Recent Advances in Plant Genetics and Culture.

      Although Gregor Mendel may have been the first to study formally the origins and transmission of specific traits in plants, the practice of selective breeding to enhance desirable traits in “domesticated” crops has been pursued by human populations since at least the beginning of recorded time. In recent years recombinant techniques have joined the arsenal of tools applied to the task.

      Recombinant DNA technology in plants has come a long way in recent years through the combined efforts of academy and industry. Improvements include new techniques for introducing foreign or modified DNA sequences into plant genomes and more efficient ways to regenerate whole plants from recombinant clones of cells cultured in the laboratory. Research goals have ranged from growing healthier grains to making plants that produce biodegradable plastic, and many of these efforts are finally beginning to bear fruit.

Potatoes for Latin America.

      In the past 50 years, U.S. potato yields have doubled through the combined successes of breeding, irrigation, pesticides, and fertilizers. Unfortunately, cultivation practices for the potato varieties common in many climates other than North America have not kept pace. Researchers in Latin American and European laboratories are closing the gap by using genetic engineering to modify varieties of potato commonly grown in Chile, Argentina, Uruguay, Brazil, and Cuba. For example, field tests are currently under way in Chile and Brazil for several genetically engineered lines that are resistant to the Erwinia bacterium, a serious potato pathogen. Additional strains engineered for resistance to insects and a variety of fungal, viral, or bacterial assaults also are in the works.

Improving the Cassava.

      Cassava is not generally considered a mainstay of nutrition in Western societies, but the leaves and starchy roots of this shrub constitute the third largest source of calories for human consumption worldwide (following rice and corn). More than 50 years ago, a group of British scientists working in East Africa initiated a program of selective breeding for cassavas that was designed to increase the size and number of edible roots per plant. Although the results of these efforts were impressive, further improvements proved difficult owing to losses from bacterial, fungal, and viral infection. Using recent improvements in plant biotechnology, however, a number of research groups are now addressing these issues. For example, one group has succeeded in creating cassava plants resistant to viral infection by engineering the plants to express replicase, an enzyme that disrupts the normal life cycle of the invading virus. If efforts such as these succeed in the field, scientists predict that yields of cassava could increase as much as 10-fold.

Engineering a Better Soybean.

      Soybeans are a source of a wide variety of food products in many countries. One problem with natural soy oil is its high content of polyunsaturated fatty acids, which makes it unsuitable for frying and cooking. Chemical hydrogenation has been used to convert these compounds to their monounsaturated form, oleic acid. One unfortunate side effect of this process is the production of increased concentrations of trans fatty acids, which have been linked to a number of health risks. As an alternative, researchers at the DuPont Co. have succeeded in genetically modifying soybean plants so that the all-cis oleic acid concentrations in natural seeds are raised from 25% to 85%, which thereby precludes the need for chemical hydrogenation. In short, they have developed a healthier soybean.

Vaccines from Potatoes.

      Recombinant vaccines, such as the popular hepatitis B series given to all children and to most adults in the U.S. and many other countries, are produced and purified from genetically modified hosts, such as yeast. These vaccines offer undeniable benefits over their predecessors, heat-killed or attenuated live virus, because there are few if any risks associated with receiving the vaccine. Unfortunately, these injectable recombinant vaccines are also expensive to produce, ship, store, and administer, so many children and adults in less-developed nations who may need them the most are least likely to receive them. In 1998 scientists in Ithaca, N.Y., engineered potatoes to express an Escherichia coli (bacterial) protein that elicited an immune response from human volunteers who ate the raw potatoes. They are now working on potatoes to provide immunity against other pathogens, such as the Norwalk virus. The benefits of such edible vaccines are clear; they should be cheap to produce, ship, and store, and no needle is needed for administration. One drawback is that the recombinant plant must be eaten raw, which has inspired researchers to look beyond potatoes for a tastier host, such as the banana.

Biotechnology—Blessing or Curse?

      Recent advances in plant biotechnology have produced a stunning array of seemingly hardier plants, growing in more climates and producing more and better fruits. Some view this second generation of modified crops as a bountiful blessing, but others see it as a disguised curse. Some fear hidden dangers to those who consume the recombinant crops, whereas others worry about damage to the environment, including potential compromises of biodiversity. Similar concerns must have been raised generations ago when the first hybrid grains and chemical fertilizers were introduced. In recent years the furor over genetically modified foods in the marketplace has been particularly keen in Great Britain and other nations of the European Union, with ripples in the U.S. and other parts of Europe.

      Public acceptance of genetically modified foodstuffs might be expected to be sluggish as long as the benefits of genetic engineering were enjoyed mainly by the producers rather than the consumers. Food prices in the developed world were already low enough that consumers had no real reason to care whether a particular crop was easier or cheaper to grow. Now that more of the benefits of genetic modification—improved taste, longer shelf life, and enhanced health benefits—are oriented directly toward the consumer, however, the public may prove more receptive.

Judith L. Fridovich-Keil

PALEONTOLOGY

      The year 1999 in paleontology included important discoveries about the earliest origins of several major groups of organisms, including vertebrates, invertebrates, plants, and fungi. Furthermore, the fossil record in general was deemed sufficiently complete to address questions of the origin and evolution of life, according to a new volume edited by Stephen K. Donovan and Christopher R.C. Paul, The Adequacy of the Fossil Record. The editors concluded that the fossil record was surprisingly complete, preserving perhaps 10% of all species that existed in the past. Previously, it had been estimated that only about 1% of species had been preserved.

      Vertebrate paleontologists reported a Devonian Period (408 million–360 million years ago) fish with bones in the front limbs that strongly resembled the fingers of a land animal. Because other features of the specimen suggested that it was not directly ancestral to primitive amphibians, the digits apparently did not evolve for walking on land. This unusual fossil also indicated that appendages with “fingers” evolved in more than one lineage of primitive fish.

      Africa continued to yield interesting dinosaur material. Suchomimus, a new theropod from Niger, possessed an extremely elongated snout in addition to a low sail-like structure on its back. Apparently the snout had been adapted for catching fish. In light of some recent discoveries from Africa and elsewhere, a new review of dinosaur evolutionary patterns suggested that there was little coevolution between dinosaur predators and prey or between herbivores and plants during the time of the dinosaurs (Mesozoic Era). Contrary to previous ideas, this study also suggested that the Mesozoic breakup of Pangaea (the ancient landmass that included all of the present continents) had little effect on dinosaur distribution patterns.

      Another significant dinosaur discovery was reported from Antarctica late in 1998. A single tooth collected from deposits approximately 65 million–70 million years old (Late Cretaceous Period) on Vega Island near the Antarctic Peninsula represented the first hadrosaurid (duck-billed) remains from Antarctica. This find indicated that hadrosaurs were more common in the Southern Hemisphere than had been previously thought. Evidence of the only other hadrosaur from a southern continent came from Patagonia in southern Argentina. Antarctica's oldest-known fossil bird was found in the same deposit that yielded the hadrosaur.

      For years paleontologists envisioned the large herbivorous dinosaurs with long necks and tails—sauropods such as Apatosaurus, Diplodocus, and Brachiosaurus—as browsers that ate foliage high in the trees. Some investigators even suggested they could rear up on their powerful hind limbs to reach the youngest leaves at the tops of the trees. A biomechanical study using articulated digital reconstructions of two sauropods concluded, however, that sauropod necks were much less flexible than previously thought. In fact, the authors of the study believed that sauropods were better adapted for ground feeding than high browsing.

      Another study involving sauropods indicated that members of this group, which included the largest land animals that ever lived, grew to adulthood surprisingly quickly. The research used growth rings in the shoulder blades of sauropods of various ages to measure rates of growth. Bones of half-sized individuals were estimated to be just 4 or 5 years old, and it was believed that these sauropods reached full size at 8 to 11 years old.

      In 1999 the same Chinese locality where spectacular specimens of feathered dinosaurs had been found the previous year yielded one of the most complete skeletons of an animal very close to the base of the mammalian family tree. Analysis of this skeleton suggested that a family (Triconodontidae) long considered to include the direct ancestors of modern mammals was not a natural group. Originally founded on characteristics of the teeth, it appeared that some triconodonts are close relatives of the mammals whereas others are not.

      Recent DNA analysis supported a radical new view that was emerging from the fossil record concerning the origin of turtles. The studies suggested that the traditional portrayal of turtles as primitive reptiles closely related to the basal reptiles of the late Paleozoic was false. Instead, turtles were now considered to be of a type similar to the more advanced archosaurs (crocodiles, birds, dinosaurs) or lepidosaurs (lizards, snakes). Although the morphological data indicated that turtles were related to lepidosaurs, DNA data suggested that they were closer kin to archosaurs. Both data sets, however, indicated that turtles were not the less-developed reptiles they had been perceived to be. The new interpretation moved the turtles to a position near the top of the evolutionary tree of reptiles, rather than very close to its base.

      A vertebrate assemblage reported from Axel Heiberg Island in the high Canadian Arctic provided evidence of very warm climates at high latitudes during the Late Cretaceous (92 million–86 million years ago). Among these fossils were several varieties of aquatic and semiaquatic freshwater vertebrates such as fish, turtles, and reptiles, including champsosaurs. Like turtles, the 2.4-m (7.8-ft)-long Champsosaurus (an extinct reptile resembling a large crocodile) was ectothermic (cold-blooded), which suggested that the environmental conditions were very mild at extreme latitudes just before the Cretaceous extinction. This assemblage was probably a better indicator of high-latitude warmth than were the Late Cretaceous dinosaurs discovered in the 1980s on the North Slope of Alaska. Unlike the case of dinosaurs, endothermy (warm-bloodedness) and migration to lower latitudes during the winter did not apply to turtles and champsosaurs.

      The field of invertebrate paleontology was not free of controversy. Fossils from India described as burrows of a wormlike animal were claimed to be 1.1 billion years old. This interpretation placed the origin of metazoans (multicelled animals) before 1.1 billion years ago, which implied that animal evolution began earlier and progressed much more slowly than had been believed and called into question the Cambrian “explosion” of animal forms that was thought to have occurred about 580 million years ago. While some believe these fossils may represent animal burrows, evidence in associated deposits placed the fossils' age at only about 600 million years. This dating placed these fossils very close in age to the Cambrian diversification of shelled metazoans.

      One of the most significant papers in the field of paleobotany published in 1999 described angiosperms, the oldest-known flowering plants. The fossils from China were first reported in 1997–98 and were dated to approximately 165 million years ago (Late Jurassic Period). For years paleobotanists had estimated the origin of angiosperms at approximately 130 million years ago. The fossil plants from China were placed in a new genus, Archaefructus, and they suggested that some of the earliest, most primitive angiosperms produced relatively large flowers and fruits. The existence of these specimens also made Asia a potential site for the origin of all flowering plants.

      The family tree of a large group of fungi was also followed farther back through time. The oldest-known ascomycetes (a class of “true fungi”), recently discovered in Scotland, pushed back the origin of the fungi to approximately 400 million years ago (Early Devonian Period) and demonstrated the level of biodiversity that existed during the fungi's early colonization of the land.

William R. Hammer

▪ 1999

Introduction

Zoology

      Scientists in 1998 uncovered intriguing new information about a number of critical stages in the life cycles of animals, including courtship (in oystercatchers), metamorphosis and development (in salamanders), and parental care and aging (in baboons and lions). A study of the association between hyenas and endangered African wild dogs offered insights that had application to conservation efforts, and an examination of fossil material resulted in a reinterpretation of the disappearance patterns of long-extinct trilobites. Fossil remains also provided support for Cope's rule that animals tend to increase in size during evolution and gave direct proof of the prey of a marine reptile.

      The European oystercatcher (Haematopus ostralegus) is a wading bird in which breeding pairs are typically monogamous. Polygyny, in which a male mates with more than one female at one time, is rare among oystercatchers. Dik Heg and Rob van Treuren of the University of Groningen, Neth., investigated polygyny within a population of European oystercatchers to determine the reproductive consequences of the behaviour when it did occur. Using data from 14 years of study, the investigators determined that vacant breeding territories for which unmated females must compete for access are at a premium. Although females that participate in polygyny are less successful at breeding than monogamous females, observations indicated that when a pair of females share the breeding territory of a male, they can use it as a stepping-stone to a neighbouring territory and to an improved chance of a monogamous relationship during the next breeding season.

      The Groningen study also revealed an unusual form of polygyny within the oystercatcher population. Among observed breeding trios (a male and two females), 57% involved the traditional form of polygyny, in which each of the females operates independently within the male's territory, aggressively defending her own portion. Among 43% of the trios, however, the researchers observed cooperative polygyny, in which both females laid their eggs in the same nest, helped the male brood eggs, and engaged in female-female copulations that mimicked male-female copulations.

      A difficulty in understanding some evolutionary processes is that the target of natural selection—the trait upon which natural selection operates—is not always obvious. To address this issue in regard to the timing of metamorphosis and maturation, Travis J. Ryan and Raymond D. Semlitsch of the University of Missouri investigated the life history of the mole salamander (Ambystoma talpoideum). Larvae of this species can undergo metamorphosis before becoming mature, as amphibians typically do, or they can bypass metamorphosis and become mature while retaining most of the features of immature larvae. The researchers took pertinent body measurements and made determinations of maturity on 864 individuals raised at either high or low population densities in experimental ponds for periods of four to eight months. They found that salamanders that skipped metamorphosis matured sexually well before those that metamorphosed. Early maturation, which maximizes reproduction and is known to be advantageous in many natural populations, appears to necessitate the retention of the larval morphology (form and structure) in these salamanders. It previously had been assumed that both metamorphosing and nonmetamorphosing forms matured at the same rate and that larval morphology was the target of selection. The experiment challenged the notion that morphological features are the chief targets, demonstrating instead that age of maturity is the principal target and that morphological changes are secondary effects.

      Abrupt declines in female fertility at an advanced age are characteristic of many mammals, including dogs, whales, rabbits, and elephants. In humans cessation of reproduction, known as menopause, has been explained in terms of evolution as an adaptation that allows grandmothers an opportunity to invest in caring for their older offspring and grandchildren and thus increase their fitness (i.e., their ability to transmit their genes successfully through successive generations). To examine such behaviour in long-lived mammals, Craig Packer of the University of Minnesota, Marc Tatar of Brown University, Providence, R.I., and Anthony Collins of Gombe Stream Research Centre, Tanzania, conducted a study of olive baboons (Papio anubis) and African lions (Leo leo), two species in which elderly females cease reproduction and engage in kin-directed behaviour. Using data from wild populations of baboons and lions that had been under continual observation for more than 30 years, the investigators examined ages of cessation of reproductive activity in females and compared infant survival patterns among young with and without interactions with grandmothers. They found no evidence that the fitness of grandchildren or older young of either species was enhanced by nurturing grandmothers. The researchers concluded that the loss of reproductive activity in older females is a nonadaptive by-product of senescence and confers no clear evolutionary advantage.

      The population densities of an endangered species, the African wild dog (Lycaon pictus), and of spotted hyenas (Crocuta crocuta) appear to have an inverse relationship in most habitats—i.e., wherever there are more hyenas, there are likely to be fewer dogs. One explanation is that hyenas steal food from the dogs, the risk of theft increasing in open habitats with high visibility, such as the Serengeti Plain, and decreasing in wooded habitats, where hyenas are rare and the killing of prey by dogs is harder to detect. Martyn L. Gorman and John R. Speakman of the University of Aberdeen, Scot., and Michael G. Mills and Jacobus P. Raath of South Africa's Kruger National Park, using an isotopically labeled water technique for measuring the metabolism of animals in the field, were able to determine the daily energy expenditure of dogs in their natural settings. From these estimates of the energy cost of hunting in real time, the researchers developed a model to determine the impact of food loss on energy balance. According to their calculations, a loss of only 25% of the dogs' food to hyena theft would more than triple their daily hunting time, which would approach the point of being physiologically untenable and thus threaten the dogs' survival. Because of the high energy cost to the dogs of food loss from theft, the investigators recommended that conservation efforts would be most effective in thickly wooded habitats, where theft was comparatively low.

      Trilobites were among the most common animals of the early part of the Paleozoic Era (540 million to 245 million years ago), being noted for their explosive evolutionary development in the Cambrian Period (540 million to 505 million years ago). After extensive diversification and specialization, trilobites appeared to falter by the middle of the succeeding Ordovician Period (505 million to 438 million years ago) such that about half of trilobite genus and family diversity was lost at the end of the Ordovician, followed by further decline of the remainder until their complete extinction near the end of the Paleozoic. An analysis by Jonathan M. Adrain and Richard A. Fortey of the Natural History Museum, London, and Stephen R. Westrop of the University of Oklahoma of 945 genera of trilobites in 56 families from the Ordovician demonstrated that scientists' impression of a steady decline of the entire trilobite group beginning in that period was simplistic. The researchers identified two major, phylogenetically distinguishable groups of trilobites that had dramatically different patterns of diversification and extinction. One group declined and completely disappeared by the end of the Ordovician, whereas the other flourished, with the surviving families showing a higher diversity of genera than did the families that became extinct. Because understanding the pattern of decline and extinction of trilobites was critical to interpretations of the marine ecosystems of the times, paleontologists considered the discovery of two groups of trilobites with contrasting patterns of development an important advance.

      An examination of fossil mammals on the North American continent provided strong support for a pattern named after the 19th-century American paleontologist Edward Drinker Cope. Cope's rule, the observation that the average body mass of animal evolutionary lineages tends to increase with time because of its survival and reproductive advantages, had not been previously documented statistically with large sample sizes of mammals. In a more detailed look at the phenomenon, John Alroy of the Smithsonian Institution, Washington, D.C., reported that new animal species evolving within a genus were 9.1% larger on average than older species. The pattern persisted throughout the Cenozoic Era (66.4 million years ago to the present), as revealed by estimates of body sizes of 1,534 species of fossil mammals analyzed in a manner to avoid sampling bias. Although the overall trend could be explained by within-lineage increases in body size, several different evolutionary mechanisms may in turn be responsible for the increases.

      Insight into the diet of a marine reptile from the Cretaceous Period (144 million to 66.4 million years ago) was provided by Tamaki Sato of the University of Cincinnati, Ohio, and Kazushige Tanabe of the University of Tokyo. Plesiosaurs had been assumed to have been marine predators, but most dietary evidence for this was circumstantial and based on morphology, particularly of the teeth. The two investigators described a short-necked plesiosaur fossil from Japan that was preserved in a way that allowed its fossilized stomach contents to be identified as ammonites, an extinct group of cephalopod mollusks. The direct evidence of prey in the diet of an extinct predator was useful in validating hypotheses of prey preference based on evidence from tooth morphology.

J. WHITFIELD GIBBONS

Entomology.

      Whereas inbreeding had been suspected to be a contributor, along with environmental and demographic factors, to the decline and ultimate extinction of small, isolated natural populations of organisms, in 1998 the first documentation of that link was provided by Ilik Saccheri of the University of Helsinki, Fin., and colleagues in studies of the Glanville fritillary butterfly (Melitaea cinxia) in Finland. In a region having more than 1,600 meadows suitable for small populations of the butterfly, the investigators found that the number of meadows in which butterfly larvae were present had decreased each year, from 524 to 320, between 1993 and 1996. In 1996, 42 populations were sampled for a determination of their genetic variability—specifically, their heterozygosity. For a given genetic trait, an individual is said to be heterozygous if the paired genes for the trait, one received from the mother and the other from the father, are different. By analyzing a sample of genes from the individual, researchers can estimate its level of heterozygosity—i.e., the fraction of its gene pairs that differ. Low heterozygosity in the individuals of a population would imply a limited gene pool and indicate inbreeding. After eliminating the influence of a variety of ecological factors that could contribute to population decline or extinction, the researchers found that the probability of extinction of a butterfly population was significantly correlated with low heterozygosity. They identified larval survival, adult survivorship and longevity, and the hatching rate of eggs as the components of the insects' life cycle adversely affected by inbreeding. The findings were relevant to management considerations for populations living in fragmented habitats in which inbreeding was likely.

      Previous evidence from fossil plants had confirmed that angiosperms, the flowering plants, were present in the Early Cretaceous Period (144 million to 97.5 million years ago), but uncertainty existed about earlier origins. The discovery in Liaoning province, China, of fossil short-horned flies, or orthorrhaphous Brachycera, in rocks of the preceding Late Jurassic (163 million to 144 million years ago) by Dong Ren of the National Geological Museum of China gave evidence of a pre-Cretaceous origin of angiosperms. Examination of the fossil flies revealed mouthparts and body hairs characteristic of those used by their modern counterparts to collect nectar and pollen. Modern members of the group are mostly flower feeders and pollinators. Confirmation of the existence of these ancient pollinators during the Late Jurassic strongly implies that angiosperms originated during or prior to that time.

      The discovery of fossil ants in amber deposits from New Jersey dating to 92 million years ago provided evidence that one major lineage of extant ants, the subfamily Ponerinae, is at least 50 million years older than previously documented. Uncertainty had existed about whether a specimen of Sphecomyrma freyi reported earlier from the New Jersey amber was actually an ant because the metapleural gland, located above the hind legs, was not identifiable. Within the insect order Hymenoptera, which includes ants, bees, wasps, sawflies, and other types, the metapleural gland is unique to ants. An examination of new specimens by Donat Agosti, David Grimaldi, and James M. Carpenter of the American Museum of Natural History, New York City, confirmed the identity of Sphecomyrma as an ant by the presence of a metapleural gland. The find was important in dating and defining phylogenetic relationships during the early evolutionary origins of ants, which were estimated to have been about 130 million years ago, during the Early Cretaceous.

ANNE R. GIBBONS

Ornithology.

      Cuckoos are well known for their habit of brood parasitism, which consists of laying the eggs singly in the nests of certain other bird species to be incubated by the foster parents, which then rear the young cuckoo. In its foster home the cuckoo chick needs as much food as a brood of five original young—say, reed warbler chicks—would have consumed had they not been ousted from the nest by the cuckoo hatchling. Consequently, it might be expected that with only one begging gape rather than five, the foster parents would not be encouraged to deliver enough food. Experiments by Nick Davies and colleagues of the University of Cambridge, however, demonstrated that natural selection (ever an optimizing process) caused the young interloper to voice as many begging cries as would have the brood that it replaced. Thus, the young cuckoo fledges at about the same weight as the combined weight of the five juvenile reed warblers.

      Another species of bird "cuckolded" by an avian brood parasite is the blue-grey gnatcatcher, in whose nests cowbirds lay their eggs. C. Groguen and N. Mathews of the University of Wisconsin discovered that some gnatcatchers recognize the egg as alien. Those birds avoid the role of surrogate parenting by dismantling the nest, leaving the cowbird's egg to addle, and then using the same materials to rebuild elsewhere.

      Birds that feed on fermenting fruit run the risk of alcoholic inebriation and, as has been observed in some species, of incapacitation. This is not the case with the starling, however, even though it is a regular summer consumer of rotting apples. According to R. Prinzinger and G. Hakimi of the University of Frankfurt, Ger., starlings avoid the problem because the birds are equipped with powerful enzymes that steady their behaviour. The researchers fed an alcohol-laced diet to captive starlings and found that within two hours the birds had fully metabolized the alcohol.

      Birds that forage on lawns—typically the song thrush in Europe and the robin in North America—characteristically run a short distance and then take up a noticeable stance in which the individual stops and appears to listen. In cocking its head, however, is the bird hunting for worms by ear or by eye? Two Canadian ornithologists, R. Montgomerie of Queen's University, Kingston, Ont., and P. Weatherhead of Carleton University, Ottawa, proved by experiment with American robins that the worm is detected not by smell, sight, or tactile means but by hearing.

      In winter, a time when both sexes of the northern shrike regularly sing, they sing a different song from that of the male in summer. Eric Atkinson showed that cold-season singing by this predatory bird includes mimicry of the begging and alarm calls of small birds such as pine siskin and song sparrow and is given from bushy cover. Individuals of the copied species are attracted—lured by deception—toward the predating shrike, which may thus more easily attack them.

      Species of living birds reported as new to science included, from Brazil, a particularly agile member of the ovenbird family named Acrobatornis fonsecai by its discoverers, José Pacheco and others of the University of Rio de Janeiro. Another Neotropical bird new to the world list was the Chocó vireo, discovered in Colombia by Gary Stiles of the University of Bogotá and Paul Salaman of the University of Oxford. From Latin America came a species of antpitta, as yet unnamed, found by Robert S. Ridgely of the Academy of Natural Sciences in Philadelphia. Ridgely heard an unfamiliar birdsong in the forest, tape-recorded it, and played back the sound; down from the forest canopy came a male bird to investigate the apparent intruder. Robert B. Payne of the University of Michigan reported from Nigeria a new kind of firefinch, which he named the rock firefinch. The tiny bird was observed to be regularly and exclusively parasitized by the Jos Plateau indigo bird, which lays its eggs in the firefinch's nest.

JEFFERY BOSWALL

Marine Biology

      In 1998 American researchers working aboard the deep-sea submersible Alvin reported their discovery of the most temperature-tolerant eukaryotic (nucleated-cell) organism on record. The polychaete worm Alvinella pompejana, living near deep hot-water vents on the East Pacific Rise, experiences temperatures as high as 80° C (176° F) within its self-constructed protective tube, in contrast to 22° C (71.6° F) at the tube entrance. Its temperature tolerance exceeds that of other known multicellular organisms, which do not normally live at temperatures above 55° C (131° F). A German study of material collected by U.K. researchers described an unusual and abundant sea anemone new to science from the Porcupine Abyssal Plain in the northeastern Atlantic Ocean. The anemone, Iosactis vagabunda, exhibits unique behaviour by intermittently vacating its burrow rather than pursuing a completely sessile lifestyle.

      Mass stranding of Cuvier's beaked whale (Ziphius cavirostris) is very unusual, but such stranding was reported in the eastern Ionian Sea of the Mediterranean. The event coincided with military acoustic tests for submarine detection, and an investigation of possible causal links was proposed. Scientists from Thailand, Spain, and Denmark presented encouraging findings for environmental managers who were concerned with halting an alarming decline of mangrove forests in Southeast Asia due to aquaculture and industrial and urban development. (See Special Report (Aquaculture: Fulfilling Its Promise ).) Their examination of a 28-year record of aerial photographs and satellite images revealed undisturbed mangroves in Pak Phanang Bay, Thailand. The mangrove edge had advanced at nearly 39 m (128 ft) per year where sufficient propagules (structures that allow the plant to spread) were available for the pioneer colonizing mangrove species Avicennia alba and Sonneratia caseolaris.

      A new technique for studies of plankton in natural habitats was developed in Sweden. Using an underwater video camera mounted at an oblique angle to a stroboscope, researchers produced dark-field images of plankton animals as small as 0.3 mm (0.01 in) in length, permitting detailed study of species interactions and distributions. Scuba divers in the Atlantic off South Carolina and in the Pacific off the San Juan Islands, Washington state, made direct observations of aggregations of marine "snow," ubiquitous oceanic material comprising detritus, microbes, and phytoplankton embedded in mucus. These aggregations were visited, often in succession, by many types of zooplankton, probably to feed on microorganisms. Other American studies demonstrated a major source of dissolved organic nitrogen (DON) in the sea to be remnants of an organic molecule called peptidoglycan derived from bacterial cell walls. The finding suggested that predation on bacteria, and thus their removal as contributor of DON, may be an important control on the long-term cycling of nutrient organic nitrogen in the sea. A U.K. study demonstrated that the planktonic copepod Pleuromamma experiences a significant lowering of nitrogen content between dawn and dusk, the period when this minute crustacean migrates downward in the sea and then back to the surface. Quantification of such losses by defecation and excretion, which at depth release particulate organic nitrogen and dissolved nitrogen, should further increase scientists' understanding of nitrogen fluxes and so enhance models that describe nutrient flows in oceanic systems.

      Molecular evidence demonstrated that the nine species of land crabs of the family Grapsidae found in Jamaica derive from a common marine ancestor that invaded terrestrial habitats only four million years ago. On an evolutionary time scale, this finding indicates a remarkably rapid diversification and specialization. A Canadian study of juveniles of the whelk Nucella emarginata assessed changes that the marine snail undergoes during development in its vulnerability to desiccation, susceptibility to predators, habitat distribution, and coloration. The study found marked changes in all four factors when juvenile whelks reach a shell length of 8 mm (0.3 in). This length demarcated a second "ecological shift," occurring later in development than the better-understood lifestyle changes that take place at metamorphosis from larva to juvenile. A joint Malaysian and Japanese study answered the question of how the mudskipper fish (Periophthalmidae schlosseri) and its eggs survive reduced oxygen conditions in what had been assumed to be water-filled burrows on tropical intertidal mudflats. The investigators observed fish on the surface gulping air into their mouths and then releasing it within the burrow to form an air store under the roof of the burrow, where developing eggs were situated.

      Using sophisticated techniques for observing inside feeding oysters as they draw in water and filter the suspended particles, U.S. researchers showed that the oysters actively select living particles for ingestion and reject nonliving particles, evidently in response to chemical and particle-surface cues. Even greater selectivity was demonstrated by Italian workers who showed that the mussel Mytilus galloprovincialis feeds selectively on living dinoflagellates rather than diatoms, with a particular preference for the toxin-producing dinoflagellate Dinophysis, the main causative agent of diarrhetic shellfish poisoning in humans in the Gulf of Trieste region of Italy.

      Global fisheries statistics from the UN Food and Agriculture Organization for 1950-94 revealed a marked change in the composition of catches over the period, attributed to overfishing. Initial catches were predominantly of long-lived, fish-feeding, bottom-living fish positioned high in the food web, but recent catches were dominated by shorter-lived invertebrates and plankton-feeding, open-sea-dwelling fish located lower in the web. The changes indicated progressively increased fishing of organisms lower down the ocean food webs, a trend considered to be unsustainable. Urgent action by fisheries' managers was recommended to protect world marine fish stocks and the food webs in which they are embedded.

      A German study highlighted a continuing decline in the numbers of coelacanth fish in the Comoro Archipelago in the western Indian Ocean. The need to prevent exploitation of this "living fossil," in the wider context of biodiversity conservation, was presented as a test case to measure the success or failure of "eco-ethics," as recently defined and called for by international ecologists.

ERNEST NAYLOR

BOTANY

      Breakthroughs in genetic engineering continued at a staggering pace in 1998. For the first time, plants were engineered with a gene from a fungus to provide them with strong resistance to fungal diseases. Even more remarkable, potatoes were genetically engineered with a vaccine against Escherichia coli disease bacteria; this achievement heralded the dawn of edible vaccines produced and delivered by plants. Scientists at the University of Maryland School of Medicine, Baltimore, tested volunteers who, after eating the treated potatoes, achieved immunity levels similar to those gained by people who underwent ordinary vaccinations by needle. Scientists were expected to place vaccines into other widely eaten foods.

      Although much progress was being made in genetic engineering, once a foreign gene had been inserted into a crop plant, it was difficult to turn it on or off safely, and the process wasted much of the plant's energy. The only effective method was to use commanding genes called promoters, but they were usually activated only by applying toxic chemicals. Recently, however, researchers devised a way to turn on promoters by using an interesting substance—alcohol. Spraying crops with alcohol could be the first safe way for farmers to switch on genes, and the levels of alcohol would be far too low for anyone to become intoxicated.

      The genetic engineering of plants was, however, becoming increasingly controversial. Fears for the safety of food derived from genetically modified crops led some protesters to dig up fields of test plants in Great Britain in illegal acts of sabotage. Concerns were also raised about the transfer of genes from modified crops to weeds; in a laboratory experiment weeds became resistant to herbicides when they acquired a gene for herbicide resistance from neighbouring genetically modified crops.

      Two separate studies revealed that plants share with animals the same sort of defenses against diseases. A team from Rutgers University, New Brunswick, N.J., showed that tobacco plants infected with a disease virus use nitric oxide to turn on special genes that attack the virus. A group at the Salk Institute, La Jolla, Calif., found that nitric oxide also plays a vital part in the hypersensitive disease response, whereby infected plant cells commit suicide in order to destroy pockets of disease before the entire plant is afflicted. The nitric oxide sets off a series of biochemical commands uncannily like that sparked off in mammals' white blood cells when they attack invading bacteria—strong evidence regarding the ancient origins of this form of disease immunity.

      Additional evidence of the ancient links between plants and animals was uncovered in hormones and their receptors. G protein-coupled receptors, or GPCRs, had been found in mammals, but Richard Hooley and his colleagues at the Institute for Arable Crops Research, near Bristol, Eng., were amazed to find a counterpart of the mammalian gene for GPCRs in cress plants. The plant receptor seems to recognize an important group of plant hormones called cytokinins, which are involved in leaf, flower, and fruit growth and development. This discovery could have a major impact on agriculture by genetically improving crop yields and food quality.

      The growth of plants also seems to be influenced by Earth's spin. Some conifer trees twist their growth in opposite directions in the Northern and Southern hemispheres, a mystery that may have been solved by Norwegian foresters. They noted that conifers tend to grow in the belt of prevailing westerly winds from latitudes 30° to 60° N and S. When the west winds buffet the trees, their trunks are stressed and the wood twists to compensate. In addition, trees grow more leaves toward the sunny side, which also helps explain the opposite twisting of some conifers in the Northern and Southern hemispheres.

      An alarming report confirmed the high rate of plants headed for extinction. In the first fully worldwide survey, the World Conservation Union published results showing that one-eighth of the world's plant species—nearly 34,000 of an estimated 270,000 total species—were now threatened. Even worse, this figure may have underestimated the problem because many areas of the world, such as Brazil and central Africa, were difficult to survey. Of the species on the so-called Red List, 91% were endemic to just one country; those species growing on isolated islands, where they were often at the mercy of foreign plants and animals introduced by human settlers, were particularly at risk. Kerry Walter, one of the report's authors, expressed the hope that the Red List would "wake people up to the fact that we spend very little on conserving plants, yet there are many more threatened plants than threatened animals." For every dollar spent on animal conservation, only a dime was devoted to plants.

PAUL SIMONS

Molecular Biology

A Promising Cancer Therapy.

      The sprouting and growth of new blood vessels is essential during embryonic development so that developing tissues can be supplied with oxygen, nutrients, and waste-disposal services provided by blood flow. At the same time, blood-vessel growth, or angiogenesis, must be limited so that an inordinate fraction of the mass of the organs will not be devoted to blood vessels. It follows that angiogenesis must be under the control of both natural stimulators and inhibitors such that the balance between them produces the proper degree of vascularity.

      This same reasoning applies to the growth of a tumour as well as to the growth of an embryo. A solid cancer, or tumour, derives from a single cell that has mutated in a way that permits it to escape from the biochemical controls that limit the multiplication of normal cells. Once that cell fails to respond normally to growth inhibitors, it starts to proliferate. When the growing tumour reaches a diameter of about two millimetres (less than one-tenth of an inch), however, simple diffusion in and out of the tumour tissue no longer suffices to supply oxygen and nutrients and remove waste. Further growth depends on angiogenesis, and the small tumour must produce factors that stimulate the ingrowth of blood vessels.

      In the early 1960s such considerations led Judah Folkman (see BIOGRAPHIES (Folkman, Judah )), then a U.S. Navy surgeon, to begin a search for angiogenic factors, a task he subsequently continued at Harvard University. An assay was essential to allow the detection of these factors and then to guide their purification, and over the years Folkman and his collaborators devised two assays that used living animal tissues to test the ability of a given substance to stimulate blood vessel growth.

      Painstaking work over several decades resulted in the isolation of not one but several angiogenic factors, including angiogenin, vascular endothelial growth factor, vascular permeability factor, and basic fibroblast g

      rowth factor. Once these were available, it was easier to search for inhibitors of angiogenesis. That such inhibitors existed was surmised from the ability of a primary solid tumour to inhibit the growth of small offspring, or metastatic, tumours. During the past few years, a number of antiangiogenesis compounds were identified, and by 1998 some of them had been given clinical trials, the goal being a generally applicable treatment for solid cancers. Moreover, because factors that stimulate the growth of cells must bind to specific molecular receptors on the cell surface in order to function, a compound that can block those receptors will prevent the action of the growth stimulators. Several such blockers, or antagonists, of angiogenic factors were also under study.

      During the year two recently isolated natural inhibitors of angiogenesis, called angiostatin and endostatin, were attracting particular attention. Folkman and his collaborators at Harvard showed that angiostatin given to mice prevented the growth of carcinoma in the lung. In a second approach they used genetic means in mice to cause their cells to overproduce angiostatin, which in turn resulted in long-lasting suppression of fibrosarcoma, ordinarily a fast-growing cancer. Importantly, there was no indication that the cancers could develop resistance to angiostatin. Researchers looked forward to conducting clinical trials of angiostatin and endostatin in cancer patients in the next year or two and, if these proved positive, to the widespread availability of this highly promising treatment.

Antifreeze Proteins.

      Certain species of fish routinely live in seawater cold enough to freeze their blood. Ocean water does not freeze at such temperatures because of its high salt concentration, but the fish blood has only a third the salinity of seawater. Why does it not freeze?

      The answer lies in antifreeze proteins present in the fish blood. It is well known that highly purified water can be cooled below its freezing point (0° C, or 32° F) without freezing. If one adds the smallest crystal of ice to such supercooled water, it rapidly freezes. Water ordinarily freezes at 0° C because it contains minute particles that initiate, or nucleate, the growth of ice crystals. The antifreeze proteins bind to ice crystals in the blood while they are still microscopic in size and prevent their further growth. In work extending back to the 1960s, scientists identified several types of antifreeze proteins from fish and determined their structures. Although all share the ability to bind to ice crystals, comparative study of their amino-acid sequences carried out in the past two years indicated that they can be grouped into four distinct families. It thus appeared that these antifreeze proteins, which have similar ice-binding functions and mechanisms, have independent evolutionary origins.

Silver Bullets for Parasitic Protozoans.

      Organisms that live in environments that are rich in some biologically essential compound can, through evolution, lose the ability to synthesize that compound themselves. For example, parasitic protozoans, including some that are important agents of human diseases, have lost the ability to synthesize purines, because they can obtain these essential organic compounds from their hosts. The enzyme, or protein catalyst, that the protozoans use to salvage purines from the host is named hypoxanthine/guanine phosphoribosyl transferase (HGPRTase). Mammals also use a form of HGPRTase but are not dependent on it, since their own cells can synthesize purines. Moreover, the protozoan enzyme differs from the mammalian one in specificity, which thus raises the possibility that a compound could be found to inhibit the protozoan HGPRTase but not the mammalian enzyme. Such a compound would constitute a specific poison, or "silver bullet," for the parasitic protozoans, without harming the human host.

      The first step in this search was the determination of the three-dimensional structures of the protozoan and mammalian HGPRTases by X-ray crystallography. Next, computer-graphics methods were used to screen the molecular structures of known compounds for those specifically complementary to the active site of the protozoan HGPRTase. Compounds selected in this way were then evaluated in test-tube experiments for their abilities to inhibit the protozoan enzyme, and the best of these were then tested in infected animals. During the year researchers reported the results of this search: compounds that inhibit the HGPRTase from Tritrichomonas foetus, a protozoan parasite of cattle, 100 times more strongly than they inhibit the mammalian enzyme. The researchers' success offered hope that effective treatments for such protozoal diseases as sleeping sickness, leishmaniasis, and Chagas' disease, which afflicted millions of persons worldwide, would soon be developed.

IRWIN FRIDOVICH

The Genetics of Human Behaviour.

      One of the most complex and interesting of human characteristics is behaviour. Like many other characteristics, such as height or weight, behaviour has come to be understood to reflect a combination of influences, some genetic, others environmental. In recent years advances in a number of techniques have allowed researchers new and provocative glimpses into the genetic basis of human behaviour. As a result, a Pandora's box has been opened, spilling questions that by 1998 were cutting right to the heart of individual human identity and behaviour and the forces that control human destinies.

      Despite its intrinsic interest, the genetic basis of human behaviour had until recently proved extremely difficult to study, as neither human genes nor the environment could be intentionally manipulated, for obvious ethical reasons. Studies aimed at dissecting the "nature or nurture" issues of human behaviour, therefore, had relied on quantitative assessments of correlation—between relatives; between biological, versus social, family members in adoption studies; and between identical and fraternal twins. Although these approaches could reveal the presence or absence of a heritable genetic component for a given behavioral trait, they provided little or no information about the actual gene or genes involved.

      For example, it is undeniable that schizophrenia runs in families, with the children of schizophrenic parents demonstrating 13 times the risk of the general population for becoming schizophrenic themselves. How much of this increased risk, however, reflects genetic predisposition rather than the result of abnormal parenting? In a classic adoption study reported in the 1960s, investigators examined 97 offspring that were all given up for adoption at birth, one group (47) born to mothers with schizophrenia, the others (50) not. Of the 47 offspring of schizophrenic mothers, 5 were eventually diagnosed with schizophrenia, compared with none of the offspring born to mothers without schizophrenia. Indeed, the apparent risk (about 11%) of developing schizophrenia for the adopted offspring of schizophrenic mothers was statistically indistinguishable from the risk (about 13%) for offspring raised by biological schizophrenic mothers.

      Subsequent evidence for a genetic component of schizophrenia came from twin studies in which the risk for schizophrenia in identical (one-egg) twins, whose genomes are identical, was compared with that for fraternal (two-egg) twins, who have no more genes in common (about half on average) than nontwin siblings. Of the sets of identical twins studied, if one twin was schizophrenic, the other had a 45% risk of also being schizophrenic. In contrast, of the fraternal twins, if one twin was schizophrenic, the other twin had only about a 15% risk of being so. Doubling the difference between these two values gives a statistical value called heritability, which for a given trait roughly describes how much of the variance seen in a population can be attributed to genetic influences. For schizophrenia, heritability is about 60%. Although the exact nature or identity of the relevant genes remained unclear from these studies, the conclusion that genetics contributes to schizophrenia was compelling.

      Equally compelling, however, was the evidence from these same studies that genetics alone does not fully account for behaviour. After all, even for the genetically identical twins in the schizophrenia study, the second twin had a little less than a one-in-two chance of being schizophrenic like the first twin. Environment accounted for at least half of the nature-nurture pie. A better understanding of these nurture factors, therefore, appeared to offer the most hope for those seeking to treat or prevent undesirable behavioral outcomes in genetically "at-risk" individuals.

      The power of these kinds of quantitative studies to explore the genetic basis of human behaviour was given a significant boost by three recently developed methodologies. One, called developmental genetic analysis, monitors change in genetic effects over a course of development, such as part or all of the human life span. For example, in research on general intelligence, many studies that did not follow their subjects over a long time (and that often involved young children) had estimated heritability at 40-50%. More recent studies that incorporated developmental genetic analysis, however, indicated that genetic contributions to intelligence become increasingly important throughout the life span, reaching heritabilities as high as 80% later in life.

      A second quantitative advance, called multivariate genetic analysis, measures the genetic contributions to two or more traits as they vary together, rather than to individual traits. For example, with regard to human cognitive abilities, studies involving multivariate analysis demonstrated that genetic influences on all specific cognitive abilities (e.g., memory, spatial reasoning, and processing speed) overlap markedly, which suggests that the same genes associated with one cognitive ability also influence others. Multivariate analysis studies also indicated that genetic contributions to scholastic achievement overlap completely with genetic contributions to general cognitive ability.

      A third methodology, called extremes analysis, attempts to examine the genetic links between normal and abnormal behaviour. Specifically, this approach tests the hypothesis that if many different genes contribute to the genetic basis of behaviour, as seems likely, a given behavioral disorder may represent the extreme of a continuous dimension of genetic and environmental variability. The latest studies employing this technique to examine depressive symptoms, phobias, and reading disability, some of which were published during the year, seemed to support this hypothesis.

      Once quantitative methods have identified behavioral traits, such as schizophrenia, that demonstrate a strong genetic component, the next step generally has been to identify and clone the gene or genes responsible. Although the potential benefits of having these genes in hand are great, not only for understanding normal behaviour but also for the diagnosis, prognosis, and treatment of abnormal behaviour, finding the correct genes can be extremely difficult. For traits that reflect principally the effects of one gene, identification of the gene usually has yielded to standard linkage approaches that track correlations between the inheritance of a given trait and the inheritance of specific regions of DNA. With few exceptions, however, most human behavioral traits appear to reflect the combined influences of many genes, which makes the standard approaches useless.

      Fortunately, methods to identify candidate gene locations for so-called complex traits underwent major improvements during the 1990s. For example, so-called nonparametric approaches became available; these do not rely on traditional parameters, or assumptions, but instead track correlations among family members who share a given trait and also share specific regions of DNA. These and other methods, combined with continuing improvements in the available genetic and physical maps of the human genome, were expected to result in the identification and cloning of genes associated with a variety of human behaviours in the near future. Indeed, in the mid-1990s each of four different research groups implicated the same genetic locus, on the short arm of chromosome 6, in the cause of schizophrenia.

      Perhaps one of the best measures of the fabric of a society is not how quickly new knowledge is uncovered but how it is used. Recent and future advances into the genetic basis of human behaviour were likely to test that fabric. By 1998 investigators had already reported evidence for strong genetic contributions to personality, vocational interests, alcoholism, and even sexual orientation. Yet another report used data collected from studies of identical twins reared apart to conclude that behavioral traits such as aggression, morality, and intelligence are substantially determined by genes. A major challenge for society will be to find ways to use this new genetic information to empower, rather than enslave, the individuals who might benefit from it.

Mammalian Cloning.

      In the year since Dolly the lamb ignited furor as the first mammal cloned from the DNA of a differentiated adult cell, the technique of mammalian cloning marched on. While scientists, politicians, religious leaders, and others debated ethics and possibilities, Dolly was joined by cloned mice, cloned calves, and another sheep that was not only cloned but also engineered with a human gene to produce blood-clotting factor IX in her milk. The clone with the added gene, in particular, illustrated that practical applications of the technology were already under way.

      Perhaps the most extraordinary application cited to date was revealed in July when scientists from China's Academy of Sciences announced a project to clone their endangered national symbol, the giant panda, by 2003. The proposed plan involved transfer of the cell nucleus of an adult giant panda into the enucleated egg of another species, perhaps the black bear. The hybrid egg would then be implanted into the uterus of a foster mother bear. Whether such transspecies cloning could actually work was the subject of considerable debate, but if it did, giant pandas would be only the first animals to benefit.

JUDITH L. FRIDOVICH-KEIL

Paleontology

      In the field of vertebrate paleontology in 1998, scientists described several significant discoveries from Madagascar. New evidence supporting the theory that birds evolved from theropod dinosaurs came from the remains of a raven-sized primitive Late Cretaceous bird, Rahona ostromi, found on that island. (The Cretaceous Period lasted from 144 million to 66.4 million years ago.) Other fossils from Madagascar—crocodiles, mammals, and dinosaurs, including one of the best-preserved and most complete dinosaur skulls known—suggested that a geographic link had been maintained to South America, perhaps through Antarctica, until late in the Late Cretaceous. Previously, it had been thought that connections between the southern landmasses emerging from the breakup of the supercontinent Gondwana had been severed by that time. (See Sidebar (Feathered Dinosaurs and Fractured Supercontinents ).)

      Investigators reported from the Early Cretaceous of Australia a partial jaw of a shrew-sized mammal with tribosphenic molars—the kind of mammal from which both placental and marsupial mammals were thought to have evolved. About 115 million years old, the specimen was interpreted to be similar to jaws of primitive placental mammals from parts of Asia and North America. Previous to this discovery, paleontologists had assumed that both placental and marsupial mammals evolved in the Northern Hemisphere but that only the latter reached Australia near the end of the Cretaceous and thus attained dominance in the mammalian species of that continent. The discovery suggested that primitive placentals may have reached Australia from the north through South America and Antarctica much earlier in the Cretaceous. If true, these early placentals then became extinct in Australia before they could give rise to more advanced groups, as they did in other parts of the world.

      Researchers at the University of Chicago published the results of a study on the origin of mammals, concluding that, unlike the origin of most other higher animal groups, the evolution of mammals was not linked to a major morphological change (such as the acquisition of feathers and wings in the hypothesized transition from theropod dinosaurs to birds). Instead, mammals evolved as a result of the gradual acquisition of a series of mammalian characteristics.

      A new fossil whale genus and species described from the Late Eocene Period (43.6 to 36.6 million years ago) of Georgia was found in association with shallow-dwelling marine mollusks and plankton. This animal, named Georgiacetus vogtlensis, was the oldest known whale that did not have the pelvis articulated with (attached to) the vertebral column. The detached pelvis is an important feature that evolved in whales to better adapt them to a fully marine habitat.

      The first record of fossilized amphibian eggs was reported during the year. The small oval-shaped eggs, 0.8 mm (0.03 in) in diameter, were discovered with fossil plants, invertebrates, and vertebrates in the Waggoner Ranch Formation of Texas. Because the eggs resembled those of modern amphibians and were of Early Permian age (286 million to 258 million years ago), it appeared likely that they were laid by dissorophoid amphibians, which belong to the order Temnospondyli. Dissorophoids were known from the Early Permian and were thought to be closely related to living amphibians.

      A report of a fish-mass-mortality fossil bed discovered at the Cretaceous-Tertiary boundary (66.4 million years ago) on Seymour Island off Antarctica suggested that the fish may have been killed by the same asteroid impact event that had been proposed for the extinction of the dinosaurs. According to the report, however, the absence of ammonites (extinct cephalopods common in Cretaceous rocks) in the fossil-fish layer and the older, underlying clay layer indicated that other environmental factors already under way may have led to changing global biotic conditions near the end of the Cretaceous prior to the impact. It suggested that, if this was true, the impact was just the final blow to the remaining members of an ecosystem already depleted of life forms by other environmental factors.

      Studies of microfossils were an important part of a major new project in the Antarctic. The Cape Roberts Project aimed to core 1,500 m (4,900 ft) of subsea sediment as much as 100 million years old near the coast of Antarctica over a period of several years. Scientists expected the study to improve their understanding of the geologic history of Antarctica prior to 40 million years ago through the use of marine microfossils, including foraminiferans, diatoms, calcareous nannofossils, and palynomorphs (fossil pollens). This project would also attempt to determine when permanent ice sheets first formed in the Antarctic.

      Researchers reported on their investigations of an occurrence of unusually detailed microfossils preserved in chert from the Doushantuo Formation of China. These prokaryotes and protists, about 550 million to 600 million years old, provided a picture of biological diversity in the oceans just prior to the rapid diversification and specialization of marine organisms observed in the Cambrian Period (540 million to 505 million years ago). The presence of 12 species of cyanobacteria (prokaryotic photosynthesizers, also called blue-green algae), 31 species of acritarchs (eukaryotic algae), 8 species of multicellular algae, and compressed macrofossils of more than two dozen species of invertebrate animals indicated a much higher level of diversity for this early period than had been previously documented. The scientists also described microfossils interpreted as multicellular-animal embryos in various stages of division.

      Notable advances in an understanding of fossil invertebrates included a major revision of the Athyridia order of brachiopods and a report on the discovery of brood pouches in trilobites of Cambrian and Ordovician age. (The Cambrian and Ordovician periods together cover from 505 million to 438 million years ago.) The later report proposed that large bulb-shaped structures on the head of these trilobites were used for sheltering larval trilobites to reduce the rate of larval mortality. The presence of the pouches in only some of the trilobites of a species suggested that they were an exclusive characteristic of females. This was the first good evidence that trilobites may have been sexually dimorphic—i.e., that males and females may have differed in body form.

      In paleobotany, studies of 400 million-year-old Rhynie Chert of the Early Devonian of Scotland provided the first conclusive evidence of lichens (cyanobacteria living symbiotically with fungus) in the fossil record. Researchers reported finding the fossil leaves, stems, and fruits of an angiosperm from the Upper Jurassic of China, the oldest known evidence for flowering plants.

WILLIAM R. HAMMER

      See also Anthropology and Archaeology ; The Environment (Environment ).

▪ 1998

Introduction

Zoology

      Advances in zoology were made during 1997 in understanding primate behaviour and the evolutionary relationship between wolves and dogs. Two independent long-term field experiments, one with lizards and one with fish, provided evidence suggesting that animals that have been introduced to new environmental situations can evolve rapidly in the wild in response to natural selection.

      Linear dominance hierarchies were known to exist among females in the social communities of some primates, such as macaques and baboons, but had not been unequivocally observed in chimpanzees. Because female dominance had been seldom observed in chimpanzee groups, especially within stable groups in the wild, many researchers did not consider the dominance rank of a female to be of particular importance to her reproductive success. To investigate the issue of female dominance in chimpanzees, Anne Pusey and Jennifer Williams of the University of Minnesota and Jane Goodall of the Jane Goodall Institute, Ridgefield, Conn., used data from 35 years of observations of a group of chimpanzees in Gombe National Park, Tanzania. The investigators were able to assess dominance relationships by analyzing "pant-grunt" responses recorded among females in the group from 1970 to 1992. A pant-grunt is accepted as an indicator of submissiveness by one chimpanzee in response to the aggressive behaviour of another. Most of the 10-18 female chimpanzees observed in the group since 1970 were able to be placed in a dominance hierarchy of high, middle, or low. When the investigators eliminated from their analyses one clearly dominant but sterile female that had been part of the group for 28 years, a dominance pattern emerged that correlated with reproductive success. A higher-ranking female was more likely to live longer, produce young more often, have a higher infant-survival rate, and have daughters that matured at an earlier age than females of lower ranking. The investigators attributed the enhanced reproductive success of higher-ranking females to better nutritional status as a consequence of acquiring more suitable areas for foraging.

      Carles Vilà and Robert K. Wayne of the University of California, Los Angeles, and colleagues used molecular genetics techniques to conclude that the wolf (Canis lupus) is the one and only wild ancestor of the domestic dog (C. familiaris). The investigators analyzed specific sequences of mitochondrial DNA that had been sampled from 162 wolves worldwide (27 localities) and from 140 dogs (67 breeds). (Mitochondria are cell organelles that contain their own genetic material, distinct from that of the cell nucleus.) They also examined corresponding sequences taken from all other wild species of the genus Canis (coyotes and three species of jackals). Dogs were found to be significantly more similar genetically to wolves than to coyotes or jackals. As observed in comparisons of fossils, wolves were distinct morphologically (i.e., in form and structure) from coyotes about a million years ago. Using molecular clock techniques to time the divergence between the species, the investigators calculated that domesticated dogs were distinct genetically from wolves as far back as 135,000 years ago. Archaeological evidence had previously suggested that dogs originated about 14,000 years ago. One interpretation of the disparity in dates of dog origin is that dogs did not become morphologically distinct from wolves until humans developed agricultural societies 10,000-15,000 years ago, even though they had become genetically distinct earlier. Hence, dog fossils found associated with preagricultural human populations would not have been distinguishable from those of wolves.

      To study the speed of evolution in a species, David N. Reznick of the University of California, Riverside, and colleagues carried out experiments on the effects of predation on natural populations of guppies in Trinidad. The investigators initially selected two streams with waterfalls. The stream sections below the waterfalls contained guppies and were determined to be high-predation habitats, whereas those sections above the waterfalls had neither guppies nor many predators because the falls served to exclude both. Guppies were then experimentally introduced to the sections above the waterfalls in both streams. Comparisons of life-history traits of the below-falls, or control, guppy populations and the above-falls, or experimental, populations were made at 4 and 7.5 years for one stream system and at 11 years for the other. After four years, i.e., after only about seven generations, the experimental males above the waterfall were seen to mature sexually at older ages and to have larger body sizes than control males. (The predators in the guppies' original habitats preferred large, sexually mature prey, which thus put selective pressure on the guppies to mature at an early age.) After 11 years both sexes in the experimental population matured later and at larger sizes than in the high-predator sites. The rapid adaptive responses to a changed environment were evaluated in the laboratory and found to have a genetic basis. Moreover, these adaptations and other traits identified in the experimental populations were the same traits found in guppies living in naturally occurring low-predation habitats and were consistent with results derived from mathematical theories of life-history evolution, which had predicted how organisms should evolve in response to external sources of mortality.

      An experiment with the lizard Anolis sagrei on islands in the Bahamas by Jonathan B. Losos of Washington University, St. Louis, Mo., and colleagues demonstrated rapid changes in morphology in response to changed environmental conditions. In 1977 and 1981 lizards were collected on relatively heavily vegetated Staniel Cay and released onto 14 nearby islands that were much smaller, had few trees, and were covered primarily by vegetation with narrow-diameter stems and branches. Previous studies of the more than 150 Anolis species in the Caribbean had revealed a positive relationship between hind-limb length and mean diameter of vegetation perches. Earlier studies also had indicated that long-legged species maximize sprinting ability whereas short-legged species are better able to maintain a grip on narrow surfaces. A comparison in 1991 of hind-limb measurements of adult male lizards on Staniel Cay and from the small islands still supporting the introduced populations demonstrated that lizard morphology had diverged in response to the magnitude of difference between a small island's vegetation and that on Staniel Cay. If the differences observed in the experimental populations of guppies and lizards were inherited genetically and brought about by natural selection, then the studies would support the conclusion that evolution in both life-history and morphological traits can occur rapidly in response to abrupt changes in environmental conditions.

      In the area of conservation ecology, investigations of the semiaquatic chicken turtle (Deirochelys reticularia) in the southeastern U.S. uncovered information suggesting that humans' traditional patterns of land use can endanger the survival of species whose evolved traits are poorly understood. Kurt Buhlmann of the University of Georgia's Savannah River Ecology Laboratory reported on the ecology of chicken turtles, which differ from most North American turtles by nesting in autumn and winter instead of spring and summer. During a four-year study the investigator documented that chicken turtles hibernate underground on land and thus spend more than half their life in the terrestrial habitat. He also found that when chicken turtle eggs are laid in the fall, as long as 20 months may elapse before the young leave the nest, enter adjacent wetland areas, and begin feeding. The dependency of this unusual species on both the aquatic habitat and the peripheral terrestrial habitat reinforces the conviction of some ecologists that large terrestrial buffer zones around wetlands are critical to the survival of some wetland species and need to be accommodated in land-development projects.

      A study of fossils from the late Precambrian in northern Russia by Mikhail A. Fedonkin of the Russian Academy of Sciences, Moscow, and Benjamin M. Waggoner of the University of California, Berkeley, revealed that large triploblastic organisms (those having three primary embryonic layers) existed and began to diversify before the start of the Cambrian Period, which began about 540 million years ago. When first discovered in the 1950s, Kimberella quadrata was thought to be a jellyfish. Recent discovery by the investigators of abundant, well-preserved fossils of the species, however, allowed them to reinterpret the earlier findings. Kimberella was actually a bilaterally symmetrical, bottom-dwelling multicellular animal that resembled a mollusk. The finding suggested an earlier origin for some higher groups of animals than previously suspected. Meanwhile, David Jablonski of the University of Chicago used fossil mollusks from about 81 million to 66.4 million years ago, near the end of the Cretaceous Period, to test the evolutionary generalization known as Cope's rule, which presupposes that evolutionary lineages will tend toward larger body sizes because of their survival and reproductive advantages. In examining 1,086 species representing 191 identifiable lineages of bivalve and gastropod mollusks, the investigator observed that directional increases in body size within a lineage occurred no more frequently than decreases or expansions in the upper and lower limits of the size; thus, Cope's rule was not supported.

J. WHITFIELD GIBBONS

Entomology.

      The discovery of the chemical substance chitin in fossil beetles in Enspel, Ger., in Oligocene shales deposited 24.7 million years ago greatly extended the known time of its persistence in fossil animals. A horny material that is chemically a polysaccharide (complex sugar), chitin is abundant in the bodies of living arthropods but had not been detected in organisms fossilized more than about 130,000 years ago. B. Artur Stankiewicz and Derek E.G. Briggs of the University of Bristol, Eng., and colleagues used analytic pyrolysis (heating) techniques and scanning electron microscopy to document the presence of chitin in the insect fossils. The findings suggested that preservation of chitin is regulated not by time per se but by the chemical nature of the environment in which fossilization occurs. The authors concluded that the chitin was preserved as the result of biochemical and geochemical factors on the lake bottom that was the source of the shale.

      New insight was provided into the previously recognized mutualism between ants and acacia trees, in which ants defend the trees from herbivorous insects and other animals while the trees provide food and shelter for the ants. Because the flowers of Acacia zanzibarica and A. drepanolobium in Africa are pollinated by insects other than ants, P.G. Willmer of the University of St. Andrews, Scot., and G.N. Stone of the University of Oxford sought to determine how such pollination is achieved when the trees are guarded by ants. The acacia trees that served in the study were pollinated mainly by solitary bees during the midday period. The investigators noted that the ants protected the flowers from insects during early development but avoided young flowers once they had matured to a stage suitable for pollination. Then, as the flowers aged and began producing seeds, the guarding ants returned. The researchers hypothesized that new flowers produce a chemical that acts as an ant deterrent; such a substance would allow the bees to pollinate the flowers without being attacked by guarding ants. To test the hypothesis, they wiped old flowers with new flowers. The ants, normally present around old flowers, avoided those that had been wiped with new flowers—a behaviour that supported the idea of a chemical deterrent.

      Sanford D. Porter of the Center for Medical, Agricultural, and Veterinary Entomology, Gainesville, Fla., and colleagues provided support for the position that the success of the imported fire ant (Solenopsis invicta) in North America since its introduction in the early 20th century resulted from the absence of many natural enemies found in its native South American habitat. The investigators examined ant mounds and colonies in the spring and fall in 13 regions in South America and 12 in North America. The areas sampled on each continent, primarily roadsides and grazing sites, included different climatic conditions. Sizes of fire ant colonies were found to be larger, mound densities higher, and ant abundances four to seven times greater in North America than in South America. Factors including climate, habitat type, seasonal variability, and ant population structure did not appear to explain the observed differences between the two continents, which bolstered the idea that natural predators, parasites, and competitors control the species in South America. Confirmation that fire ants' success in North America is primarily a consequence of escape from natural enemies was an important objective when biological control of this exotic pest was considered.

ANNE R. GIBBONS

      This article updates insect1 (insect).

Ornithology.

      In a review of major significance published in 1997, Sharmila Choudhury of the Wildfowl and Wetlands Trust, Slimbridge, Eng., compared various hypotheses that had been advanced to explain "divorce" in birds. Most avian mating systems are monogamous; the key to understanding the circumstances under which divorce occurs lies in determining the costs and benefits of both pair fidelity and divorce. Individuals can be expected to divorce when the benefits outweigh the costs. Hypotheses included incompatibility, preference for a better-quality mate, accidental loss of mate, and intrusion of a third party.

      As part of a study by G.L. Kooyman and T.G. Kooyman of the Scripps Institution of Oceanography, La Jolla, Calif., adult emperor penguins in Antarctica were fitted with time-and-depth recorders to monitor their ocean dives while foraging. Most dives were found to be to depths of 20-40 m (65-130 ft) for times between four and five minutes. The deepest individual dive was 534 m (1,752 ft), and the longest was 15.8 minutes. The closely related king penguins dive similarly, but the breaths they snatch while briefly resurfacing are not enough to restore their oxygen fully. Yvon Le Maho of the Centre for Ecology and Physiology Energetics, Strasbourg, France, suggested that submerging king penguins cope by deliberately creating hypothermia. In depressing their core temperature, they reduce their oxygen need.

      Birds were known to have two complex navigation systems, one that relies on the position of the stars and another that uses the Earth's magnetic field. It had been thought that either system was adequate to guide migrating birds. Nevertheless, according to Wolfgang Wiltschko and co-workers of Johann Wolfgang Goethe University, Frankfurt am Main, Ger., garden warblers, at least, cannot navigate by the stars alone when flying south for the winter; they also need information from the Earth's magnetic field if they are to fly off on exactly the right heading. At the end of each summer, central Europe's garden warblers set off southwest to the Iberian peninsula, then south to Sierra Leone, and finally southeast toward South Africa. Although born with those instructions, the birds need an external reference system to lay in the correct flight path. The researchers raised two groups of warbler chicks to about six weeks of age. Both groups were exposed to an artificial sky with 16 fake stars rotating once per day to mimic the motion of real stars. While one group experienced the Earth's magnetic field, the other group was exposed to artificial fields, which canceled out the natural field. In August, at the onset of migratory restlessness, the birds' activity was recorded to determine the direction in which they intended to fly. Warblers that had been exposed to the stars and the Earth's magnetic field oriented themselves in the correct southwesterly direction. The other birds, however, prepared to set out wrongly, almost due south.

      The spectacled eider, a species of sea duck, was classified as threatened in 1993 after populations in western Alaska had declined more than 90% in 30 years because of unknown causes. The species spends the summer and breeds in the coastal tundra, but its wintering sites had been unknown. To discover where the eiders went in winter, about two dozen individuals were fitted with radio transmitters and tracked until the batteries became too weak to send strong signals. At that time the eiders were dispersed in the Bering Sea south of St. Lawrence Island, where the ocean had not yet frozen solid. Unexpectedly, after six months of inactivity a transmitter emitted a freak signal. U.S. Fish and Wildlife biologists Greg Balough and Bill Larned chartered a plane and flew in search of the source—300 km (190 mi) within the Arctic ice pack. They discovered first hundreds and then thousands of ducks jammed into tiny holes in the Bering Sea ice pack, which the birds kept open to the ocean by their own body warmth and movements. A rough count gave about 150,000 spectacled eiders, estimated to be at least half the total wintering population.

JEFFERY BOSWALL

      This article updates bird.

Marine Biology

      Concern over the enlargement of ozone holes—thinned regions of the Earth's protective stratospheric ozone layer—above the polar regions generated interest in the effects on marine organisms of the associated increase in solar ultraviolet (UV) radiation reaching the surface. To study such effects researchers cultured algae known as diatoms under six spectrally different light regimes near Palmer Research Station, Antarctica. Under conditions simulating daily exposure to ambient UV radiation, the diatoms showed a 34% reduction on average in carbon fixation (the organism's essential assimilation of carbon into organic compounds via photosynthesis).

      In the tropical seas a challenge was made to the common assumption that damaging UV wavelengths penetrate to considerable depths in oligotrophic (clear) waters, posing a potential threat to coral reefs. A U.K. study that made use of a semisubmersible scanning spectroradiometer at various sites around the central Indian Ocean and Andaman Sea demonstrated that damaging UV radiation attenuated very rapidly with depth, even in very clear waters around the Maldives.

      The International Year of the Reef was declared for 1997 to focus attention on the current and increasing plight of coral reefs, particularly the damage being caused by human activity. .) (Coral Reefs: The Forgotten Rain Forests of the Sea ) The bleaching and consequent death of corals following the disruption of the association with their pigmented symbiotic microorganisms (zooxanthellae) was one major concern, and the causes of bleaching were being actively sought. During the year the phenomenon was reported in the Mediterranean coral Oculina patagonica after infection of its zooxanthellae by a species of the bacterium Vibrio. Building on the recent discovery that corals can act as host for more than one species of zooxanthellae, another study showed that bleaching might be reversible, since some zooxanthellae are resistant to bacterial infection.

      Colonies of species of massive corals (notably Favites abdita, Montastrea curta, and M. annuligera) on reefs at Heron Island, northeastern Australia, were found to be regularly spaced. Studies revealed that they formed a structural matrix, each colony releasing a chemical that inhibited settlement and growth of neighbouring colonies within a certain distance. A novel method of artificial transplantation of corals was reported by German investigators, who inserted pieces of living coral into a steel mesh that was positioned at the new underwater site and made to function as the cathode in a electrolytic circuit. Passing direct current through seawater between the electrodes induced the accretion of calcium and magnesium minerals at the cathode and thereby generated in situ a new coral substrate having a limestone character. A unique feeding strategy was reported for a soft coral, Gersemia antarctica, which grows upright to a height of 1-2 m (3.3-6.6 m). Instead of feeding on suspended plankton, assumed to be the normal feeding mode, observed specimens flexed the upper body downward, which brought polyps into feeding contact with bottom sediment.

      Spread of the introduced tropical alga Caulerpa taxifolia into the western Mediterranean continued to cause concern along the coasts of France, Spain, and Italy. Reported at new record depths near 100 m, the alga was penetrating far deeper than expected for a photoautotrophic alga (one requiring light and using only inorganic compounds as nutrients), which suggested that it also employs heterotrophic metabolism—i.e., that it can live off organic compounds. A Spanish study reported that close proximity to Caulerpa inhibited the growth of native algae such as Cystoseira and Gracilaria. The inhibitor, called caulerpene, was found to be a secondary metabolite produced by Caulerpa, which also made the alga repellent to grazing marine animals and to colonization by epiphytes (plant species that rely on other plants for physical support). A grazing-activated chemical defense was reported for the first time in a single-celled planktonic alga, Emiliania huxleyi, when grazed by the protozoan Oxyrrhis marina. Feeding resulted in the production of dimethyl sulfide by means of an enzyme-mediated reaction. When experimentally offered algal cell mixtures, the protozoan selected algae showing low activity of the enzyme involved in the reaction.

      A Canadian study reported different daily patterns of vertical migration in populations of the veliger larval stage of the sea scallop Placopecten magellanicus. Each pattern favoured transport of the veligers by currents back to their particular parental scallop beds.

      Historical data on catch localities of the sperm whale (Physeter macrocephalus) in the 19th century were compared with contemporary satellite-derived data on the distribution of chlorophyll in the ocean, which can be interpreted as a measure of productivity. On large spatial scales the abundance of chlorophyll, measured by ocean colour, was found to be a good predictor of areas of ocean where sperm whales should be abundant. In a Ukrainian study humpback whales (Megaptera novaeangliae) of the Arabian Sea were reported to remain in the same area year-round. Having no northern outlet from the Arabian Sea, they did not migrate to high latitudes in summer for feeding, as did other Northern Hemisphere stocks of humpbacks.

ERNEST NAYLOR

      This article updates fish.

Botany

      In 1997 the genetic engineering of plants continued to make impressive contributions to the development of improved agricultural crops. The gene in baker's yeast that allows the cells to revive after being totally desiccated was introduced into tobacco plants; when the plants' leaves were cut and left to dry, they were still fresh a day later. The advance opened up a new way to protect crops from both severe drought and frost. Plants were also being engineered with greater tolerance of aluminum, the cause of a problem that afflicts 40% of arable land, mainly in the tropics, where acid soils release toxic aluminum ions into the groundwater. Tobacco plants were genetically altered such that their roots released citric acid, an organic acid that tied up aluminum ions in the soil, preventing the aluminum from entering and damaging the roots.

      The importance of engineering corn (maize) was highlighted when the U.S. Congress announced plans to analyze the entire genetic makeup, or genome, of the plant, the first crop plant designated to have all its genes mapped and DNA sequenced, in a $40 million project considered to be as significant as the Human Genome Project. The corn genome comprises three billion pairs of bases, the molecular building blocks of DNA, and 30,000 genes, which makes the task comparable in size to unraveling the human genome. By helping to unravel the genetic mysteries of corn, the project could help researchers engineer other major grain crops. The Japanese government pledged to map and sequence the rice genome, six times smaller than the corn genome.

      Making productive decisions about the genetic engineering of plants requires a thorough understanding of plant physiology. Biotechnologists had been eager to eliminate a process in plants called photorespiration, a side reaction of photosynthesis that seems to waste a plant's synthesized food by turning it back into carbon dioxide. Akiko Kozaki and Go Takeba of Kyoto (Japan) University, however, discovered that photorespiration actually protects plants from the harmful effects of strong light. Using genetically modified tobacco plants, they reported that the more a plant photorespires, the better it withstands high-intensity light.

      Because plants are rooted to one spot and unable to run from danger, they have evolved an immense array of self-defense systems against pests. Investigators took genes that had been discovered to give both wild beets and snowdrops the ability to repel nematode soil worms and introduced them into grapevines to protect their roots. Commercial spin-offs of the achievement could be considerable; currently, vines infected with nematodes were treated with methyl bromide, a fumigant that was scheduled to be banned in the U.S. in the year 2001.

      Since the early 1990s an astonishing airborne communication system between plants had been deciphered. Researchers learned that plants under attack by pests send out messages in the form of volatile compounds to their still-unassaulted neighbours that tell them to prepare their defenses against the insects. Work during the year by Vladimir Shulaev and colleagues of Rutgers University, New Brunswick, N.J., showed that the chemical message system extends to viral attacks. Plants infected with tobacco mosaic virus release methyl salicylate, better known as the fragrant oil of wintergreen, which switches on the defense mechanisms of nearby healthy plants.

      Work on plant defenses had also revealed that plants under attack from such insects as caterpillars release airborne insect repellants or broadcast chemical signals to predatory wasps, which attack the pests. Recently, a plant called molasses grass was discovered giving off such signals when unmolested. In field trials in Kenya during the year, molasses grass planted with corn and sorghum cut massive pest devastation to those crops by 95% and thus offered a promising alternative to chemical pesticides.

      Knowledge of the ways that plants and animals can cooperate advanced with the discovery, in mangrove trees in Belize, of the first known symbiosis between sponges and trees. Large sponges were found attached to the exposed roots of the trees, with both parties benefiting. Roots with attached sponges were almost four times the size of roots without sponges, and the attached sponges grew faster, perhaps by feeding off nutrients drawn up by the roots.

      Some root symbioses had enormous potential for improving crop yields. Legume plants are nourished by root-dwelling Rhizobium bacteria that take nitrogen from the air and turn it into nitrate compounds on which the roots feed. For decades a holy grail of crop-plant research had been to find a way to feed other crops in the same manner to boost their growth, and during the year plant scientists found such promise in rice. One group of researchers uncovered a species of Rhizobium growing symbiotically in rice plant roots, and a second group discovered previously unknown nitrogen-fixing bacteria of the genus Azoarcus that can colonize rice plants. The finds opened up enormous possibilities for reducing the amount of chemical fertilizers currently used in rice farming.

PAUL SIMONS

      This article updates plant.

Molecular Biology

Toward a Therapy for CGD.

      Chronic granulomatous disease (CGD) is an inherited loss of the ability to ward off infection by bacteria and fungi. Affected persons suffer a series of life-threatening infections to which they finally succumb.

      The seat of the problem in CGD is a subset of the white blood cells called phagocytes, which normally engulf and kill invading microorganisms. When they become activated, normal phagocytes dramatically increase their consumption of oxygen in a process called the respiratory burst. The increase is actually accomplished by a chain of chemical reactions, some catalyzed by enzymes (protein molecules that regulate specific reactions), that ultimately yield hypochlorite (OCl^-). Hypochlorite is the active ingredient of laundry bleach and is intensely lethal to the engulfed microorganisms. Phagocytes from people with CGD cannot mount a respiratory burst and are defective in their microbicidal activity.

      The first step in the respiratory burst is the activation of a membrane-associated enzyme called NADPH oxidase. The active enzyme requires the interaction of two proteins in the cell fluid, or cytosol, with two proteins in the cell membrane. A defect in any one of those four proteins disarms the respiratory burst. CGD can be caused by a mutation of any one of the four genes that code for the four components of the active NADPH oxidase. In fact, medical researchers have identified cases of CGD that are traceable to defects in each of the four genes.

      It should be possible to cure CGD by replacing the defective gene with a normal one. Investigators recently tested the validity of that approach, using cultured lymphocytes taken from a CGD patient. When DNA bearing a normal copy of the defective gene responsible for the CGD was introduced into the lymphocytes, the cells regained the ability to mount a respiratory burst. The next step would be to attempt this gene replacement therapy in the living body. A lasting cure would depend on genetic modification of the body's stem cells. Located in the bone marrow, the stem cells are the long-lived progenitors of the circulating phagocytes. Toward this end, researchers sought to develop an animal model of CGD so that the best therapeutic approach could be worked out prior to attempting it in humans.

      One way to create an animal model—for example, a mouse model—of a genetic disease is to eliminate the function of a specific gene. The method involves the introduction of a modified, dysfunctional form of the desired gene into cells that have been derived from an early-stage mouse embryo. Those cells in which the modified gene has successfully replaced the normal gene are injected into early mouse embryos, which are placed into the uterus of a mouse so that development can proceed. Those resultant mouse pups that express the modified gene are used to develop a breeding colony. In this way researchers produced mice that lacked one of the cytosolic components of the NADPH oxidase and whose phagocytes thus could not mount the respiratory burst. The mice exhibited the hallmarks of CGD, being extremely susceptible to infection.

      In 1997 the animal-model research was extended to humans when five patients with GCD were treated at the National Institutes of Health, Bethesda, Md., with their own stem cells into which functional genes had been introduced. In each case the outcome was encouraging, with the genetically engineered stem cells producing functionally normal white blood cells for an average of three months.

One Protein, Several Functions.

      Why are most enzymes in nature so much larger than their substrates—i.e., the molecules that they act upon? The question had long puzzled enzymologists, who thought that smaller catalysts would be more efficient at facilitating the many reactions that go on in cells. One answer is that many enzymes do much more than simply speed up a specific chemical reaction.

      An example of the multiple functions that a single protein can serve recently came to light. That protein is glyceraldehyde-3-phosphate dehydrogenase (GDH). It was first isolated in the 1930s as the enzyme that functions in cell metabolism to catalyze the oxidation of glyceraldehyde-3-phosphate (which possesses one phosphate group) in the presence of inorganic phosphate to yield 1,3-diphosphoglycerate (which possesses two phosphates). This reaction is particularly important in that it conserves the energy that is liberated during oxidation of the aldehyde group in the energy-requiring synthesis of a high-energy phosphate bond. An abundant enzyme, GDH plays a crucial role in the process by which the nutrient sugar glucose is converted in the cell to lactic acid, with concomitant production of high-energy phosphate bonds that are used to power cellular processes.

      In the 1990s, however, GDH was found to serve other, unrelated roles. One was the repair of defects in DNA that, if left unattended, would result in mutation. DNA normally contains the four nitrogenous bases adenine, thymine, guanine, and cytosine. It should not contain the base uracil, which is a normal component of RNA, but its cytosine base can slowly and spontaneously lose ammonia, or deaminate, and thus be converted to uracil. This instability is compensated by enzymes, collectively called uracil glycosylases, that remove uracil from DNA so that other enzymes can then replace it with cytosine. When the major uracil glycosylase was isolated from human cells and characterized, it proved to be identical to GDH.

      Yet another function served by GDH was found to be the transport of transfer RNA (tRNA) out of the cell nucleus. Molecules of tRNA are made in the nucleus but used in the cell cytoplasm (the protoplasm outside the nucleus) during protein synthesis. A carrier protein serves to conduct tRNA from the nucleus into the cytoplasm. When characterized, it too proved to be GDH. Moreover, the versatility of GDH is not exhausted by the foregoing functions. GDH was found to be one component of the complex structure required for the replication of DNA. It also proved to be one of the microtubule-associated proteins that regulate the assembly and function of this ubiquitous element of the cytoskeleton, the network of protein fibres that gives shape and support to the cell.

      These multiple functions of GDH should be reflected both in the regulation of GDH and in its location within the cell. The amount and the intracellular location of any protein can be assessed by the use of antibodies that have been prepared to bind specifically to the protein of interest and tagged with a fluorescent substance that stands out distinctly under the microscope. When researchers applied this technique to human cells in culture for visualization of GDH, they observed that nongrowing cells had GDH only in the cytoplasm, in keeping with its role in glucose metabolism and its binding to microtubules. By contrast, growing and dividing cells had GDH in both the nucleus and the cytoplasm, as predicted by its additional roles in tRNA transport, DNA repair, and DNA replication. Such functional versatility may well turn out to be a common feature of proteins. Given the potential for many of the approximately 50,000 different cellular proteins to perform multiple functions, the life of the cell may prove to be even more complicated than previously thought.

IRWIN FRIDOVICH

A Lamb Named Dolly.

      In 1997 cloning became a household term, thanks to Ian Wilmut and colleagues of the Roslin Institute, near Edinburgh, who reported in February the first successful cloning of an adult mammal. The centre of attention, a Finn Dorset ewe named Dolly, by her very existence dispelled decades of presumption that adult mammals could not be cloned and ignited a debate concerning the many possible uses and misuses of mammalian cloning technology.

      The concept of cloning in mammals, even in humans, was nothing new. Naturally occurring genetic clones, or individuals genetically identical to one another, had long been recognized in the form of monozygotic (identical) twins, triplets, and so on. Unlike Dolly, however, such clones are derived, as their scientific name indicates, from a single zygote, or fertilized egg. Moreover, clones had been generated previously in the laboratory, but only from embryonic cells or from the adult cells of plants and "lower" animals such as frogs. Decades of attempts to clone mammals from existing adults had met with repeated failure, which led to the presumption that something special and irreversible must happen to the DNA of mammalian cells during the animal's development. Indeed, until 1997 it had been generally accepted dogma that adult mammalian cells are no longer genetically totipotent, or capable of giving rise to all of the different cell and tissue types (e.g., liver, brain, and bone) required for making a complete and viable mammal. It was presumed that somatic-cell differentiation, the process by which a single fertilized egg is converted into all of the different cell types found in an adult, involved some irreversible step. That Dolly remained alive and well long after her birth—that she had a functional heart, liver, brain, and other organs, all derived genetically from the nuclear DNA of an adult mammary-gland cell—proved otherwise. At the very minimum, the specific tissue from which Dolly's nuclear DNA was derived must have been totipotent. By extension, it was reasonable to suggest that the nuclear DNA of other adult tissues also remains totipotent. With the success of Dolly, this speculation became a testable hypothesis.

      To appreciate more fully the ramifications of Dolly's existence, it is necessary to consider in some detail the circumstances of her creation. Dolly did not spring from the laboratory bench fully formed but developed to term normally in the womb of a Scottish Blackface ewe. Although the DNA in her cell nuclei was derived from a mammary-gland cell taken from an adult Finn Dorset ewe, that DNA had to be fused by electrical pulses with an unfertilized egg cell, the nucleus of which had been removed. The egg cell was taken from a Scottish Blackface ewe, and later another sheep of the same breed served as a surrogate mother. Furthermore, in order for the DNA to be accepted and functional within the context of the egg, the donor mammary-gland cells first had to be induced to abandon the normal cycle of growth and division and enter a quiescent stage. To do this, researchers deliberately withheld nutrients from the cells. The importance of this step had been determined experimentally, and although a number of hypotheses had been raised to explain its necessity, which, if any, of them was correct remained unclear. Nevertheless, a number of fused couplets formed embryos, which were transferred to surrogate ewes. Of 13 recipient ewes, one became pregnant, and 148 days later, which is essentially normal gestation for a sheep, Dolly was born.

      Dolly's unusual conception and normal birth raised a host of questions—some scientific, others social, ethical, or even religious. Some of the questions were answerable, and others were not. Of the scientific questions, at least two were thought to be experimentally approachable from studies of Dolly or her offspring.

      The first question addressed the issue of X-chromosome inactivation, the process by which normal mammalian females limit the expression of most of the genes located on their X chromosomes. In brief, a normal mammalian male receives an X chromosome from the mother and a Y chromosome from the father and so carries only one X chromosome; a female, on the other hand, receives an X from each parent and so carries two. To avoid the overexpression of genes that would occur with two active X chromosomes, a female effectively shuts down nearly all of the genes on one of her two X chromosomes very early in embryonic development. Which X is inactivated in each individual cell of the female, however, appears to be a matter of chance. Some cells inactivate the maternally derived X; others, the paternally derived X. As the embryo grows and develops and the cells divide and differentiate, the progeny of each cell "remember" the original decision, so that normal adult females end up as mosaics, with some of their cells expressing genes only from their maternally derived X chromosomes and others only from their paternally derived X chromosomes.

      The implication for cloning using DNA from adult female cells is that unless the X-chromosome inactivation that exists in the donor cell is somehow reversed and then randomly reestablished in the cells of the developing embryo, the resultant female clones will not be mosaic. All of their cells will express only those genes on the X chromosome that had not been inactivated in the donor cell. If that chromosome carries any abnormal genes, the female clones could fail to express the normal equivalents of those genes present on their other (inactivated) X chromosome and, as a result, be afflicted with any of a range of biological abnormalities early or later in life. That Dolly appeared healthy suggested either that the X-chromosome inactivation was reversed and rerandomized in her cells or that none of her essential X-chromosome genes were abnormal. This was a testable distinction.

      A second scientific question raised by Dolly's creation involved the mitochondria, cell organelles that carry their own set of genes distinct from the nuclear genes and that exist outside the nucleus in the cell cytoplasm. Even though the two sets of genes exist independently, they must operate interdependently for the cell to function normally. Since Dolly's mitochondria were derived from a Scottish Blackface donor egg and nuclei from a Finn Dorset mammary-gland cell, an important question was whether there would be any incompatibility. Clearly, Dolly's good health suggested otherwise. An extension of this question remained, nevertheless. Could mammalian cloning technology be applied to study experimentally the effect of mitochondrial DNA mutations on whole organisms, rather than only on cultured cells, as had been done in the past?

      Finally, both scientists and nonscientists were confronting the social and ethical Pandora's box of questions raised by mammalian cloning. On the positive side, cloning of nonhuman animals may greatly simplify the otherwise cumbersome manipulation of domestic livestock currently required for engineering genetic improvements in resistance to disease. It may also facilitate the production of lifesaving pharmaceuticals for human use—e.g., the production of human insulin in nonhuman animal milk. In addition, the application of cloning to the creation of founder individuals in a breeding population of animals could aid in saving endangered species otherwise doomed to extinction.

      On the other hand, would racehorse owners attempt to clone champions rather than breed them? If so, how would this approach be regarded by the horse-racing industry? Much more important, what of cloning humans? Does the concept of cloning violate the sanctity of the individual? During the year some observers voiced concerns about misguided zealots attempting to clone political or religious leaders; others envisioned hope for desperate parents of children in need of a perfectly matched donor for a bone-marrow transplant, pointing out that some parents were already opting to pursue pregnancy after pregnancy in an attempt to create such a donor. In 1997 human reproductive technology allowed for in-vitro fertilization, genetic characterization of early embryos prior to implantation, and a multitude of genetic and other forms of both pre- and postnatal presymptomatic testing. One could only wonder to what new accepted practices human cloning might lead. Special Report.) (Uses and Ethics of Cloning )

JUDITH L. FRIDOVICH-KEIL

      This article updates heredity.

Paleontology

      The year 1997 was an active and exciting one for paleontology, as new discoveries and interpretations of fossil vertebrates, invertebrates, and plants advanced scientists' understanding of past ecosystems. In vertebrate paleontology the efforts of the joint expeditions of the American Museum of Natural History and the Mongolian Academy of Sciences to the Gobi Desert of Mongolia continued to produce exceptional vertebrate fossils from the Cretaceous Period (approximately 144 million to 66.4 million years ago). In total the skeletons of more than 150 dinosaurs and 300 lizards and the skulls of 240 mammals were collected. The most famous was a complete skeleton of the small theropod dinosaur Oviraptor preserved on top of a nest of eggs. Identification of the embryonic skeletons within the eggs confirmed that the oviraptor was actually sitting on its own nest rather than stealing eggs from the nest of another dinosaur species. This finding was in contrast to interpretations of fossil evidence found in the 1920s, which had incorrectly assumed that the animal was raiding nests for food and hence had led to the genus name Oviraptor, meaning "egg thief."

      Important new vertebrates were also discovered by an expedition of the State University of New York (SUNY) at Stony Brook to Madagascar. Birds and primitive mammals of the Mesozoic Era (about 245 million to 66.4 million years ago) were among the new fossils under study from this collection. Many of the fossils represented new species within extinct groups of mammals previously known only from South America.

      One of the more unusual finds was described in a preliminary report from Chinese paleontologists given at the Jurassic Symposium held at the Museum of Northern Arizona in late 1996. Intriguing pictures showed small theropod dinosaurs that appeared to have feathers preserved along the vertebral column (backbone), although some experts argued that the features in question might be connective-tissue fibres that supported a midline structure down the back of the animal. In another development, researchers at Dinosaur National Monument, Utah, reported that a number of small theropods possessed furculae (wishbones in birds). The two discoveries further strengthened the theory that birds evolved from small carnivorous theropod dinosaurs.

      In mid-1997 a paleontologist from the University of Notre Dame, Ind., reported to the media the finding of what might be the largest skeleton of Tyrannosaurus. The ownership of the specimen, from Montana, was under dispute, and most of it had yet to be collected; hence, the significance of the find remained unclear. Ownership of the largest and most complete tyrannosaur specimen known to date, which was discovered in 1990 in South Dakota and nicknamed "Sue" had also been a subject of controversy for several years. The courts finally resolved the issue, which cleared the way for the designated owner to auction off the skeleton to the highest bidder. Sue was bought by Chicago's Field Museum of Natural History in October for $8,360,000.

      A new skeleton representing one of the very earliest groups of mammals in the fossil record was reported for the first time from the Late Triassic Period (230 million to 208 million years ago) of Greenland by paleontologists from Harvard University. In addition to the fact that fossil vertebrates from Greenland were relatively rare, the new specimen suggested that these early mammals were not closely related to the multituberculates, an extinct primitive group of rodentlike mammals, of the later Mesozoic Era, as had been thought.

      Laboratory studies made significant contributions to paleontology during the year. Researchers at Montana State University reported that for the first time organic molecules had been discovered preserved in dinosaur bone. In particular, molecules resembling collagen, a type of protein, were found in nonpetrified samples of Tyrannosaurus bones. In a second study, analysis of the rates of genetic change by researchers working on fossil invertebrates at SUNY at Stony Brook suggested that the origin of the major animal phyla may date to 1 billion-1.2 billion years ago. This age is much older than previous estimates, since the first known fossils of metazoans (multicelled animals) date back only to about half that age.

      Other invertebrate studies focused on using fossils to document ecological change. For instance, the distribution of corals, which are very sensitive to temperature variations, was being used to track global climate change through portions of the Earth's history (particularly more recent times). The results of this research were stimulating the growth of projects designed to find links between the distribution of climate-sensitive shallow-water marine organisms and environmental change.

      Interesting new discoveries of Paleozoic invertebrates reported during the year included a unique group of soft-bodied Silurian fossils from northeastern Iowa and southern Wisconsin. These unusually well-preserved specimens included numerous arthropods, annelid worms, and fish, some of which represented new genera. Like the famous assemblage from the Burgess Shale of the Canadian Rockies, this could turn out to be one of the most significant collections of soft-bodied-animal fossils from North America.

      Exopaleontology, the study of ancient organisms from other planets, emerged as a new field of research following a report in 1996 of fossil evidence of primitive life preserved in a Martian meteorite from Antarctica. The report was controversial, and experts continued to debate whether life existed on Mars some 3.6 billion years ago. Nevertheless, it stimulated studies by NASA scientists on ancient underwater vent and seep sites on Earth that produced fossils and represent environments in which organisms derive energy from chemical compounds rather than sunlight. Such sites were of interest to NASA because they may be similar to the type of environment that would allow life to originate on a planet like Mars.

      In paleobotany the content of coprolites (fossil dung) was increasingly being employed to determine the nature of early terrestrial plant communities. One such study indicated that the very earliest trees of Middle to Late Devonian age (387 million to 360 million years ago) grew in types of soils very different from those of later ages. The origin and early evolution of the flowering plants was another topic of considerable recent interest to paleobotanists. A paper published in a recent book on the topic identified the oldest angiosperm (flowering plant) known to date, from very Early Cretaceous deposits of Israel.

      Permian and Triassic age plants from Antarctica also continued to add to scientists' understanding of plant evolution. Permineralized peat deposits from the Transantarctic Mountains contained some of the best-preserved cell structures of any fossil plants. Reproductive structures from these peats provided vital new information about the early evolution of seed plants.

WILLIAM R. HAMMER

      See also Anthropology (Anthropology and Archaeology ); The Environment (Environment ).

      This article updates evolution, theory of (evolution).

▪ 1997

Introduction

ZOOLOGY

       Zoological research during the past year contributed to an improved understanding of the relationships between genetics and the aging process, further explored some of the intricacies of internal physiology, and uncovered the first known example of eusociality in a marine organism. A new species of mammal was discovered in the rain forests of the Philippines, and studies of turtles and lizards provided insight into current conservation issues. Molecular techniques established that the guinea pig is not a rodent, as had been thought.

      Bernard Lakowski and Siegfried Hekimi of McGill University, Quebec, presented evidence that four genes, named the Clock genes , interact to determine the life span of the nematode Caenorhabditis elegans, a microscopic, wormlike soil animal used extensively in genetic studies. The Clock genes appear to extend life span by a mechanism distinct from that of other Caenorhabditis genes, the dauer genes, that previously had been found to affect life span. Nematodes containing mutations in both a Clock gene and a dauer gene lived nearly five times longer than normal wild-type nematodes—the greatest increase in life span over the species average that had been achieved by any means in any organism. The Clock genes also were found to affect other timed processes, including the length of development and the cell cycle. The study showed that Clock-gene mutations affect the rate of development and adult life span in a similar manner, which suggests that the long life of the mutant nematodes may be a consequence of a "slower rate of living," possibly due to a slower rate of metabolism. The Clock genes may be regulatory genes that control metabolic rates and influence a general physiological clock in nematodes.

      Lawrence C. Rome and Stephen M. Baylor of the University of Pennsylvania and colleagues investigated the physiological mechanisms that allow muscle fibres involved in sound production in vertebrates to have contraction cycles 10-20 times faster than most vertebrate locomotory muscles. The tail muscles causing the rattling of western diamondback rattlesnakes (Crotalus atrox ) contract repeatedly at about 90 hertz (Hz; cycles per second), whereas muscles that surround the swim bladders of the oyster toadfish (Opsanus tau ) and are used in creating a mating call contract at about 200 Hz, the fastest known rate for any vertebrate. The investigators found in both instances that calcium, the trigger for muscle contraction, cycles in a manner that allows the muscle fibres to activate and relax at a rapid rate. Movement of calcium through toadfish bladder muscle is as much as 50 times faster than through most muscles used for locomotion. In addition, the myosin-filament cross bridges, whose repeated binding to actin filaments and subsequent release generate the force in muscle contraction, attach and detach about 50 times faster as well. One significant revelation of the study was that the physiological traits necessary to permit muscle fibres to move rapidly evolved independently in the rattlesnake and toadfish.

      A study of the rubber boa (Charina bottae ), a nocturnally active snake, by Michael E. Dorcas and Charles R. Peterson of Idaho State University revealed that the internal temperature of the animal's head is significantly warmer than either its internal body temperature or cool nighttime air temperatures. Precise regulation of temperature in the head region of an organism is presumed to be advantageous in optimizing functions of the central nervous system. Although differential temperatures in parts of a reptile body had been reported for other species, the findings in the rubber boa represented the first instance of the phenomenon in a reptile active at night. The study suggested that some reptiles may have greater versatility in regulating temperatures in different bodily regions than formerly suspected.

      Social insects, such as ants, honeybees, and termites, and the naked mole rat, a mammal, are considered eusocial, with reproduction often being limited to a single female, or queen, within a colony. Additional characteristics of eusociality are cooperative care of the young and division of labour among nonreproductive members of the colony. The discovery by J. Emmett Duffy of the Virginia Institute of Marine Science, Gloucester Point, Va., of eusociality in a coral-reef shrimp (Synalpheus regalis ) was the first such report in a marine organism or a crustacean. S. regalis lives in the internal canals of sponges. Duffy dissected more than 30 sponges from the coast of Belize, each of which housed a shrimp colony with a single reproductive female and usually with multiple generations of her offspring. Examination of the shrimp colonies supported previous hypotheses that altruistic behaviour among nonbreeding members of a colony can be favoured as a result of kin selection in species living in enclosed habitats that provide protection against predators and an adequate food supply.

      In the area of conservation ecology, investigators found evidence that the use of turtle excluder devices (TEDs) by shrimp trawlers indeed did result in reduction of the numbers of sea turtles killed in trawling operations. TEDs are grid attachments within trawl nets that retain shrimp but allow most turtles to escape. Without TEDs, shrimpers can unintentionally drown turtles in their nets. Larry B. Crowder and J. Andrew Royle of North Carolina State University and Sally R. Hopkins-Murphy of the South Carolina Department of Natural Resources completed a statistical analysis of the numbers of dead loggerhead sea turtles washed ashore in South Carolina in a 15-year period. In years when shrimping was under way, 44% fewer dead turtles turned up on shore when TEDs were in use than when they were not. TED use also reduced the rate of decline in the population of nesting females along South Carolina beaches and, according to the investigators, had the potential for allowing the loggerhead population to expand by a factor of 10 by the year 2055.

      In a continuation of a long-term study on islands in The Bahamas, Thomas W. Schoener and David A. Spiller of the University of California, Davis, experimentally demonstrated the way in which introduction of a predator (an anole lizard) into a system can have devastating effects on the diversity and abundance of prey species (web spiders). The investigators ran a seven-year experiment in which they selected four groups of three islands each, one inhabited by lizards and two without lizards; all of the islands were inhabited by spider species. In each trio of islands, lizards were introduced onto one of the two lizard-free islands. Within two years the islands onto which lizards had been introduced were almost identical in spider diversity and abundance to those with natural lizard populations. The proportion of spider species becoming extinct on islands with introduced lizards was 12.6 times higher than on islands with no lizards, and most rare species disappeared. The study underscored the impact that predator introductions can have in some situations by severely threatening species composition and integrity of natural systems.

      The order Rodentia traditionally has been divided on the basis of morphology into several suborders, one of which, Caviomorpha, includes such animals as chinchillas, degus, agoutis, porcupines, capybaras, and guinea pigs. On sequencing the complete genome, or genetic endowment, of the mitochondrion (a DNA-containing cell organelle) of the guinea pig (Cavia porcellus ) and using three distinct analytic methods, Anna Maria D'Erchia and Cecilia Saccone of the University of Bari, Italy, and colleagues provided evidence supporting an earlier contention that guinea pigs are in a separate phylogenetic line from the rodents. They concluded that guinea pigs should be placed in a new order of mammals distinct from Rodentia.

      A new mammalian species from the Philippine rain forest was reported by Robert Kennedy of the Cincinnati (Ohio) Museum of Natural History & Planetarium and Pedro Gonzales of the National Museum of the Philippines. Named the Panay cloudrunner (Crateromys heaneyi ), the tree-dwelling, squirrellike rodent has soft brown fur, small ears and eyes, and a long black tail and weighs about 1 kg (2.2 lb).

      (J. WHITFIELD GIBBONS)

Entomology.

      Anne-Geneviève Bagnères of the Laboratory for Neurobiology-Chemical Communication, Marseille, France, and colleagues reported on the way in which one species of paper wasp, Polistes atrimandibularis, which is incapable of building a nest or producing a worker caste, persists as an obligatory social parasite on a related host species, P. biglumis bimaculatus. Social insects characteristically produce chemical signatures that enable colony members to recognize each another. Annually in late June a fertile parasitic P. atrimandibularis queen searches for the nest of her host species. At that time the chemical signatures of the two species differ, with the cuticle of the parasite producing a family of hydrocarbons distinctive from the composition of hydrocarbons produced by the host. On colonizing the nest, however, the parasite ceases producing the distinguishing hydrocarbons, and a month later her signature, based on gas chromatography and mass spectrometry, is indistinguishable from that of the host queen. For the remainder of the colonial cycle before the emergence of adult wasps and mating in late summer, P. biglumis bimaculatus workers feed and care for parasite offspring as they do the offspring of their own species. The study demonstrated the versatility of the parasite in adjusting its chemical signature at a critical time in its colonial cycle and supported the idea that, in addition to a simple role as an enclosure and a barrier, the cuticle of insects functions as a true gland.

      Researchers used training techniques to explore the ability of honeybees to distinguish between symmetry and asymmetry, a critical skill for pollinators in that the symmetry of a flower may indicate its quality. Martin Giurfa, Birgit Eichmann, and Randolf Menzel of the Free University of Berlin presented bees with different stimuli designed to be distinguishable only on the basis of their bilateral symmetry or asymmetry. One group of bees was rewarded for selecting symmetrical patterns, the other for selecting asymmetrical ones. Afterward, both were presented with either symmetrical or asymmetrical patterns that they had not seen before. Individual performance was measured by means of a microphone apparatus, adjusted to detect the bee's flight noise. The investigators recorded how often a bee chose the novel symmetrical or asymmetrical patterns, how close the bee went, and how long it hovered. The results indicated that bees could easily be taught to favour either symmetrical or asymmetrical patterns and could transfer that learning to patterns not seen before. Although bees could be trained to prefer symmetrical or asymmetrical patterns, they showed a predisposition for symmetrical ones. Previous studies had shown that bees are attracted to symmetrical shapes, but the new study demonstrated that they recognize symmetry as a property and respond to it on the basis of their experience.

      Mary E.A. Whitehouse and Klaus Jaffe of Simón Bolívar University, Caracas, Venez., studied leaf-cutting ants of the species Atta laevigata to investigate two laws of combat strategy. The linear law proposes that a few good fighters are a better strategy than many poor fighters in a series of one-on-one conflicts. The square law holds that if all individuals are equally susceptible to attack, many poor fighters are better than a few good ones. During manipulative field experiments the investigators staged battles between ants from one colony and those of another or against vertebrate predators. The ants responded to vertebrate threats according to the linear law, by recruiting specialized soldier ants from their colony. On the other hand, their response to threats from other ant colonies followed the square law; they recruited large numbers of smaller individuals. Thus, leaf-cutting ants alter their mode of fighting according to the threat and follow the combat strategy law most effective for the situation. (ANNE R. GIBBONS)

      This updates the article insect1 (insect).

Ornithology.

      Scientists regarded birds' use of tools as mostly stereotyped and their manufacture of tools as involving only limited modification of material objects. In 1996 Gavin R. Hunt of Massey University, Palmerston North, N.Z., reported that to assist in capturing insect prey, New Caledonian crows make and use two different types of hook tools from twigs and one kind of stepped-cut tool from the barbed leaf of the pandanus tree. According to Hunt, these instances of tool manufacture by a bird species had three features new to tool use in nonhuman animals: a high degree of standardization, distinctly discrete tool types with a definite imposition of form in the shaping of the tool, and the use of hooks. During the course of human evolution, such features first appeared in stone and bone tool-using cultures only after the Lower Paleolithic Period (about 2.5 million to 200,000 years ago).

      The foraging success and habits of pelagic (open-ocean) seabirds were largely unknown. Using satellite transmitters attached to the birds in conjunction with recorders for measuring feeding times and the weight of ingested food, researchers found that wandering albatrosses on foraging trips from the nest encountered prey on average every 4.4 hours and consumed 2.1 kg (4.6 lb) of food daily. Birds traveled as far as 3,600 km (2,200 mi) from the nest in search of scarce prey, mostly pelagic squid.

      Ornithologists had long hypothesized that seagoing birds such as petrels use their sense of smell to find food in the open ocean. Research in the past year showed that petrels indeed can sniff out minute amounts of a telltale chemical released by plankton. Gabrielle Nevitt of the University of California, Davis, and Richard Veit and Peter Kareiva of the University of Washington staged a number of experiments in the waters around the sub-Antarctic island of South Georgia. They created small "slicks" of vegetable oil laced with small amounts of the compound dimethyl sulfide (DMS). Microscopic plants in plankton release DMS when consumed by small animals such as krill. Because petrels and their relatives eat such animals, the researchers reasoned that the birds might be able to detect DMS. In fact, DMS turned out to be highly attractive to several seabird species, including Wilson's storm petrels, black-bellied storm petrels, and prions. As storm petrels and their allies often hunt by night, they would gain from their sensitivity to DMS. Furthermore, some areas of the open ocean, where plankton thrive, tend to have higher concentrations of DMS than others. Birds may be able to detect these chemical patterns and use them to help navigate over the otherwise featureless oceans.

      Two fossil discoveries prompted paleontologists to rethink theories about the diversity of bird life in the age of the dinosaurs. The beautifully preserved bones of Vorona berivotrensis, a new, very primitive bird species unearthed in Madagascar, was the first specimen from the Mesozoic Era (245 million to 66 million years ago) to be found in a large portion of the ancient continent of Gondwana (mainly present-day South America, Africa, India, Australia, and Antarctica). It was also the first pre-Holocene bird (older than 10,000 years) found in Madagascar. The lower limb of the crow-sized fossil indicated a close relationship to the extinct Enantiornithes, the most common group of birds contemporary with the dinosaurs.

      The second fossil, Eoalulavis hoyasi, from Spain, showed that birds had evolved their efficient, modern style of flight as early as 115 million years ago. According to Luis Chiappe of the American Museum of Natural History, New York City, who helped describe the Madagascan and Spanish fossils, "The diversity of early birds was much larger than we thought five years ago." E. hoyasi was about the size of a goldfinch. Its remains included a well-preserved wing with many feathers in their original positions and showed a crucial stage in the evolution of flight. It lived only about 30 million years after the first bird, Archaeopteryx, but already possessed the alula, or bastard wing, that allows modern birds to maneuver among trees. (JEFFERY BOSWALL)

      This article updates bird.

MARINE BIOLOGY

      The discovery of a species of marine animal that appeared to constitute an entirely new phylum was reported in the science journal Nature as the "zoological highlight of the decade." Two Danish investigators proposed that their newfound invertebrate species, Symbion pandora, be attributed to a new phylum, Cycliophora, related to the phyla Ectoprocta (Bryozoa) and Entoprocta. Symbion is an acoelomate metazoan—i.e., a multicellular animal lacking an internal fluid-filled body cavity. Its sessile stages were found abundantly on the mouthparts of the Norway lobster (Nephrops norvegicus), where they capture food being ingested by their host.

      Vertical migration rhythms in plankton living in the open sea typically show a daily pattern. However, a U.K. study of newly hatched larvae of the shore crab Carcinus maenas demonstrated endogenous rhythms geared to the tides. Upward swimming during ebb tides evidently disperses the larvae offshore and thus prevents their premature stranding onshore in the intertidal area. In a Polish study two species of mid-water lantern fish from the Atlantic, Hygophum macrochir and H. taaningi, were shown to avoid vertical migration at night during the new moon lunar phase. The fish stayed in cold water below 400 m (1,300 ft) at new moon and did not, as during other lunar phases, rise to warmer surface waters at night. The lunar variations of vertical migration were found to be recorded in the animals' otoliths, so-called ear stones used in maintaining balance. The microstructure of the otolith shows a pattern of daily growth rings, which varies according to the sea temperatures experienced by the fish. A similar record of carbon isotope ratios was detected in baleen plates taken from stranded southern right whales from South Africa. Changes of isotope ratios along the length of the plates provided the first direct evidence of seasonal migrations of the whales north and south of the Subtropical Convergence.

      French and German researchers fitted five albatross of the species Diomedea exulans with miniature sea-temperature recorders and satellite transmitters and released the birds to forage over the Southern Ocean. During frequent pauses on the sea surface, the birds transmitted, via satellite to a tracking station, the sea-surface temperature where they rested. The technique could be useful for verifying the accuracy of satellite-measurement data and for obtaining data from remote areas when cloud cover precluded direct satellite measurement. Caulerpa taxifolia, a green alga with a circumpolar distribution, was observed for the first time in the Mediterranean Sea in 1984. During 1996 the alga was reported to occur in the Mediterranean over an area of 1,000-2,000 ha (2,500-5,000 ac) and to be spreading annually by a factor of 2-10.

      The marine coccolithophore Emiliana huxleyi is a single-celled alga that undergoes massive blooms, or rapid population increases, worldwide. Researchers estimated that once the algal masses die off and sink, they transport 800 million tons of carbon as calcite (a form of calcium carbonate) and 500 million tons of carbon as organic compounds to the seabed each year, which confirms the major role of the blooms in regulating global ocean carbon flux. The blooms also emit into the atmosphere dimethyl sulfide, a greenhouse gas, which was shown by European researchers to derive from death of the algal cells following viral infection, which contributes to the termination of the blooms.

      A laboratory study carried out in the U.S. showed that the tropical flatfish Bothus ocellatus can adjust its pigment patterns for camouflage purposes with surprising fidelity in two to eight seconds to blend with different backgrounds. It even was able to adapt to a black-and-white checkerboard pattern put into the laboratory tank. U.S. and Australian investigators marked coral-reef damselfish (Pomacentrus species) with fluorescent dyes and tiny, implanted, code-carrying tags, which for the first time allowed long-term recognition of individual reef fish in studies of immigration and emigration. Related studies around Apo Island in the central Philippines provided evidence of the emigration of adult fish from protected reserves to fished areas, justifying the establishment of reserves.

      Larvae of vestimentiferans, gutless worms that live around deep-sea hydrothermal vents and cold seeps, were cultured and described for the first time. The larvae resemble trochophores, the free-swimming larvae characteristic of polychaete annelid worms, which places the vestimentiferans phylogenetically closer to that group than hitherto recognized. An investigator reported the first known case of eusociality in a marine invertebrate, analogous to the social behaviour of bees and termites. A sponge-dwelling shrimp, Synalpheus regalis, was found to live in colonies of more than 300 individuals. A single reproductive animal functions as a queen, while other members serve to protect the colony against intruders. (See Zoology, above.) Living specimens of the sea anemone Gerardia, obtained from a depth of 620 m (2,034 ft) off The Bahamas, were revealed by means of carbon-dating techniques to have been alive for 1,500-2,100 years. (ERNEST NAYLOR)

      This article updates crustacean; fish; mollusk.

BOTANY

      The remarkable similarities between plants and animals became more evident in 1996 as scientists unraveled details of the hormonal communication system used by plants to regulate their physiological activities. The natural organic compounds known as steroids play major roles as hormones in animals, but their functions in plants have been much less clear. During the year researchers in California discovered that a plant steroid called brassinolide, which in its molecular structure closely resembles the human male androgen sex hormone, is used by plants as a hormone, although not for sex. Joanne Chory and her team at the Salk Institute, La Jolla, Calif., examined a stunted form of thale cress (Arabidopsis thaliana), a small, fast-growing plant often used for genetics experiments. The stunting was caused by the plant's failure to respond to light, and the problem was traced to a defective gene involved in making brassinolide.

      Animals use another hormonal communication system based on fairly large, complex molecules called peptides, which are short chains of linked amino acids, the building blocks of proteins. Plant researchers from The Netherlands and Germany, led by Karin van de Sande, reported their discovery that a peptide in legume plants carries signals involved in building special nodules on the plants' roots, where symbiotic nitrogen-fixing bacteria live. Communication in plants previously had been thought to be the work of small molecules, but if peptide signaling turns out to be widespread, it would challenge scientists' current view of the sophistication of plant physiology. (See Molecular Biology, below.)

      Genetic research revealed some startling insights into plant development. Two separate discoveries showed that a simple genetic switch is all that is needed to transform ordinary green shoots into flowers. Working with A. thaliana, Detlef Weigel and Ove Nilsson of the Salk Institute demonstrated that by jamming into the "on" position the "master switch" gene that controls the other genes involved in flowering, they could not only turn side shoots into flowers but also make the plant flower much sooner than normal. In subsequent experiments they switched on the flowering genes of aspen trees and thereby cut the time to flowering from years to months. Similar results, although by means of a different gene, were achieved by Alejandra Mandel of the University of Arizona and Martin Yanofsky of the University of California, San Diego. A third gene was revealed by biologists at the John Innes Centre, Norwich, Eng., to direct the location at which plant flowers sprout. Normally the gene stops the main stems of snapdragons from producing flowers at their tips, but by interfering with the gene they made each plant bloom only at the tip of its stem.

      Genetic engineering of plants continued to make progress. Tobacco plants, normally killed by salty water, were given a gene that allowed them to survive brackish waters. This achievement helped to open the way for the development of new crop plants that can grow in arid, salty areas of the world. Potatoes were programmed to commit suicide if they became afflicted with an infectious disease; the intent was to limit disease spread, which in turn would reduce pesticide use. On the other hand, fears for the safety of genetically engineered plants found some support. Danish scientists conducting field trials on oilseed rape (Brassica napus) discovered that a gene inserted into the crop spread alarmingly fast to a wild relative, B. campestris. This raised concern that weeds could accidentally be genetically modified.

      Paradoxically, while scientists engineered new varieties of crops, the natural genetic diversity of the world's crop plants was rapidly vanishing, leaving the remaining varieties prone to pests and plague. In June 150 government representatives meeting in Leipzig, Ger., pledged to halt the decline in crop varieties, many of which dated back thousands of years. The statistics were alarming; for instance, since 1900 the U.S. had lost most of its 20,000 varieties of agricultural plants. Governments were responding with an international network of gene banks that made use of refrigerated seed-storage facilities and farms to conserve threatened varieties. One big step in plant conservation was the announcement by Kew Gardens, near London, that it would build the world's largest seed bank for wild plants. It would cost $32 million and eventually could be expanded to house as much as 10% of the world's wild plant species, many of which were on the verge of extinction.

      One of the great attractions of conservation was the potential for finding new drugs and other useful compounds in plants. Scientists studying watercress, for example, discovered compounds that counter the cancer-causing effects of nicotine. Other researchers discovered a protein in snowdrop (Galanthus nivalis) that reduces appetite in sap-sucking pests; the gene that codes for the protein was being introduced into potato and tobacco plants to combat aphids. In a search for new biologically active substances, Hermann Niemeyer and colleagues at the University of Chile, Santiago, collected some 400 plant species. Among them was the yellow-flowered Calceolaria andina, from the foothills of the Chilean Andes, which was found to contain two powerful insecticides. These so-called napthoquinones selectively target a range of highly damaging sap-sucking insects, including a virulent strain of the tobacco whitefly, a serious global agricultural pest that was resistant to many current commercial sprays. (PAUL SIMONS)

MOLECULAR BIOLOGY

Self-Defense in Plants.

      Rooted to the ground and thus unable to flee, plants need defenses against a variety of predators and disease-causing microorganisms. While obvious structural features such as thorns can deter large animal predators, more covert defenses are required against plant-eating insects and microorganisms. When organic chemists first began analyzing the chemical composition of plants, they found a bewildering array of compounds whose functions were totally unknown. The compounds were collectively termed secondary metabolites, which seemed to imply that they were not of great importance. Since the early 1990s it has become increasingly clear that most of these compounds function as part of a remarkably sophisticated passive-aggressive defense system, which ongoing work in 1996 continued to explore.

      The interaction of the disease-causing fungus Phytophthora with a tomato or tobacco plant can serve as an example of the way that part of the defense system was found to work. In the immediate vicinity of contact with the fungus, the plant dramatically changes its metabolism so as to prevent the growth of the fungus. It increases its local production of certain highly reactive, oxygen-derived chemical species—namely, hydrogen peroxide and groups of atoms called free radicals. It also steps up local production of toxic compounds called phytoalexins. The oxygen-derived species and phytoalexins cause local cell death. This activity leads to a spot of dead tissue on the leaf, but it also impedes the spread of the fungus. Concomitant with the local reaction, the plant produces chemical signals that circulate systemwide throughout the plant and induce changes leading to general resistance.

      As Phytophthora attempts to infect the plant, it secretes small proteins, called elicitins, that ultimately serve a structural role for the fungus. It is the elicitins that turn on the defensive responses of the plant. In fact, it was shown experimentally that a light touch of a dilute solution of pure elicitins induces both the local acute response and the systemic response. The signal within the plant that mediates the systemic changes leading to resistance is carried by salicylic acid, which is made in response to elicitins. This simple compound serves several kinds of signaling roles in plants and is more familiar to people in the form of a chemical derivative, aspirin.

      Recent research also revealed that plants mount other types of defenses to ward off plant-eating insects like caterpillars and beetles. The response may involve the production and release of compounds distasteful or toxic to the insect. In some cases the plant releases volatile compounds that attract predators or parasites of the insect. In addition, the mechanical injury caused by the insect sets off a signaling cascade that induces the entire plant to adapt to the attack. The first element in the cascade is a short chain of amino acids, or oligopeptide, called systemin, which is produced in response to the mechanical damage. Systemin activates the production of jasmonic acid, which in turn signals the entire plant to prepare for attack. This systemic call to arms includes the production of lignin and a protease inhibitor. Lignin is a woody polymer that caterpillars and beetles find indigestible. The protease inhibitor prevents digestive enzymes called proteases from breaking down proteins in foods and thus keeps insects from benefiting from the plant protein that they ingest. Protease inhibitors, which are proteins themselves, are abundant in such seeds as soybeans as a defense against seed eaters. Humans circumvent natural protease inhibitors in foods by cooking, which inactivates them and renders the food digestible.

      The recent discoveries about plant defense systems uncovered parallels between them and the defensive responses and signaling reactions of mammals. For example, the phagocytic white blood cells of the human body respond to invading organisms by producing hydrogen peroxide and a free radical called superoxide, similar to the response of plants. Furthermore, the human body produces signaling molecules, called prostaglandins, made from the polyunsaturated fatty acid arachidonic acid; plants produce jasmonic acid from a similar fatty acid, linolenic acid.

      The existence of chemical defenses in plants is a powerful argument for the maintenance of maximum biological diversity. Scientists have only begun to explore the compounds involved in these systems, and the same can be said for the defense systems of insects, amphibians, and many other organisms. Unraveling these secrets may provide as great a benefit to human beings as have the discoveries of the major antibiotics, like penicillin and streptomycin, which are defensive antimicrobial compounds made by molds and bacteria.

Lou Gehrig's Disease.

      Advances continued in the past year in the understanding of the molecular and genetic basis of amyotrophic lateral sclerosis (ALS), or Lou Gehrig's disease. ALS is a degenerative disease of the motor neurons—the nerve cells that control muscular movements. The inexorably progressive paralysis that results usually begins during the third or fourth decade of life, and victims of ALS usually die within a few years after the appearance of symptoms. ALS occurs in two forms, one familial (FALS) and the other sporadic (SALS). Except for the heritable character of FALS, the two forms are symptomatically indistinguishable.

      The search for a genetic defect involved in the cause of FALS led first to chromosome 21 and then, in the early 1990s, to a gene called SOD1. The gene was found to encode—i.e., to carry the genetic code for making—an enzyme called superoxide dismutase. The enzyme protects the body's cells against the destructive effects of accumulating superoxide radicals by catalyzing their conversion into molecular oxygen and hydrogen peroxide.

      FALS is genetically dominant, which means that one copy of the defective gene is sufficient to cause the disease. The corollary is that one copy of the normal gene cannot prevent the disease. In theory, mutations in the SOD1 gene could cause FALS by specifying a superoxide dismutase product that has modestly decreased activity or, alternately, by giving the enzyme a novel deleterious activity. The latter mechanism recently was shown to be the case in experiments that involved mice genetically engineered to carry a normal or defective human form of the SOD1 gene in addition to the natural mouse form of the gene. When the normal human SOD1 gene was expressed in mice, they did not develop paralysis. On the other hand, when genes coding for FALS-associated mutant forms of SOD1 were expressed, the mice did become paralyzed. Since the transferred human genes were expressed against a background of normal mouse SOD1 genes and the mice did indeed show normal levels, or even somewhat greater-than-usual levels, of superoxide dismutase, their paralysis could not have been due to a lack of the enzyme.

      What toxic property of mutant superoxide dismutase could cause degeneration of motor neurons? As of 1996 two possibilities had been put forward, with data supporting each. One is that the mutant enzyme catalyzes novel oxidation reactions that ultimately destroy the motor neurons. The other is that it catalyzes the addition of nitrate groups to tyrosine, one of the amino-acid building blocks of proteins. In fact, tests devised specifically to detect the nitrated tyrosine product found it in the spinal cords of ALS patients but not in those of persons free of the disease.

      Although many aspects of ALS remained mysterious, given the impressive gains in understanding in the past few years, investigators looked forward to a time in the near future when they would be able to predict, prevent, or at least slow the progress of the disease. Of course, the sporadic form of ALS does not involve mutations in the SOD1 gene. Nevertheless, because its symptoms are so similar to those of FALS, there is likely some similarity in causation.

      (IRWIN FRIDOVICH)

DNA Vaccines.

      If treating a disease is good, preventing it is better. For the past several generations, through the widespread practice of vaccination, that concept has been realized for a growing number of serious and often fatal infections. Indeed, organized vaccinations of children worldwide against smallpox led to the eradication of the known natural reservoirs of its causative virus in the 1970s.

      While the concept of vaccination—exposing an individual to some modified form of a disease-causing microorganism in order to generate an immune response—has been around for many years, vaccines themselves have undergone a stepwise evolution toward greater safety. Thus, vaccination has progressed from infection with a related but less virulent microorganism (e.g., cowpox virus in place of smallpox virus) to exposure to a live but attenuated (partially crippled) or heat-killed form of the virulent organism to injection with benign preparations of immunity-triggering proteins derived from the organism (e.g., the modern three-part vaccine against the hepatitis B virus). Along the way, vaccines against polio, tetanus, diphtheria, mumps, measles, rubella, and other devastating diseases have saved the lives and preserved the health of innumerable children and adults.

      Two fundamental and interconnected problems have remained, however. The first is that not all disease organisms have proved susceptible to control by conventional vaccines. Some viruses and other infectious agents possess the ability to mutate, or alter their surface proteins over time, such that antibodies generated by exposure to the surface proteins of one variety or strain become useless against future infections.

      The second problem is that the safer heat-killed or protein-based vaccines can be less effective at stimulating immunity than their more dangerous predecessors. In brief, this loss of efficacy reflects the fact that a human body exposed solely to a foreign protein will generate antibodies against that protein, whereas a human body whose cells are infected by a live virus—and thus tricked into making that same foreign protein as part of the process of viral replication—will generate both antibodies and killer cells (a type of white blood cell) that recognize the protein. As their name implies, killer cells retain the ability to target and kill any virally infected cells that make the foreign protein. A combined immune response of antibodies and killer cells not only offers a surer defense against infection but also allows the body to develop immunity against both the surface proteins of an infectious organism and its normally hidden internal proteins, which become visible to the body's immune system after the organism infects the cell. This point is a key one, because many disease agents are able to change their surface proteins, but few, if any, can change their internal proteins as well.

      In recent years a number of research groups, notably Margaret Liu and her colleagues of Merck Research Laboratories, West Point, Pa., and Stephen Johnston and his colleagues of the University of Texas Southwestern Medical Center at Dallas have developed an alternative approach to vaccines that may provide the best of both worlds—safety and long-lasting immunity against, at least in theory, almost any disease agent.

      The new vaccines are actually preparations of DNA, not protein, designed to be taken up by the cells of the recipient. The DNA consists of nonreplicating plasmids, or DNA loops, that correspond to either specifically chosen or random fragments of the DNA of the disease organism. The fragments are flanked by additional regulatory DNA sequences intended to encourage the host cells to make the proteins or protein fragments encoded by the foreign DNA. As the cells synthesize these foreign proteins, parts of them make their way to the cell surface and thereby attract the attention of that part of the immune system responsible for generating killer cells. Because each plasmid carries only a small fraction of the total DNA of the disease organism, there is essentially no risk of infection. Furthermore, because the plasmids carry DNA for both internal and surface proteins of the disease organism, immunity can be elicited even against those organisms that have learned to change their surface proteins.

      As of 1996, tests of the new vaccines in animals had produced results better than anticipated. In addition, studies designed to test for potential risks associated with the new vaccines, such as permanent integration of the plasmids into the DNA of the host cell or complications arising from an immune response against the introduced DNA, detected no evidence of such events. Clinical trials in humans were under way.

Yeast Genome Project.

      Much of what is known about living systems and the way that they function has been learned not from the study of humans but from the study of so-called model organisms, including bacteria, yeast, flies, worms, and mice. Indeed, the founders of the Human Genome Project so valued these other organisms and their contributions to biomedical science that obtaining the whole genome of each—i.e., establishing the exact sequence of DNA for the organism's entire genetic blueprint—was established as an important goal in addition to obtaining the whole genome of humans. The past year witnessed the completion of the first of these whole-genome sequencing efforts for a eukaryote—i.e., for a cellular organism whose cells contain a distinct nucleus. The target was the genome of the yeast Saccharomyces cerevisiae, strains of which are the familiar baker's, brewer's, and vintner's yeasts.

      The yeast genome project was initiated in 1989 by the European community of yeast researchers, but the effort soon expanded into a global collaboration involving laboratories in the U.K., continental Europe, the U.S., Canada, and Japan. Their combined efforts enabled the complete sequence of the S. cerevisiae genome to be published in April as a database on the Internet's World Wide Web (http:/ /genome-www.stanford.edu).

      Both the short- and the long-term benefits of the Saccharomyces genome database (SGD) promised to be enormous. For example, in terms of genome anatomy, data from the SGD revealed that the yeast genome is highly compact, with its genes tending to be much smaller and much less dispersed than those of the human genome. The data also predict that about 70% of the yeast genome encodes various protein molecules, specifically about 6,000 different proteins. Of this number, only about 40% had been identified previously in genetic studies. Of the remaining 60% (roughly 3,700 proteins), more than half bear no significant sequence similarity to any previously identified sequences for proteins of known function from any other organism. The sheer numbers of these "orphan" proteins stood as humbling testimony to how little scientists yet knew about so "simple" an organism as yeast.

      Perhaps the most obvious benefit of biomedical relevance to emerge from the availability of the SGD is the ability to quickly find yeast counterparts, or homologues, of genes in humans that are associated with specific diseases. In recent years researchers have made significant advances in identifying those genes that, when either absent or present in defective form, are responsible for a number of hereditary human diseases—for example, Huntington's disease, Batten disease, and fragile X syndrome. Although the identification of a disease gene can offer powerful new tools to aid in diagnosis, appropriate treatment requires at least some fundamental understanding of the normal function of the gene and the protein product that it encodes.

      Unfortunately, knowledge of the sequence of a given gene may offer little insight into its function, especially if no similar sequences of known function have been found, as is the case for many human disease genes. It is in such cases that a yeast homologue can provide a major benefit, since the ease with which yeast can be genetically and biochemically manipulated allows studies of gene function to be conducted more quickly in yeast than in human or other mammalian cells. The insights gained in studying the yeast homologue of a gene may then be transferred back, either wholly or partly, to the corresponding human disease gene. Indeed, oftentimes the functions of homologous human and yeast genes are so similar that a human sequence can be substituted successfully for a missing homologous sequence in yeast and thus enable direct studies of both normal and defective forms of the human sequence in a genetically and biochemically amenable yeast model system.

      (JUDITH L. FRIDOVICH-KEIL)

      This article updates heredity.

PALEONTOLOGY

      In 1996 students of fossils continued to provide new insights about past life that resulted in new philosophical challenges. A major event was the sixth North American Paleontological Convention (NAPC), held in June in Washington, D.C., and attended by 650 paleontologists, about 120 from outside North America. The meeting opened with discussions by J. William Schopf and Bruce Runnegar of the University of California, Los Angeles, about Precambrian life (before about 545 million years ago) and the oldest known fossils on Earth—3.5 billion-year-old bacterial filaments.

      Two months later David McKay (see BIOGRAPHIES (McKay, David Stewart )) of NASA and colleagues announced the finding of organic residue and bacteria-like structures about 3.6 billion years old in a meteorite thought to be from the planet Mars. The findings may be the first indications of life on another planet and the first real data available to the science of exobiology. Debate over the interpretation of the findings was just beginning. For example, Schopf (an expert in very ancient microfossils) reckoned, "I think it's very unlikely they [McKay and colleagues] have remnants of biological activity."

      Another notable event at the NAPC was the firm placement of conodont animals among jawless vertebrates and closer to lampreys than to amphioxus. Conodonts are known mostly from abundant disarticulate toothlike microfossils. The most recent work meant that conodonts finally yielded the title "fossils of unknown affinities." They had eyes, an asymmetrical ray-supported tail fin, and a notochord (the forerunner of the spinal column of higher vertebrates), as reported by M.A. Purnell of the University of Leicester, Eng., and I.J. Sansom and M.P. Smith of the University of Birmingham, Eng., and colleagues. Twenty-nine researchers from around the world devoted a full day to the origin and evolution of whales. Eocene fossils (about 50 million years ago) provide the missing links documenting the transition of land mammals to amphibious whales that lived along rivers to marine whales, as reported by J.G.M. Thewissen of Northeastern Ohio Universities College of Medicine and colleagues.

      Other advances in the study of vertebrates included new information on dinosaurs. Gregory M. Erickson of the University of California, Berkeley, and colleagues reported that according to the results of their experiments, Tyrannosaurus rex had very strong, impact-resistant teeth that could withstand the stresses associated with struggles during prey capture. Their data did not resolve the debate as to whether T. rex was a hunter or a carrion feeder; they did show that T. rex was not mechanically limited by its dentition to scavenging carrion. John A. Ruben of Oregon State University and colleagues reported that their analyses of the nasal regions of four dinosaur species indicated that dinosaurs had metabolic rates significantly lower than those in modern warm-blooded animals. Their data were derived from the study of the cross-sectional area of the nasal passages and the presence or absence of nasal turbinate bones, which in warm-blooded animals are involved in warming and cooling the blood during respiration. As the Washington Post noted in its Sept. 2, 1996, issue: "Paleontology: Cold-Blooded Idea Ahead by Nose." Paul Sereno of the University of Chicago and colleagues announced the discovery of two large carnivorous dinosaurs from Cretaceous rocks (about 90 million years ago) of Morocco. The larger dinosaur, Carcharodontosaurus saharicus, had a skull measuring 1.63 m (64 in), which may be larger than that of the largest known T. rex. The other dinosaur, Deltadromeus agilis, had long, slender limbs, which suggested agility and speediness.

      Paleobotanists held their twice-a-decade international meeting in Santa Barbara, Calif. A major theme was early land plants and the environments of early terrestrial ecosystems. C.L. Hotton and F.M. Hueber of the Smithsonian Institution, Washington, D.C., discussed evidence for environmental partitioning among Lower Devonian (about 400 million years ago) plants with embryos in the rocks of Gaspé, Que. T.N. Taylor of the University of Kansas and colleagues reported that in the Lower Devonian rocks of Scotland, fungi functioned as saprophytes (living on decayed material), parasites, and various types of mutualists (two organisms living together for the benefit of both). Lichen terrestrial mutualism is also present in these rocks. William Shear of Hampden-Sydney (Va.) College and Paul Seldon of the University of Manchester, Eng., noted that terrestrial arthropods are known to occur with vascular and nonvascular land plants in rocks ranging in age from Late Silurian to Late Devonian (about 410 million to 360 million years ago) in both North America and Europe. Shear and Seldon indicated that none of the arthropods known to date are herbivores but rather are detritus feeders or predators. Thus, in early terrestrial ecosystems, plants and animals were decoupled in the food chain, and primary productivity flowed through detritivores. At the NAPC, C.C. Labandeira of the Smithsonian presented data showing that by Late Pennsylvanian time (about 295 million years ago) insect herbivores were partitioning food use of plant tissues in major and essentially modern ways.

      David A. Grimaldi of the American Museum of Natural History, New York City, was directing the collecting of rich deposits of amber-preserved fossils in the Cretaceous rocks of New Jersey; the amber is about 90 million to 94 million years old. To date, about 100 previously unknown species of insects and plants were identified. Included in this amber treasure trove were a mushroom, a bee, a mosquito, a moth, a blackfly, flowers, and a feather.

      The year was one of festivals celebrating fossils. In addition to the standard professional and amateur gatherings, Dinofest International was held in April at Arizona State University, Tempe, and Fossilfest at the Museum of Natural History and Science in Cincinnati, Ohio. In November the Florida Museum of Natural History, Gainesville, served as host for Paleofest 96. In part, all three festivals were sponsored by the Paleontological Society. They were designed to increase the public's knowledge about fossils and to give hands-on experience with collecting and identifying fossils. The three festivals attracted at least 250,000 people. (JOHN POJETA, JR.)

      See also Anthropology (Anthropology and Archaeology ); Earth Sciences ; The Environment (Environment ).

      This article updates evolution, theory of (evolution); dinosaur; geochronology.

▪ 1996

Introduction

ZOOLOGY

      A keener awareness of global conservation issues emerged during 1995 from research involving a variety of animal groups. In addition, scientists discovered new reproductive traits related to mate selection, parental care, and the induction of egg hatching in several species.

      To examine factors that control the presence and absence of animal species on islands and that contribute to the success of colonization, Thomas W. Schoener and David A. Spiller of the University of California, Davis, conducted a difficult but informative experiment. Their objective was to test the relative influence of island size and the presence of predators on the colonization success of prey species. They selected islands from a chain in The Bahamas, of which five each were large ones with predatory lizards, Anolis sagrei; large ones without lizards; and small ones without lizards. They selected a common orb spider, Metepeira datona, native to the region but absent on all of the test islands, as the artificially introduced prey species. In the first year of the experiment, spiders of both sexes were released on each of the 15 islands, and in the following year three times as many were released. By the end of the five-year experiment, the introduced spiders were extinct on all islands with lizards. One small island still had spiders, and three of the large lizardless islands had enormous spider populations. The investigators concluded that the presence of predators strongly influenced survival success and persistence of spiders. In some ways island size was less important, suggesting that more emphasis in conservation ecology should be given to studying predation effects on islands.

      Zoological studies in temperate climates offered evidence that threats to biological diversity and to the environment are global in scale and not confined to tropical terrestrial ecosystems or restricted to less developed countries. Charles Lydeard and Richard L. Mayden of the University of Alabama reported on imperiled aquatic animals of the rivers and streams of the Mobile Basin in the southeastern U.S. They showed that the biodiversity of native fish, aquatic snails, mussels, and turtles in the temperate-zone ecosystem rivals that of many higher-profile tropical systems. The extraordinarily high species diversity found there was attributed to circumstances of the area's surface features and river-drainage history. Many species in the region remained undescribed, and the ecology and life history of the majority were poorly known. Numerous snails and mussels and at least two fish species in the region were known to have become extinct in the past century. Because declines and extirpations of species populations can be directly attributed to general habitat degradation, Lydeard and Mayden emphasized the importance of strengthening environmental protection regulations to safeguard entire ecosystems rather than just particular species.

      Karen A. Kidd and David W. Schindler of the University of Alberta and colleagues offered an explanation for the presence of unusually high levels of the pesticide toxaphene in fish from a subarctic lake. The use of toxaphene as an agricultural insecticide and fish-killing agent was discontinued in the U.S. and Canada in the 1980s but continued in Mexico, parts of South America, Africa, and Asia. The chlorinated compound is transported via the atmosphere to Arctic regions and in 1991 was detected in lake trout (Salvelinus namaycush), burbot (Lota lota), and whitefish (Coregonus clupeaformis) from Lake Laberge, Yukon Territory, at levels considered hazardous to human health. Although the same fish species in other lakes in the region also contained toxaphene, levels in the Lake Laberge fish were higher. Kidd, Schindler, and co-workers showed that the higher levels were caused by biological concentration, or biomagnification, of toxaphene up an unusually long food chain. In the other lakes the fish in question eat mostly invertebrates, whereas those in Lake Laberge feed heavily on other fish. By occupying the top of a longer food chain, the Lake Laberge fish accumulate more toxaphene than do the same fish species in other regional lakes.

      Bill Amos of the University of Cambridge and colleagues found evidence of mate fidelity in the gray seal (Halichoerus grypus), generally considered a purely polygynous species in which males compete with each other for territories and mates. Seals born on the island of North Rona, Scotland, were genetically analyzed to determine which pups were full siblings—i.e., shared the same father and mother. The number was high, although dominant males in the breeding colony fathered an unexpectedly small proportion of the full sibs, indicating significant partnering between females and subordinate males. The investigators concluded that although some males are polygynous, many show partner fidelity, mating with the same female year after year. Partner fidelity should increase survival rates of seal pups by reducing fights between males over females, which by disturbing the clan are often a major cause of pre-weaning deaths.

      Rudolf Diesel and Gernot Bäurle of the University of Bielefeld, Germany, and Peter Vogel of the University of the West Indies, Kingston, Jamaica, reported the first known instance in which frogs breed in caves and transport their fully developed young to the outside. The investigators observed male Jamaican frogs (Eleutherodactylus cundalli) calling from as far back as 87 m (285 ft) from the cave entrance to attract females. After mating, females laid eggs in the cave in total darkness, attended them for about a month, and then carried the hatchlings out of the cave on their backs. Egg laying in caves and the subsequent transport of newborn frogs were theorized to have evolved as a way to maintain developing eggs in a relatively predator-free environment and then to place the young in a productive habitat after birth.

      Karen M. Warkentin of the University of Texas reported the first known instance in which egg hatching is induced by a predator. The red-eyed tree frog (Agalychnis callidryas) of Middle America lays eggs on leaves overhanging temporary ponds. At hatching, which occurs 5 to 10 days after the eggs are laid, the tadpoles drop into the water below. The primary predator of the eggs is the cat-eyed snake (Leptodeira septentrionalis), whereas fish and other animals prey on the tadpoles. Older hatchlings were found to be less vulnerable to aquatic predators than younger ones; therefore, later hatching favoured the survival of tadpoles. In experiments comparing the timing of hatching, most undisturbed eggs hatched late, but eggs under attack by snakes were seen to hatch within minutes, sometimes seconds. The escaping tadpoles entered the water at a more vulnerable stage but managed to avoid egg predators. Such plasticity in an animal's life history can be highly advantageous. A hatching age that changes with conditions to maximize survivorship should increase fitness in a variable environment.

      (J. WHITFIELD GIBBONS)

      This updates the articles animal behaviour; biology; mammal; evolution, theory of (evolution).

Entomology.

      Most modern insects possess wings and can fly. The evolutionary development of wings and flight in insects, however, is obscure because of the lack of transitional forms between earlier wingless and later winged forms in the fossil record. To demonstrate a possible intermediate step in wing evolution, James H. Marden and Melissa G. Kramer of Pennsylvania State University investigated primitive aquatic insects called stoneflies that have wings but do not fly. Taeniopteryx burksi move across water by surface skimming, with the body supported by water but propelled forward by the wings. Allocapnia vivipara sail across water by raising their wings to catch the wind. In experiments comparing surface-skimming and sailing abilities in stoneflies having artificially shortened wings of various sizes, the researchers found that greater wing area generally resulted in greater speed. They proposed that ancestral stoneflies and other semiaquatic insects used gill structures to move across the water's surface and that over time the advantages of faster surface skimming or sailing favoured an increase in the size of those structures, ultimately leading to wings and wing-powered flight.

      The defensive behaviour with which some honeybees (genus Apis) respond to attacks by hornets may be an example of coevolution, according to a study carried out in Japan by Masato Ono and colleagues of Tamagawa University, Tokyo. Giant hornets (Vespa mandarinia japonica) make orchestrated attacks on other social hymenopterans such as honeybees, which they kill with their mandibles and feed to their larvae. The investigators confirmed that an individual giant hornet marks a bee colony with a pheromone (chemical attractant) from specialized glands. Additional giant hornets then congregate and initiate a slaughter attack. Introduced European honeybees (A. mellifera) appear defenseless against the hornets and are killed at rates as high as 40 per minute. Native Japanese honeybees (A. cerana japonica), however, can detect the hornet pheromone and change their behaviour by increasing the number of defending bees. More than 500 bees swarm around an attacking hornet, forming a ball whose internal temperature reaches 47° C (117° F), high enough to kill the hornet but not the bees. European bees seem unaware of the hornet pheromone and do not respond effectively to the hornet attacks. The differential responses of the two bee species suggest that the Japanese honeybees have coevolved with the predator and developed an effective defense.

      The evolutionary history of symbiotic relationships between fungus-growing ants (tribe Attini) and their fungi was investigated by Ulrich G. Mueller and colleagues of Cornell University, Ithaca, N.Y., and the U.S. Department of Agriculture. Using ribosomal DNA analysis and morphological characteristics to compare phylogenies, or evolutionary family trees, for the ants and their fungi, they concluded that whereas the ants originated from a single ancestral form, the cultivated fungi had more than one ancestral line, which indicated that ants developed symbiotic relationships with different fungal lineages. They also found that the less primitive, generally more specialized species, including the leaf-cutting ants, have been associated with the same fungal lineage for at least 23 million years. In a related study Mitchell Sogin of Woods Hole (Mass.) Marine Biological Laboratory and colleagues, using ribosomal RNA analysis, concluded that the less-primitive leaf-cutting ant species and their fungal symbionts have undergone long-term coevolution. A notable feature of the relationship that exists between ants and fungi is that one symbiont may be inconspicuous yet be essential to the survival of the other. (ANNE R. GIBBONS)

      This updates the article insect1 (insect).

Ornithology.

      Identifying the factors that regulate the number of birds of a particular species breeding in a particular area has been a difficult task but is one of fundamental importance in the study of the natural regulation of animal numbers. I. Newton of the Institute of Terrestrial Ecology, part of the U.K. Natural Environment Research Council, reviewed the results of experiments on the limitation of the densities of breeding birds. In general, densities can be limited by resources such as food or suitable nest sites or held at a lowered level by predators, parasites, or other natural enemies. Among a group of 18 experiments in which supplementary winter food was provided by the experimenter, 11 showed an increase in nesting-season densities compared with control areas. Four experiments in which the summer food of insect-eating forest birds was depleted by the use of insecticides resulted in no reduction in the density of nesting pairs. In a third group of experiments in which additional nest sites (boxes) were provided, density increased in 30 cases out of 32. Among experiments in which natural predators of the birds were removed, 14 out of 15 led to increased hatching success, 4 out of 8 to higher post-breeding numbers, and 6 out of 11 to increased breeding density. Taken together, the experiments confirmed that all major potential external limiting factors can affect breeding density of one bird species or another. They also confirmed that a particular species limited by one factor in certain years or areas may be limited by a different factor in other years or areas.

      It was well known that birds act as important dispersers of plant seeds by voiding not only the seeds of consumed fruit but also the remains of the fruit material, which has been converted into useful fertilizer. That a fruit has evolved to contain a laxative for speeding the seed through the bird's digestive system was revealed for the first time by Greg Murray of Hope College, Holland, Mich. He showed that the fruits of Witheringia solanacea, a Central American bush, pass quickly through the gut of the black-faced solitaire (Myadestes melanops) of Panama and Costa Rica and are thus more likely to germinate.

      Newly discovered bird species included the chestnut-bellied cotinga (Doliornis remseni), a thrush-sized fruit eater from the Andes of Ecuador, and the diademed tapaculo (Scytalopus schulenbergi), a small, secretive, fast-running bird of the cloud forest, which was discovered near La Paz, Bolivia, but later was shown to be common at 900 m (3,000 ft) altitude and above in Bolivia and neighbouring Peru. In a semideciduous Brazilian forest was found a previously unknown member of the Tyrannidae (the tyrant flycatcher family), which was named the Bahia tyrannulet (Phylloscartes beckeri). Brazil also yielded a new nighthawk, Chordeiles viellardi, a bird of the caatinga vegetation common in the state of Bahia. From Africa was reported a new nightjar (a close relative of the nighthawk, both groups being insect feeders active at dawn and dusk) dubbed the Nechisar nightjar (Caprimulgus solala). Nechisar is a plain in southern Ethiopia. The Indian Ocean revealed a new long-winged seabird, the Mascarene shearwater (Puffinus atrodorsalis).

      The monumental, nine-volume The Birds of the Western Palearctic (i.e., Europe, North Africa, and the Middle East), easily the most detailed reference to the birds of any major region of the Earth, was completed with publication of its last two volumes. The first volume had appeared in 1977. In total the series covered 770 species. (JEFFERY BOSWALL)

      This updates the article bird.

MARINE BIOLOGY

      Studies reported from the Finnish research vessel Aranda in 1995 demonstrated a reversal of a recent trend detected in the Gotland Basin of the Baltic Sea toward reduced oxygen and increased hydrogen sulfide concentrations in the water of the basin. In 1993 a major inflow of North Sea water had occurred, and it was thought that the event created favourable preconditions for even small subsequent inflows to increase oxygen concentration drastically in the basin. Another report focusing on northern Europe presented the results of a comprehensive analysis of the effects of offshore gas and oil exploration and production on bottom-living animals of the Norwegian continental shelf. The study showed that oil-based drilling fluid resulted in severe depletion of key species, some of which serve as food for bottom-living fish. Whereas replacement organisms were abundant, they were too small and too deeply burrowing to serve as substitute food for fish. In work in the U.S., researchers reported that the oyster population in the Maryland portion of Chesapeake Bay had fallen 50-fold since the early 1900s. The decline was blamed on habitat destruction and overfishing, not on worsening water quality and disease, as had been previously thought.

      Phytoplankton is the plantlike part of the community of the generally minute, drifting organisms called plankton that live at or near the water's surface. Japanese workers showed for the first time that phytoplankton growth can be inhibited by cell-to-cell contact with another organism. The phytoplanktonic flagellate Gyrodinium instriatum was observed to be killed by contact with a species of Heterocapsa, one of the dinoflagellates responsible for the toxic blooms called red tides that occasionally discolour the ocean. A recently described dinoflagellate, Pfiesteria piscicida, was implicated as a causative agent of major fish kills in the southeastern U.S. Pfiesteria responded to as-yet-unidentified substances secreted from fish schools by producing toxins that in laboratory assays proved lethal to 19 species of native and exotic finfish and shellfish. U.S. scientists reported a severe decline (80% since 1951) in the biomass of zooplankton, the animal-like component of plankton, in the ocean off southern California. The decline was correlated with a rise in ocean surface temperatures of more than 1.5° C (2.7° F) in some areas. The warming was thought to have caused zooplankton reduction, and a consequent decline in abundance of fish and some birds in the region, by slowing cold-water upwellings that replenish the surface waters with nitrates and other nutrients.

      In a U.K. study miniature acoustic transponders wrapped in bait were released on the seafloor from an autonomous vehicle at depths of 1,500-4,000 m (4,900-13,100 ft) in the Porcupine Sea Bight of the North Atlantic. Photographs of deep-sea fish taking the baits and sonar records of signals from the swallowed transponders showed that the fish moved out of range of the sonar in three to nine hours, which indicated that they had no home range or territorial behaviour. New threats to the survival of the so-called living fossil known as the coelacanth (Latimeria chalumnae) were reported. In its protected habitats around the island of Grande Comore in the Comoros, the rare lobe-finned fish was being illegally taken by local fishermen who were unable to move their motorized canoes out from shore beyond the designated coelacanth-protection zone.

      Maltese studies revealed the first occurrence in the Mediterranean Sea of "imposex," a phenomenon observed in marine snails whereby females became masculinized and unable to reproduce. This threat to species survival was caused by tributyl tin, until recently a commonly used antifoulant compound in marine paints. Such findings continued to spur searches for less toxic antifoulants, including compounds made naturally by marine organisms. The presence of such a natural antifoulant was suggested in studies in the U.K. of the egg cases of a shark species, the dogfish Scyliorhinus canalicula, which were found to survive cleanly in the sea for as long as 300 days before hatching. Similar properties of resistance to marine fouling were also reported for ascidians, marine animals commonly called sea squirts.

      In Australia the large herbivorous marine mammals known as dugongs (Dugong dugon) were seen to practice "cultivation grazing." They fed intensively in large herds in a manner that favoured rapidly growing, nutritious algae such as Halophila ovalis. As a result, those algal species thrived at the expense of slower growing, normally dominant species such as Zostera capricorni, which the dugongs favoured less. A female green turtle (Chelonia mydas) was fitted with a radio transponder and tracked by satellite in the South China Sea from its nesting beach to its normal foraging grounds more than 600 km (370 mi) away. The final 475-km (295-mi) leg of the journey brought the turtle directly to its goal with pinpoint accuracy, the animal maintaining constant speed and direction by day and night. The best explanation for such precise orientation seemed to be a geomagnetic compass similar to that previously reported in birds, honeybees, and other animals. (ERNEST NAYLOR)

      This updates the articles crustacean; fish; mollusk.

BOTANY

      The publication in June of Botany for the Next Millennium by the Botanical Society of America (BSA) was a landmark in the history of botany. The report, the result of a collaboration of the BSA and nine other scientific societies that represented diverse interests ranging from mycology and lichenology to taxonomy and chemical ecology, established a framework for identifying research and educational goals, priorities, and opportunities in the botanical sciences on the eve of the 21st century. One set of goals addressed what the report called “vigor of the profession” and suggested actions that would help maintain botany's record of achievement and discovery. A second set addressed “continuity of the profession” and included actions designed to attract future botanists and support their training. A third set spoke to “integration into the community” in order to broaden the relevance of botanical knowledge beyond academia. The report was needed because, although plants and plant products were ubiquitous in daily life, botany was underemphasized in biology curricula at all levels of education. The brief (54-page), easily readable publication was expected to spur improvements in botanical education and help focus research efforts in the coming years.

      Within the space of eight months, the U.S. journal Science produced two special issues focusing on subjects important to botany. One reviewed recent discoveries in plant biotechnology, and the other dealt with aspects of developmental biology. By 1995 plant biotechnology had become an emerging and rapidly expanding field in which the techniques of the molecular biologist were being applied to practical problems in plant science. Reports in one of the special issues included descriptions of plants that had been genetically engineered to synthesize specific substances that stimulate the animal immune system, with the eventual goal of making plant-produced vaccines against human diseases. There was even speculation that vaccines could be built into plants that then would be eaten as part of a normal diet. Other reports described ways in which biotechnology was helping to transform the concept of plant-based raw materials. For instance, researchers engineered cotton plants to produce natural fibres incorporating some of the desired characteristics of synthetic polyesters. Other potential products from engineered plants included naturally produced biodegradable plastics and industrial lubricants. Among the more promising studies were those aimed at a better understanding of plant disease-resistance mechanisms and the pertinent genes so that plants might be better equipped to protect themselves against the wide array of disease organisms that attack them. At the very least, such studies could lead to novel strategies for plant-disease control and less reliance on traditional chemical pesticides.

      No matter how complex the form of a mature organism, all sexually reproducing organisms, after fertilization, begin as single cells called zygotes, which are nearly spherical in shape. In higher plants, once fertilization has been achieved, the zygote begins to differentiate into an embryo. Polarity, the development of recognizably different ends, begins with the first cell division. Although the precise genetic and molecular mechanisms that result in embryo formation and polarity were still poorly understood, major insights were gained from recent studies of plant embryo mutants and others that used molecular approaches. These were summarized in the Science special issue on development by Robert Goldberg and collaborators of the University of California, Los Angeles. Taken together, the research suggested that the structure of a developing plant embryo is modular, with several regions forming independently.

      Parasitism, an association between two different organisms in which one benefits at the expense of the other, is widespread in nature. Mutualism, an association in which both organisms benefit, is also widespread although less commonly noticed. Mutualism is a characteristic of a large group of plantlike organisms called lichens, which develop as an association between an alga and a fungus. The alga photosynthesizes and produces food for the fungus, which in turn absorbs water and perhaps supplies other benefits to the algal partner. It had been accepted by many that lichen associations probably started as parasitic associations, which in the course of evolution transformed into mutualistic ones. A collaborative study among scientists of the Smithsonian Institution, Washington, D.C., Graz (Austria) University, and the University of Stockholm challenged that idea. By comparing DNA sequences taken from different fungus species, including lichen-derived fungi, and using the relationships to construct a fungal family tree, they showed that the alga-fungus association originated independently at least five times and involved very different groups of fungi whose lifestyles range from benign to parasitic. On the basis of such results, the authors found no support for the frequently expressed notion that mutualism must begin with parasitism. (PHILIP D. REID)

      This updates the articles biosphere; conservation; plant.

MOLECULAR BIOLOGY

Mitochondria, Aging, and Disease.

      Aptly called the powerhouses of the cell, mitochondria are the organelles responsible for most of the cell's respiration and energy production. They possess their own DNA that is distinct from that which makes up the chromosomes of the cell nucleus. In 1995 more evidence of the importance of this "other genome" came to light as researchers continued studying mitochondrial genetics and its role in aging and disease.

      About the size and shape of bacilli, mitochondria are bound by two membranes, as are some bacteria. Indeed, it is likely that mitochondria arose from an ancient symbiosis between a bacterium and a primitive amoeba-like cell. As befits an association that has lasted a billion years, there have been accommodations. For example, the bacterial symbiont lost its cell wall, which was superfluous in the protected environment provided by the host cell. Also, its inner membrane became corrugated, increasing greatly in surface area to accommodate the extra molecular machinery required for meeting the energy needs of the host cell. The host cell, for its part, took over most of the biochemical chores for the symbiont's replication and maintenance. The host benefited from the ability of the symbiont to trap the abundant energy released during aerobic respiration—the oxygen-dependent breakdown of foodstuff molecules—while the symbiont benefited from the stable environment and nutrients supplied by its host. The association has proved highly successful, as evidenced by its numerous and diverse progeny, which constitute all macroscopic life on Earth.

      There are hundreds of mitochondria in the average cell. Perhaps as a relic of its symbiotic origin, each mitochondrion retains a bit of its own DNA, which codes for 13 different proteins and 24 different RNA molecules that assist in protein synthesis. It retains the ability to replicate its DNA and make its proteins, which are essential components of its energy-producing and energy-trapping functions.

      Both egg and sperm cells contain mitochondria. During fertilization in humans and in nearly all other animal species, however, the mitochondria of the sperm are not incorporated into the fertilized egg. Consequently, mitochondrial genes are transmitted to offspring only by the mother.

      DNA, the repository of the genetic information of the cell, is not a perfectly stable storage medium, and changes can creep in for a variety of reasons. Cells go to great lengths to minimize such changes and to repair those that do occur. Yet some changes persist and, if they are transmitted to progeny, are the cause of mutations, which often are deleterious.

      Mitochondrial DNA is at greater risk of mutation than is nuclear DNA. The reasons remained to be fully understood, but it was clear that damage accumulates in mitochondrial DNA 10-20 times faster. Such damage, as investigators were learning, is involved in senescence—i.e., the biological changes related to aging—and disease.

      A decrease in the usable energy available to cells and tissues as they age would necessarily undermine their function, and a decline in mitochondrial integrity would certainly curtail that energy supply. When researchers looked for evidence to support the idea that mitochondrial integrity declines with age, they found it. Mitochondria isolated from aged animals were seen to be enlarged, full of cavities (vacuoles), and lacking in the degree of inner-membrane corrugation seen in the mitochondria of young animals. Senescent mitochondria also were fragile and less likely to survive the isolation procedure itself, which means that the most severely affected mitochondria were likely underrepresented in the observations.

      In spite of the difficulty in isolating senescent mitochondria, scientists detected a number of age-related losses of function, including less-efficient coupling of respiration to the production of useful energy and a decline in the activities of enzymes crucial to respiration. Furthermore, they found that mitochondrial DNA from aged animals contain a variety of genetic damage, which can reduce or destroy mitochondrial function.

      How is it that the cell can tolerate such damage at all? The answer lies in the large numbers of mitochondria in each cell and of DNA molecules in each mitochondrion. Damage thus has a graded effect, with a little cumulative damage causing little loss of function for the cell and more damage causing more loss. Different tissues are dependent to different degrees on the metabolic energy production by mitochondria and will reflect to different degrees the cumulative damage to their mitochondria. Whereas damage to mitochondrial DNA in somatic (nonreproductive) cells may be a problem for the individual, it will not be passed on to offspring. On the other hand, damage that occurs in egg cells may or may not be transmitted to progeny. Why is this the case?

      The fertilized egg contains about 200,000 molecules of mitochondrial DNA. In the early stages of development, however, cells divide without replicating their mitochondrial DNA; consequently, the number of copies per cell falls dramatically. Each cell destined to give rise to different tissues in the developing embryo thus receives a relatively small number of molecules of mitochondrial DNA. If that DNA is seriously defective, the result is death; if it is moderately defective, the result is transmission of a genetic disease.

      The first mitochondrial disease was described in the 1960s by investigators who were attempting to understand the symptoms of an extremely emaciated, weak, feverish patient who consumed enormous amounts of food and liquids and sweated profusely. Her basal metabolic rate was nearly double the normal value. After eliminating hyperthyroidism as the possible cause, the investigators realized that her symptoms might be explained by "uncoupled" mitochondria—that is, mitochondria in which respiration was liberating energy from foodstuffs but not trapping it in metabolically useful form. Indeed, when mitochondria from muscle tissue of the patient were isolated, they were found to be uncoupled.

      This pioneering discovery opened the door to the field of mitochondrial diseases. Other investigators followed the lead, linking mitochondrial defects with maladies such as Kearns-Sayre syndrome, chronic external opthalmoplegia, and myoclonic epilepsy and ragged red-fibre disease. In 1995 medical science knew of about 120 mitochondrial diseases. As expected, they are maternally inherited, and they tend to affect specific tissues because of the different dependence of various tissues on mitochondrial energy production. Many make their appearance only later in life because the defects that accumulate with age add to those that are inherited and must reach a critical threshold level before symptoms appear.

      Knowledge of the cause of a problem often leads to a solution. Consequently, researchers were optimistic that their growing appreciation of the complexities of mitochondrial genetics would eventually produce practical benefits.

      (IRWIN FRIDOVICH)

Telomeres: New Beginnings from Old Ends.

      A major distinction between prokaryotes, or bacteria, and eukaryotes, or so-called higher organisms made of nucleated cells, is the manner in which they arrange the DNA of their genetic endowment, or genome. Bacteria generally maintain their genomes as circular molecules, whereas animals and plants maintain their nuclear genomes as collections of linear molecules, called chromosomes. Although a linear architecture has its benefits, it also presents problems—perhaps most notably, what to do about the ends.

      The trouble with having ends is at least twofold. First, free ends on DNA molecules are notoriously unstable; they degrade chemically and undergo recombination much more often than their non-end, protected counterparts. Second, the DNA polymerase enzyme that is responsible for replicating the nuclear genome during cell proliferation has difficulty copying the very ends of DNA molecules, so without special precaution the end molecular sequences tend to be lost in the copies. To circumvent the problems, eukaryotic cells cap their chromosomes at both ends with specialized structures called telomeres. New evidence gathered in a number of laboratories suggests that telomeres and the enzyme or enzymes that create and maintain them play key roles in cellular aging and the immortalization of cells so often associated with cancer.

      The first telomere was isolated in the 1970s from the single-celled ciliated protozoan Tetrahymena thermophila. It was found to consist of 50-70 tandem copies of the short DNA base sequence TTGGGG (T is the base thymine; G is guanine). Both the structure and sequence were considered peculiar at the time, but subsequent work with many different organisms served to validate and extend the observations. For example, all mammals examined as of 1995, including humans, carry the repeated sequence TTAGGG (A is adenine) in their telomeres. Indeed, not only do telomeric sequences from different organisms look alike, but they also may function alike. This was first demonstrated when linear pieces of DNA carrying ciliate-derived telomeres were put into cells of yeast, an extremely distant relative of ciliates. The ends of the DNA remained stable; in other words, the ciliate telomeres worked in yeast. With time, however, the ciliate-derived telomeric sequences eroded and were replaced by the corresponding yeast sequences.

      The gradual erosion of the ciliate-derived telomeric sequences suggested that telomeres do not escape the fate of unprotected DNA ends during cell replication; they simply buffer the loss. Indeed, telomeric sequences made of multiple repeats are, in retrospect, very logical; such sequences can be at least partially sacrificed without losing genetic information. That the eroded ciliate sequences were replaced by yeast counterparts indicated that the repeats are not only expendable but also renewable by means of a cellular activity that is independent of the sequence of existing repeats. That activity was found to be carried out by a most unusual enzyme, named telomerase, that consists of both RNA and protein. Subsequent experiments involving the ciliate Tetrahymena revealed that if telomerase is inactivated (by a mutation, for example), the telomeres in the mutant cells grow shorter and shorter; moreover, the single-celled organisms, which normally do not have a finite life span, eventually die. In other words, in the absence of functional telomerase, the length of a cell's telomeres appears to have an inverse relationship to its age.

      The mortality of human beings and most other living creatures is a characteristic not only of the body as a whole but also of most of the body's cells, even cultured cells. Although aging is clearly a complex process that likely reflects the interactions of many genes and environmental factors, a variety of recent observations imply a role for telomeres and telomerase in the replicative senescence of cells. For example, it was found that proliferating cultured human fibroblasts, which normally die after a finite number of divisions, lack telomerase activity; as they age, their telomeres become gradually, and ultimately profoundly, shorter. Similar observations were made for somatic cells functioning normally in the body; in other words, chromosomes from the cells of younger people tend to have longer telomeres, while those from older people tend to have shorter ones. This is consistent with the general observation that cells taken from younger donors tend to live longer in culture than do cells from older donors. Indeed, in a study of cultured fibroblasts from 31 donors ranging from newly born to 93 years in age, investigators saw a striking correlation between initial telomere length in the donated cells and ultimate proliferative capacity. In addition, they found that telomeres from fibroblasts donated by Hutchinson-Gilford progeria patients, who experience abnormally rapid aging, were unusually short.

      In contrast to the findings in somatic cells, studies of normal human and nonhuman cells that are naturally "immortal," namely, germ, or reproductive cells, revealed that the telomeres of those cells appear to be stable with time. For example, telomeres from normal human sperm do not shorten with age, which suggests that some mechanism such as telomerase activity maintains telomere length. Consistent with this idea, telomerase activity was directly observed in frogs' eggs. All this information taken together suggests that telomerase activity is generally absent from normal mortal cells, which thus experience replicative telomere shortening, but is found in normally immortal cells, such as protozoans or germ cells. If this is true, what then of normally mortal cells that become immortalized, such as cancer cells?

      The potential role of telomeres and telomerase in cancer has been revealed in at least two ways. First, experiments were conducted in which normal cultured cells, which lacked telomerase activity, were exposed to oncogenic (cancer-inducing) DNA sequences derived from tumour viruses. Whereas most of the cells died after a number of replications, some continued to divide without limit, presumably owing to transformation by the viral DNA. Under examination the telomeres of the newly immortalized populations were found to be stable with time, and telomerase activity was observed. The results suggested that the switching on of telomerase activity, presumably by activation of one or more otherwise silent genes, might be part of the process whereby normal cells are transformed into cancer cells.

      A second line of information came from studies of cells taken directly from tumours removed from patients. Again, in sets of matched cells derived either from tumours or from the corresponding normal tissues, telomerase activity was found in tumour cells but not in normal cells.

      These studies offered considerable food for thought about how telomere shortening interacts with other factors involved in the aging process, for both replicating and nonreplicating cells. On a much more practical note, they pointed to telomerase as a possible new target for anticancer drugs. Indeed, during the year researchers worked to develop specific and effective inhibitors against the enzyme, following the logic that if telomerase can be inactivated in tumour cells, the cells may become mortal again and eventually die. Because most normal cells already lack telomerase activity, a truly specific inhibitor should cause them little if any harm. (JUDITH FRIDOVICH-KEIL)

      See also Chemistry (Mathematics and Physical Sciences ).

      This updates the articles cancer; cell; disease; heredity; biological development; reproduction.

PALEONTOLOGY

      Paleontological discoveries during the year shed light on the phylogeny of living and extinct organisms. Neil H. Shubin of the University of Pennsylvania and Farish A. Jenkins, Jr., of Harvard University reported the discovery of the fossilized remains of a jumping frog from the Early Jurassic Kayenta formation of Arizona. The species, which the investigators named Prosalirus bitis, lived about 190 million years ago. Although the fossil retained some of the primitive characters of earlier amphibians, certain features of the pelvic girdle, legs, and feet were clearly indicative of saltatory, or leaping, locomotion. The investigators concluded that the species unquestionably belonged within the Anura, the frog and toad order, and was the order's earliest known member.

      The question of whether dinosaurs were endothermic or ectothermic (warm-blooded or cold-blooded) has long been an issue of intense debate. The physiology of endothermic animals enables them to maintain a nearly constant body temperature in the face of varying environmental conditions and, as a result, to maintain high levels of activity. Among ectothermic animals, on the other hand, body temperature and, consequently, level of activity vary significantly under changing conditions. Although dinosaurs originally had been thought to be ectothermic, like living reptiles, some paleontologists had argued, largely on the basis of skeletal characteristics, that they were endothermic, like birds and mammals.

      John Ruben of Oregon State University and his former student Willem Hillenius took a stride toward possibly resolving the issue. They proposed that the key lay in the respiratory turbinates, which are found in the nasal passages of living mammals and birds but are absent in living reptiles. These bony plates, which in life are covered with a thin membrane, provide a large surface of exposed tissue. The tissue and the blood that it contains are cooled by inhaled air and then are reheated as warm air is exhaled, thus providing an important mechanism for maintaining body temperature. Because the turbinates are delicate, they are seldom preserved in fossils. According to Ruben and Hillenius, however, the turbinates are attached to a distinct ridge of bone in the nasal passage, which the investigators identified in fossil mammals dating back to the origin of the class in the Jurassic Period some 160 million years ago. On the other hand, they found no such evidence in the dinosaur fossils that they examined. Opponents argued that dinosaurs still may have had turbinates or may have evolved other mechanisms of thermoregulation. Whatever the outcome of the debate over dinosaurs, the findings of Ruben and Hillenius were an important contribution to scientific knowledge of the physiology of fossil vertebrates.

      Paleontologists have long been puzzled by the so-called Cambrian explosion some 540 million years ago, when most of the great phyla that were to dominate the subsequent history of life appeared, in geologic terms, quite suddenly. Whereas many Precambrian fossils were known, none of them contributed much to an understanding of the remarkable increase in animal diversity at the start of the Cambrian Period. During the year a report drawn from knowledge of developmental biology offered an explanation for the lack of any evidence that would presage the dramatic events that were to occur at the beginning of the Cambrian. E. Davidson and R.A. Cameron of the California Institute of Technology and K. Peterson of the University of California, Los Angeles, argued that before such wide diversification could occur, organisms had to overcome a barrier that limited their size and complexity by restricting to about 10 the number of times a fertilized egg could divide. According to the authors, the solution lay in the appearance of cells, of a kind found in the embryos of some organisms living today, that are not destined to develop into a particular kind of tissue. It was those cells, the authors contended, that may have provided an opportunity for the evolution of much larger and more complex organisms. Davidson and his colleagues concluded that this crucial development would have taken place in soft-bodied, embryolike animals, which almost never would have left any trace, hundreds of millions of years before the Cambrian explosion.

      Clusters of fossil dinosaur eggs presumed to be the remains of nests have been known for decades. A discovery in south central Mongolia, however—an example of the recent continuing Chinese-U.S. cooperation—appeared to demonstrate that dinosaurs not only laid eggs in clutches but tended to them as well. In a press conference at the American Museum of Natural History, New York City, it was announced that the skeletal remains of an oviraptor, a toothless predaceous dinosaur about the size of an ostrich, were found positioned with at least 15 eggs in such a way as to suggest that the animal had been sitting on the eggs when it was killed in some catastrophic event, perhaps a sandstorm, in the late Cretaceous Period about 80 million years ago. The eggs, which measured about 18 cm (7 in) long, were found arranged in a circular pattern under the skeleton. Mark Norell of George Washington University, Washington, D.C., who found the specimen, pointed out that, judging from the skeleton, oviraptors were more closely related to birds than to other meat-eating dinosaurs—which perhaps accounted for the birdlike brooding behaviour. (DAVID B. KITTS)

      See also Geology and Geochemistry (Earth and Space Sciences ); The Environment (Environment ); Libraries (Libraries and Museums ) and Museums (Libraries and Museums ).

      This updates the articles evolution, theory of (evolution); geochronology.

▪ 1995

Introduction

ZOOLOGY

      Ecologically oriented research in zoology in 1994 revealed the potential sensitivities and responses of populations of animals to human-caused alterations in the environment. Wolves and Atlantic cod were at the focus of work that addressed the spatial ecology and movement patterns of animals. The fossil record provided further support for the evolution of whales from a terrestrial ancestry to the marine environment. New species of mammals were reported from Indonesia and Vietnam.

      Andrew R. Blaustein and colleagues of Oregon State University conducted experiments to test the sensitivity of amphibian eggs to ultraviolet (UV) radiation. Scientists had suggested that increased UV radiation levels as a consequence of the destruction of the ozone layer in the Earth's upper atmosphere could be a reason for reported declines in amphibian populations in many regions of the world. In a study of the developing eggs of three species of frogs from the Cascade Range in Oregon, the investigators used a light-filtering apparatus to modify the amount of UV light to which eggs in experimental enclosures were exposed. Eggs of two of the species showed significantly greater hatching success in the treatments in which UV light had been blocked than did eggs receiving natural sunlight. Eggs of the frog species that appeared resistant to UV light were found to contain high levels of photolyase, an enzyme that repairs UV-damaged DNA. The findings supported a hypothesis that amphibian eggs are sensitive to UV light and that human-induced increases in levels of UV radiation were contributing to a decline in amphibian populations.

      Michael C. Newman and Margaret Mulvey of the Savannah River Ecology Laboratory, Aiken, S.C., and colleagues provided evidence that snail populations that have been exposed to high levels of lead in the environment for long periods sequester the toxic metal differently from snails exposed for shorter periods. The investigators sampled populations of the common garden snail Helix aspersa in England and northern Wales and conducted laboratory analyses to determine the level of exposure of each population to lead. To estimate the duration of exposure, they used the isotopic signatures of lead (ratios of the isotopes making up the lead) to determine the proportion of the metal at a site that had been derived from recent human sources (e.g., automobiles) compared with that from older mines and smelters. Thus, they were able to determine the time period over which a snail population had been exposed to high lead levels. In snail populations experiencing long-term exposure (as long as 2,000 years at sites mined since the Roman occupation), the proportion of lead in the shell compared with soft tissues was higher than that in populations experiencing shorter exposure periods (no more than a few decades). One implication of the study was that the sequestering of lead in biologically inert tissues (the shell) provided protection from a toxic material and had been enhanced owing to the continued exposure of the populations, through either genetic selection or physiological adaptation.

      The importance of snails in the food web of a forest ecosystem was revealed by Jaap Graveland of the Netherlands Institute of Ecology, Heteren, and colleagues, who examined the ecological effect that acid precipitation on soils has had in parts of The Netherlands during the past several years. The eggshells of great tits (Parus major) have become increasingly thinner and more porous. Concomitantly, desertion of clutches by the birds has become more common. In the regions that they studied, the investigators documented declines in the species diversity and abundance of snails that are strongly correlated with acidification of the soil by acid rain. They further established that snail shells in the diet are critical for eggshell production in great tits and many other bird species owing to the need for calcium during egg production. High soil acidification in regions with poor soils could reduce bird populations by causing a decrease in reproductive success due to lack of snail shells.

      The patterns in which animal species are spatially distributed are a complex of historical circumstance, response to environmental conditions, and intraspecific and interspecific interactions. George A. Rose of the Department of Fisheries and Oceans, St. John's, Newfoundland, used echo sounders to discover that Atlantic cod (Gadus morhua) migrating across the Newfoundland Shelf followed a deep highway of slightly warmer water (2° -2.5° C, or 35.6° -36.5° F) that flows under colder surrounding ocean water (less than 0° C, or 32° F). The investigator concluded that the fish, which sometimes numbered in the hundreds of millions over many kilometres, were led by larger, presumably older, scouts and that they veered from the narrow band of warm water when prey species were encountered. Mid-water spawning above the warm oceanic highway was also observed. If the migration routes are learned by older fish and used annually, the recent decline in the numbers of Atlantic cod may turn out to have a disruptive effect on cod migration patterns.

      Mark A. Lewis of the University of Utah and James D. Murray of the University of Washington used a simple, mechanistic mathematical model to explain the pattern of territoriality in gray wolves (Canis lupus) and the interactions between wolves and deer. Their model assumed that the direction and distance of wolf dispersal are mediated by the presence or absence of wolf scent markings characteristic of raised-leg urination. They demonstrated that a stable, steady-state condition is reached naturally among individuals and packs of wolves in their responses to scent marks. The model is based on assumptions that upon encountering a foreign scent mark, a wolf tends to increase its own scent marking and move toward the organizational centre of the pack. Thus, the levels of scent marking are greatest between adjacent packs, and buffer zones arise. Deer, and presumably other prey species, are most abundant in the buffer zones, where wolf densities are lowest. A significant feature of the study is that the seemingly complex formation of wolf territories can be reduced to a relatively simple formula involving scent marking.

      Evidence was gathered on the evolutionary origin of swimming and an unusual adaptation for feeding in whales (order Cetacea). Two independent discoveries helped clarify and further define the evolutionary connection between the terrestrial ancestors of whales and their modern relatives. Two new fossil species were found in Eocene sediments (about 50 million years old) in Pakistan. One species, Rodhocetus kasrani, was described by Philip D. Gingerich of the University of Michigan and colleagues, and another, Ambulocetus natans, by J.G.M. Thewissen of the Northeastern Ohio Universities College of Medicine, S.T. Hussain of Howard University, Washington, D.C., and M. Arif of the Geological Survey of Pakistan, Islamabad. The structure of the pelvic and sacral regions of R. kasrani were intermediate between structures designed for terrestrial locomotion and for ocean swimming. Evidence of a terrestrial ancestry in A. natans included the termination of the toes in a convex hoof, the presence of a long tail, and presumably the absence of the tail fluke present in modern cetaceans. By determining the form and structure of the appendages, the investigators concluded that A. natans was able to walk on land in a manner similar to that of sea lions and could swim by moving its feet up and down like an otter.

      The description of a toothed whale (Odontocete) in Lower Pliocene sediments (about five million years old) of southern Peru by Christian de Muizon of the French Institute for Andean Studies, Lima, Peru, provided evidence of evolutionary convergence and specialization in the feeding apparatus. The newly described fossil whale, Odobenocetops peruvianus, whose closest living relatives are the beluga whale and narwhal (family Monodontidae), had orbits (eye sockets) that faced dorsally (upward), possibly indicating binocular vision. The species apparently lacked the melon, a rounded organ in the head of some cetaceans that is used in echolocation. The structure of the anterior portion of the skull suggests that the species had a muscular upper lip, lacked teeth in its upper jaw, and presumably had an adaptation for feeding similar to that of walruses, which feed mainly on mollusks. These structural modifications for suction feeding are extreme among the cetaceans and suggest that O. peruvianus occupied an ecological niche previously unknown among toothed whales and comparable to ones occupied by Pliocene walruses in the Northern Hemisphere.

      New species of large mammals came to the attention of zoologists during the year. Tim Flannery of the Australian Museum, Sydney, and a team of Australian and Indonesian colleagues described a marsupial previously unknown to science—a tree kangaroo found dwelling on a remote forested mountainside in central Irian Jaya, an Indonesian province on the island of New Guinea. The animal, about as large as a medium-sized dog, is thickly furred with unique black-and-white patterns. Scientists from the World Wildlife Fund and the Vietnamese Ministry of Forestry reported the discovery of a new species of muntjac, or barking deer, in a rain forest of the Vu Quang Nature Reserve in central Vietnam. The new species, which was identified by its remains rather than by means of a living specimen, is larger by half than any other known muntjac species, weighs about 45 kg (100 lb), and has a red grizzled coat and long tusklike canine teeth. The animal was the second new species to be discovered in the Vu Quang reserve in recent years. In 1992 a large bovid, the Vu Quang ox (Pseudoryx nghentinhensis), also had been described from an examination of its remains. In June 1994 the World Wildlife Fund reported that after a two-year search, a living example of the Vu Quang ox had finally been located. (J. WHITFIELD GIBBONS)

      This updates the articles animal; animal behaviour; biology.

Entomology.

      During the year scientists employed wing patterns in insects as a means of understanding genetic development. The application of advanced technology gave insight into the mechanisms of prey capture by a predatory ant and the detection of magnetic fields by honeybees. A study involving butterflies that became dependent on human-caused changes to their habitats raised questions about the risks of even more rapid environmental change.

      Sean B. Carroll of the University of Wisconsin and colleagues identified molecular processes involved in the developmental organization of wing patterns in butterflies. They examined the genes responsible for wing patterning in the butterfly Precis coenia and compared them with those of the fruit fly Drosophila melanogaster, about which the molecular events of early development are known better than for any other plant or animal. The investigators established that the organization of butterfly wing patterns is partitioned into two spatial coordinate systems. One comprises a regulatory network that provides information on positioning of elements with respect to the entire wing and operates in a manner similar to that found in fruit flies and possibly other insects. The second system involves some of the same genes and provides genetic instructions during development that elaborate specific elements of the pattern, such as eyespots, on the wing. This second system in butterflies appears to have been modified from one that governs development of other anatomic components and has no counterpart in fruit flies. A significant feature of the research is the prospect of identifying in one group of organisms a molecular process with a function that has evolved from a process with a separate function in another group.

      Wulfila Gronenberg, Jürgen Tautz, and Bert Hölldobler of the Theodor Boveri Institute, Würzburg, Germany, reported that a trap-jaw mechanism used by a Neotropical ant (genus Odontomachus) when hunting prey may lead to a better understanding of the evolution of predator efficiency in prey capture. Using electrophysiological recordings, the researchers demonstrated that trigger hairs located on the inner edge of the ant's mandibles are associated with large sensory cells and function as mechanoreceptors, sensing mechanical stimuli. When prey animals touch the trigger hairs, the jaws close reflexively in less than 8 ms (milliseconds; thousandths of a second), and the actual jaw strike may take as little as 0.33 ms. The underlying neurons are among the thickest and fastest-conducting sensory cells in insects. Such rapid neuronal conduction supports one of the fastest known reflexes and thus leads to one of the fastest movements measured to date in an animal.

      Honeybees are known to use the Earth's magnetic field for such activities as comb building and navigation. The existence of a magnetic-field receptor in the insects had been supported by the finding of magnetite (magnetic iron oxide crystals present in animals that can detect magnetic fields) in the abdomens of dried honeybees. Using high-resolution transmission electron microscopy, Hsu Chin-Yuan and Li Chia-Wei of the National Tsing Hua University, Hsinchu, Taiwan, found iron-containing granules located in the trophocytes, cells surrounding the abdominal segments, and examined their fine structure. The granules were seen to contain tiny magnetite particles, 10 nanometres (10 billionths of a metre) or less in diameter, leading the investigators to suggest that the granules are the magnetoreceptors of the honeybee. They also determined that trophocytes are innervated by the nervous system, thus providing a neural pathway for signals initiated in the bee's magnetoreceptors.

      Michael C. Singer and Camille Parmesan of the University of Texas and Chris D. Thomas of the University of Birmingham, England, reported that two independent populations of a rare butterfly, Euphydryas editha, underwent rapid evolution in diet in response to human manipulation of habitats. At a California site the butterflies had fed primarily on a plant, Pedicularis semibarbata, that was killed as a result of logging operations. Following logging, another plant, Collinsia torreyi, became the preferred host plant for E. editha; during the 1980s the butterflies colonized this new host and rapidly evolved the habit of laying eggs on it. At a separate site in Nevada, a European weed, Plantago lanceolata, that had been introduced by cattle ranchers proved more suitable for E. editha than its traditional, native host plant, Collinsia parviflora. Whereas in 1983 most female butterflies preferred to lay eggs on the native plant, by 1990 most preferred the introduced weed. Experiments showed that this change was genetic and that the preferences in the insect population were evolving rapidly. By 1990 some butterflies refused to accept their traditional host, thus rendering themselves dependent on the modified habitat. If entire populations were to evolve a dependence on the continued existence of a habitat that had been changed by humans, still more human modification could result in elimination of those species in which evolution could not keep pace with the habitat changes. (ANNE R. GIBBONS)

      This updates the article insect1 (insect).

Ornithology.

      Egg production in birds was the subject of a lecture given by C.M. Perrins of the University of Oxford at the 21st International Ornithological Congress, held in Vienna in August. Egg size can vary markedly within a species, and it is not uncommon for some birds to lay eggs that are 50% larger than those of others of the species. The differences in quantities of egg nutrients between small and large eggs appear to represent very small differences in a bird's daily energy budget. In a study of great tits (Parus major), larger eggs were found to be associated with warm weather, low breeding densities of great tits, and low densities of blue tits (P. caeruleus) occupying the same region. Each associated factor can be interpreted as a set of conditions in which food is likely to be more plentiful or in which the laying female is likely to need less food for her own bodily maintenance and so have more available for egg formation. Thus, although the differences in nutrient quantity between small and large eggs may appear tiny, it is possible that they result from responses of the birds to different feeding conditions. It is important for birds to lay large eggs. Larger eggs have a higher hatching success than small ones, a higher fledging success, and a higher weight for chicks that fledge, the increased weight improving the chances of survival. Hence, it remained to be understood why, if large eggs are so advantageous and require so little extra nutrients, birds lay small eggs under many circumstances.

      The evolution of feathers remained an area of ongoing debate among researchers. Did feathers evolve originally for flight or for another purpose, such as heat regulation? Walter J. Bock of Columbia University, New York City, and Paul Bühler of Germany argued in support of the recent theory that feathers evolved for heat regulation, possibly initially to insulate the animal from the heat of the sun and secondarily to prevent the outward escape of body heat. Primitive feathers were most likely similar to contour feathers, not the specialized down feathers found in modern birds. Feathers are associated with obligatory homoiothermy (warm-bloodedness as a sole mode of life), which is energetically expensive; hence, the evolution of feathers must have been allied with important selective advantages. Moreover, the origin of homoiothermy in animals is believed to be connected with lethargic, rather than vigorous, activities. It was thus suggested that the evolution of homoiothermy, and thus of feathers, in the ancestors of birds was coupled with arboreal dwelling and incubation of eggs in a tree nest.

      Bearded vultures (Gypaetus barbatus) that live in the wild have a strikingly rufous colour to the head, neck, and underparts. On the other hand, birds reared in captivity develop pure white plumage. David C. Houston of the University of Glasgow, Scotland, and colleagues reported that caged bearded vultures that were presented with intensely red damp soils became excited and enthusiastically rubbed their belly and head feathers in the soils, acquiring within an hour the characteristic rufous coloration of wild birds. The bearded vulture was the only bird species known to use cosmetic coloration from soils to such spectacular effect.

      Sperm competition, a recently emerged and rapidly evolving concept in avian behavioral ecology, had changed in meaning, according to a report by T.R. Birkhead of the University of Sheffield, England. The term was used initially in a narrow sense to describe the events taking place in a female's reproductive tract following insemination by two or more males. Subsequently it came to encompass all the behaviours associated with copulation, including multiple mating and paternity guards (various means by which a male attempts to ensure that he will be the father of the resulting offspring). Since its inception the term sperm competition had emphasized the male, but with increasing attention being given to female-driven phenomena, such as the fact that females may control which sperm fertilize their eggs, the term could no longer be considered strictly accurate. Theories of selection advanced the idea that because of the fundamental differences between males and females, the interests of individuals of each sex differ, even within socially monogamous pairs, and, thus, so also will their attempts to maximize fitness.

      Lars Dinesen and co-workers of the University of Copenhagen reported the discovery of a new genus and species of bird in Tanzania. Determined to be a distinctive kind of partridgelike bird and named Xenoperdix udzungwensis, the bird is a relict Afro-tropical form with Indo-Malayan affinities. An up-to-date count of the world's known birds, provided by Richard Howard and Alick Moore in their Complete Checklist of the Birds of the World (3rd ed., 1994), listed 9,522 species, subdivided into 26,898 races, in 1,916 genera. (JEFFERY BOSWALL)

      This updates the article bird.

MARINE BIOLOGY

      A study published during the year examined the effects of cleanup procedures on shore recovery following the Exxon Valdez tanker disaster of 1989, when some 38 million litres (240,000 bbl) of crude oil were spilled into Prince William Sound, Alaska. It was observed that although the addition of fertilizers significantly increased rates of oil degradation by naturally occurring microorganisms, the areas most intensively cleaned by this technique and by hot water sprayed at high pressure showed the slowest recovery of the brown alga Fucus gardneri. The finding confirmed earlier studies that intense cleaning of rocky shores after oil spills may not be justified environmentally.

      After the accidental discharge in 1986 of some 15 million litres (95,000 bbl) of medium-weight crude oil into fringing mangrove areas of Bahía Las Minas on the central Caribbean coast of Panama, mangrove muds in the region showed unexpected persistence of the full range of aromatic hydrocarbon residues. Researchers estimated a time scale of at least 20 years for catastrophic oil spills trapped in muddy coastal habitats to lose their toxicity.

      The date mussel Lithophaga lithophaga, which bores into calcareous rocks, in recent years had been intensively harvested for human consumption by scuba divers in the Mediterranean Sea off the coast of southern Italy. Exploitation involved demolition of the rocky substratum, often with the help of underwater vehicles. As a result, the entire bottom-living community of animals disappeared, and tens of kilometres of coastline were "desertified."

      Advanced very high-resolution radiometer (AVHRR) satellite images and simultaneous ship transects in the Baltic Sea revealed increased sunlight absorption at the surface by blooms of cyanobacteria (blue-green algae), raising water temperatures by as much as 1.5° C (2.7° F)—a rare quantified example of direct influence of a biological process on ocean physics. Scientists discovered that the noise and light emitted by remotely operated vehicles in the sea on scientific and exploratory missions adversely affected the behaviour of lobsters; the finding had clear implications for the future design of such vehicles for behavioral studies. Fibre-optic microprobes developed to measure the amount of light penetrating to various depths in sandy sediments permitted, for the first time, investigations of the interaction of light with the physiology of sediment microorganisms at a level comparable to that of open-sea phytoplankton (the plant and plantlike component of plankton).

      Scientists characterized methane-seep habitats in sediments of the southern slope of the central Skagerrak off Denmark. In association with very high concentrations of methane gas and dissolved sulfide were found abundant populations of the pogonophoran worm Siboglinum poseidoni and the bivalve mollusk Thyasira sarsi. Each animal is dependent for food on internally living symbiotic bacteria, which, in the case of the worm, consume methane and, in the case of the mollusk, derive energy from the oxidation of sulfur. How such nutritionally restricted animals have crossed the vast distances between methane seeps and between related communities around hydrothermal vents to become dispersed around the world remained an unanswered question. The deep-diving research submersible Alvin, however, revealed similar communities associated with decaying whale skeletons at depth. It was concluded that "whale falls," which are widespread in the ocean, may nurture substantial sulfide-dependent communities on the deep seafloor and that some species may be dispersing to hydrothermal vents from whale-fall "habitat islands."

      The effects on penguins of the flipper bands commonly used for marking the birds were quantified, and the use of the bands was questioned. Banded birds were shown to expend 24% more power than unbanded birds during swimming, with detrimental implications for performance and survival. In another study researchers attached transmitters to king penguins (Aptenodytes patagonica) near the Crozet Islands in the southwestern Indian Ocean and tracked the birds by satellite. Swimming distances ranged from 33 to 95 km (20 to 59 mi) daily, much greater than previously assumed. Late in the year observers reported a mysterious die-off of about 20,000 king penguin chicks on the island of South Georgia in the South Atlantic. Suspected causes included unseasonably heavy snow, which may have smothered the birds, and a food shortage. (ERNEST NAYLOR)

      This updates the articles crustacean; fish; mollusk.

BOTANY

      Flowering plants exhibit a remarkable ability to sense different colours, or wavelengths, of visible light and then use the light energy that is absorbed by particular pigments in the plant to carry out specific processes. For example, photosynthesis is most effectively promoted by wavelengths that are absorbed by the pigment chlorophyll and include wavelengths in both the blue and red regions of the spectrum. The pigment phytochrome, which is used to signal a wide variety of developmental events, including flowering in some plants, absorbs most strongly in the red and far-red regions of the spectrum. Other light-influenced events such as phototropic bending—i.e., bending toward a light source, as is observed in most plants—is most influenced by blue light. Additional responses to blue light include formation of chloroplasts, the cell organelles that serve as the sites of photosynthesis in green plants, and opening of stomata, or leaf pores. It has been difficult, however, to ascribe the signaling effect of blue light to a particular pigment.

      During the year plant scientists reported on their search for the blue-light receptor pigment via an approach in which by various means they manipulated the amount of a suspected receptor, the carotene-like pigment zeaxanthin, in tips of maize (corn) seedlings. The seedlings that were rendered devoid of the pigment did not show phototropic bending, whereas those in which the pigment was present did bend. The scientists thus suggested that zeaxanthin may be a blue-light receptor for this response.

      Introductory biology textbooks list a number of characteristics that distinguish a "typical" plant cell from a "typical" animal cell. Included is the fact that most mature, living plant cells possess a large, membrane-bound central space, called the vacuole, that is not present in animal cells. For many decades the vacuole, which often comprises more than 95% of the plant cell volume, had been considered simply a site for the accumulated waste products of cell metabolism. As early as the 1960s, however, reports that plant vacuoles function as protein storage centres began to appear. At that time it was pointed out that at certain stages of plant development, such as embryo formation and seed maturation, proteins accumulate in the storage vacuoles of certain cells in the cotyledons, or seed leaves. Later, when the seed begins to germinate, enzymes called proteases are made in the cytoplasm and then transported to the vacuoles. There they break down stored protein, their action resulting in the release of amino acids needed by the entire plant to make new proteins. Further, it was demonstrated that other molecules such as carbohydrates are also stored in the vacuoles of some cells.

      An important question for plant cell biologists has been how plant cells are able to sort out specific proteins and other molecules to ensure their delivery to the vacuole. Several recent papers added to an understanding of the mechanism by which specific molecules such as proteins are targeted for delivery to a specific cell location. During the processing of these molecules in the Golgi apparatus, a complex organelle involved in molecular modification and transport, the molecules are packaged into membrane-bound vesicles, and specific chemical messages called targeting sequences are added. Functioning much like the zip code on a package, the targeting sequences allow the vesicle to recognize and bind to a docking molecule on the membrane of the vacuole. As a result, the molecules shipped to the vacuole for sequestering are specific rather than random ones. Included among proteins often found in the vacuoles are those involved in defense against leaf-eating insect predators. When the cell is damaged by an insect, the molecules are released from the vacuole and discourage further insect feeding.

      A second distinguishing characteristic of a typical plant cell is its cell wall, which is composed mainly of polysaccharides—i.e., polymers of sugar molecules, such as cellulose, hemicellulose, and pectin. Proteins are also present in plant cell walls and include molecules such as extensin, which confers some of the elastic properties of the wall. The walls provide mechanical support for cells but also are involved in other important processes, including cellular defense against disease-causing organisms, particularly fungi. The chemistry of cell-wall architecture is complex, and both the elucidation of pathways of molecular synthesis involved in the construction of cell walls and the listing of cell-wall composition have changed often in recent years.

      During the year researchers seeking a better understanding of plant cell walls produced mutants of Arabidopsis thaliana (a small, fast-growing plant of the mustard family often used in genetics experiments) that lacked the sugar fucose as part of their cell-wall composition. Plants that lacked fucose, a component of both hemicellulose and pectin, were dwarfed compared with normal nonmutated plants and possessed cell walls more fragile than normal. The achievement suggested a useful approach for studying the synthesis, structure, and function of plant cell walls. (PHILIP D. REID)

      See also Botanical Gardens and Zoos ; Environment .

      This updates the articles botany; biosphere; conservation; plant.

MOLECULAR BIOLOGY

Hyperthermophiles: Beneficial Relics of a Hotter Earth.

      Boiling as a means of sterilization is based on the expectation that heating to 100° C (212° F) kills virtually all microorganisms. Yet there are bacteria that not only survive exposure to such temperatures but also grow optimally at, or even above, 100° C. They are the extreme thermophiles, or hyperthermophiles, and many of their names—for example, Pyrococcus furiosus or Methanothermus fervidus—reflect the sense of amazement that they aroused in their discoverers. These organisms are usually found in naturally hot environments, such as hot springs or deep-sea hydrothermal vents, but they also occur in human-made environments, such as hot water tanks.

      Hyperthermophiles are interesting for several reasons. First, there is the question of whether their adaptation to heat represents a primitive characteristic retained from their origin on a once hotter Earth or whether it is a recent adaptation to the limited hot environments that currently exist. Second, there is the question of how the organisms maintain the structural integrity of their components, particularly since protein, DNA, and RNA are generally considered to be quite heat-sensitive. Finally, there are the commercial advantages of the high-temperature stability, or thermostability, of the enzymes made by such organisms.

      Evolutionary relationships between organisms are commonly deduced from features of form, function, or both that are observed in creatures living today or in fossils of extinct life. From such observations it is clear, for example, that whales evolved from land-dwelling animals. Direct observations of size and shape, however, are of little use in revealing relationships between microorganisms. Since the earliest inhabitants of Earth were microscopic, scientists had long been totally ignorant of the long course of evolution that preceded the appearance of larger, multicellular organisms.

      In recent years methods for determining the precise sequences of the building blocks of protein, DNA, and RNA—the molecular carriers of genetic information—have opened a window on early evolution. The basic tenet is that evolutionary relatedness is revealed by similarity in sequence. If the sequences of, say, corresponding genes or RNA molecules taken from two different organisms are very similar, then the organisms are closely related. Conversely, great sequence differences reflect early evolutionary divergence. This relationship between sequence similarity and evolutionary relatedness is well-founded in theory and is in accord with a wealth of data, both molecular and traditional.

      On the basis of such sequence data, all life on Earth can be grouped into three domains: the eubacteria, the archaea (or archaebacteria), and the eucarya (or eukaryotes). The more familiar kingdoms, such as the plants, fungi, and animals, are subdivisions of these domains. The hyperthermophiles are members of the archaea, and the sequence differences in their genetic material compared with that of the eubacteria and the eukaryotes suggest that they appeared early in the course of biological evolution. Their tolerance for heat thus likely represents a retained primitive characteristic.

      Metabolism is another indicator of evolutionary history. The Earth contained little molecular oxygen prior to the advent of true photosynthesis carried out by cyanobacteria (blue-green algae), which occurred over a billion years ago. Hence, organisms that developed prior to the photosynthetic cyanobacteria must have been anaerobes—organisms that live in the absence of free oxygen. Significantly, hyperthermophiles are anaerobes. Volcanic vents and other environments heated by geologic processes are often rich in sulfur. The hyperthermophiles usually make heavy metabolic use of sulfur; most reduce sulfur to hydrogen sulfide, while others use nitrate to oxidize sulfur to sulfuric acid.

      Enzymes are proteins that function to promote, or catalyze, biochemical reactions in living organisms. The enzymes that have been isolated from hyperthermophiles are remarkably thermostable, some retaining catalytic activity up to 140° C (284° F). Scientists had hoped that comparing heat-resistant proteins from hyperthermophiles with their heat-sensitive counterparts from mesophiles—organisms that live in moderate-temperature environments (such as Escherichia coli bacteria or human beings)—would reveal the structural basis for thermostability. Unfortunately, the situation proved more complex than expected. As of 1994, comparisons of proteins on the basis of their amino acid sequences had not revealed striking differences. On the other hand, comparisons of native three-dimensional structure, i.e., the shape into which the amino acid chain folds to form the functional protein, did provide a clue.

      The native conformation of a protein depends on a collection of many weak interactions, such as van der Waals interactions, hydrophobic bonding, hydrogen bonding, and electrostatic, or salt, bonding. The total effect of these weak bonds is a substantial net stabilization. However, once a few of the weak bonds are overcome, say, by the addition of heat energy, the entire structure can unfold and lose its functional properties, a phenomenon called denaturation. This explains why a small increase in temperature, above some critical value, can cause a large increase in the rate of denaturation of a protein. In research carried out in 1993, the structures of the enzymes called rubredoxins from mesophiles and hyperthermophiles were compared; the former enzyme was seen to contain an unattached amino terminal end, whereas the latter did not. It appears likely that the amino terminus is the Achilles' heel, the point of unfolding, of the mesophilic enzyme, whereas it is tied down by hydrogen bonding, and thus protected, in the thermophilic version.

      Enzymes, nature's catalysts, are more efficient and more specific than any human-made catalysts devised to date. By the mid-1990s they had found use—and in the future may become even more useful—in synthetic and analytic chemistry, biotechnology, food processing, and even laundering, to name a few applications. The problem of poor heat stability, an impediment to many possible applications, is solved by the enzymes in hyperthermophiles. For example, protein-containing food stains on clothing can be removed by enzymes called proteases, which digest protein. Such enzymes, however, must resist hot water and detergents. Proteases from hyperthermophiles do exhibit the necessary stability and were being studied for such use.

      (IRWIN FRIDOVICH)

Fragile X and the Genetics of Anticipation.

      Most known genetic disorders, such as cystic fibrosis, exhibit traditional, or Mendelian, patterns of inheritance. Some are transmitted as recessive traits, so that two carrier parents, themselves unaffected, may produce an affected child; some as dominant traits, so that one affected parent may produce an affected child; and some as sex-linked traits, passed from either an affected father or an unaffected mother to sons but generally not to daughters. Numerous factors complicate the picture for certain diseases; e.g., diseases that depend on the inheritance of more than one gene, that arise from new mutations, or that reflect a combination of genetic and environmental influences.

      In marked contrast to the traditional patterns of inheritance, however, stand a growing list of serious human genetic disorders that exhibit patterns of inheritance far too complex to be explained in simple Mendelian terms. Examples include fragile X syndrome, the most common known form of inherited mental retardation, and myotonic dystrophy, the most common known form of adult-onset muscular dystrophy.

      Fragile X syndrome affects about one in 1,500 males and one in 2,500 females. As the name implies, affected individuals almost always display, in addition to a collection of characteristic cognitive and physical traits, an unusual chromosomal constriction, known as a fragile site, which is visible microscopically under defined conditions on their X chromosomes. Although the gene associated with fragile X can be passed from one generation to the next by members of both sexes, the risk of someone in a subsequent generation being affected is much higher if the carrier parent is the mother rather than the father. Moreover, for any individual in a fragile X family, the risk of being affected depends not only on the degree of relatedness to any other known affected or carrier individual but also on one's position in the pedigree, or ancestral line. In brief, the farther down a pedigree a person is located, the greater is the risk of being affected. For example, the brothers of unaffected carrier males (dubbed NTMs, for normal transmitting males) run a low risk (about 9%) of being affected, while the grandsons and great-grandsons of NTMs run a much higher risk (about 40% and 50%, respectively). This unusual pattern of inheritance was first described by Stephanie Sherman of Emory University School of Medicine, Atlanta, Ga., in the mid-1980s and is named the Sherman paradox.

      In 1991 a candidate gene associated with fragile X syndrome, called FMR-1, was identified and cloned as a result of work in the laboratories of several different investigators, including Stephen Warren, Emory University School of Medicine; C. Thomas Caskey, Baylor College of Medicine, Houston, Texas; and Ben Oostra, Erasmus University, Rotterdam, Neth. Subsequent studies of this gene region in normal and affected individuals in the laboratories of the researchers named above, as well as in those of Grant R. Sutherland, Adelaide (Australia) Children's Hospital, and Jean-Louis Mandel, National Institute for Health and Medical Research, Strasbourg, France, revealed the molecular nature of the defect ostensibly responsible for the disease and provided a novel and unexpectedly intriguing resolution of the Sherman paradox.

      A gene carries information for the synthesis of a specific protein in the sequence of building block molecules, called nucleotides (abbreviated A, G, C, and T, for the constituent bases adenine, guanine, cytosine, and thymine), that make up DNA. This sequence information is ultimately translated into information specifying the sequence of amino acids that form the protein. In fragile X syndrome the apparent molecular defect takes the form of an expansion, or amplification, of tandem repeats of the triplet base sequence CGG near the beginning of the FMR-1 gene. Such a defect, in which the extra repeats range in number from one to more than 1,000, represented a novel form of mutation to be associated with human disease.

      A molecular survey of the FMR-1 CGG repeat regions in normal and fragile X families revealed a startling pattern. Normal individuals had on average about 29 repeats, spanning a range from 6 to 52 repeats, while unaffected carrier individuals had between 50 and greater than 200 repeats. Affected individuals could have as many as 1,000 repeats or more. Perhaps most striking, however, was the finding that of the FMR-1 genes studied in families, those containing 46 repeats or fewer showed no instability, or tendency to change, when passed from parent to child, while those greater than 52 repeats showed complete instability. Genes carrying large numbers of repeats, i.e., those associated with affected individuals, were so unstable that even different cells within a blood sample from a single individual could show different repeat sizes. In families having intermediate, or "premutation," numbers of repeats in the FMR-1 gene, it was not uncommon to see expansion from, for example, 66 repeats in the mother to 80 repeats in one child, 73 in another child, and 110 in a third child.

      Furthermore, the risk of expansion to a full mutation (greater than 230 repeats) on passage from mother to child increased with the number of repeats already present in the mother. For example, women with premutation numbers of repeats in the 60-69 range had about a 17% chance of transmitting a full mutation to a child, whereas women with premutation numbers of repeats greater than 90 had a 100% chance of transmitting a full mutation. Therefore, in a typical fragile X family one would often see repeats in the premutation range move from small to large numbers in one or two generations and then to full mutations in subsequent generations, thereby providing a molecular explanation for the Sherman paradox.

      Among the early benefits to be realized from discovery of the FMR-1 repeat expansion was a gain in the ease and reliability of diagnosing fragile X for both the affected and carrier states. Previously diagnosis could be confirmed only by an expensive, labour-intensive procedure specifically designed to visualize the fragile sites in the patient's X chromosomes. While this method reliably detects affected individuals, it does less well for carrier females, whose fragile sites are not always discernible. With the identification of the FMR-1 gene and the discovery of the fragile X-associated repeat expansion came the prospect of diagnosing affected and carrier individuals with molecular methods, which were faster, cheaper, and in many cases more informative. Indeed, given the observed patterns of expansion risk as a function of premutation size, molecular methods could be used not only to distinguish probable carriers from probable noncarriers but also to distinguish particularly high-risk carriers from comparatively low-risk carriers.

      Although the CGG triplet repeat expansion associated with fragile X syndrome was novel and unexpected when first identified, its discovery paved the way for similar discoveries about other disorders. For example, it was subsequently learned that myotonic dystrophy, an autosomal (non-sex-linked) dominant neuromuscular disease, also is associated with repeat expansion of a triplet base sequence located near one end of a newly identified gene for the enzyme myotonin kinase. Indeed, the discovery provided a molecular explanation for the unusual inheritance pattern, termed anticipation, observed earlier for myotonic dystrophy; namely, that although the disease is passed in an autosomal dominant manner, the age of onset decreases and severity of symptoms increases with each generation in an affected family. As with fragile X, the more severely the individual is affected with myotonic dystrophy, the larger the triplet repeat expansion appears to be. By 1994 a number of other disorders, many characterized by anticipation, also had been linked to triplet repeat expansions, and the list was expected to grow. Included were spinobulbar muscular atrophy, Huntington's disease, spinocerebellar ataxia type 1, and FRAXE mental retardation (a disorder resembling fragile X syndrome caused by a similar defect at a different site on the X chromosome).

      The identification of triplet repeat expansion as a mechanism of mutation answered some important questions about human genetic disease, but it also raised some new ones. Why, for example, are some triplet repeat genes unstable and others not? If "normal-sized" triplet repeats are completely stable, where do the premutation sizes come from? What are the origins of repeat expansion? Is the observed instability perhaps a normal form of evolution, sometimes associated with disease but other times not? What mediates and controls the process in humans and other species? How does repeat size expansion cause the observed traits of the disorder?

      Finally, what are the normal roles of the identified genes and gene products in healthy individuals? Recent work indicated that the product of the FMR-1 gene is likely to be a protein that binds RNA. The gene product associated with spinobulbar muscular atrophy functions as a molecular receptor for androgen (male sex hormone). Genes and gene products associated with the other disorders were under study. (JUDITH L. FRIDOVICH-KEIL)

      See also Chemistry .

      This updates the articles biology; cell; heredity; reproduction.

PALEONTOLOGY

      The origin of life, evolutionary time, and the nature of the early atmosphere and oceans are a direct concern of paleontology. The old model of the oceanic broth of organic "soup" as the birthplace of life has given way to speculation that life emerged in more limited, protected environments such as the systems of hydrothermal vents observed today on the ocean floor. There in geothermally heated, mineral-rich waters thrive hydrothermal bacteria, which together with the highly anaerobic methane-producing bacteria form a major division of extant life—the archaea. The way in which hydrothermal bacteria use chemical reactions to make the molecules needed for life, i.e., their means of chemosynthesis, is considered to be the most primitive among organisms. In the March 1994 issue of Geotimes, Everett L. Shock of Washington University, St. Louis, Mo., supported the assumption that primordial chemosynthesis utilized elemental sulfur and hydrogen sulfide found at hot springs around deep oceanic trenches. (See Molecular Biology (Life Sciences ), above.)

      Paleobotanists use microscopic fossil spores, pollen, and dinoflagellate cysts as indicators of past biogeography, floral diversity, and extinction. By means of such tools, Paul Colinvaux of the Smithsonian Tropical Research Institute, Balboa, Panama, and his collaborators were reconstructing changes in global climate and the history of tropical Amazon Basin vegetation. It was believed that fossil plants lived under climatic restraints similar to those of recent plants and that detailed studies of stomata in fossil leaves could help determine past concentrations of gases in the atmosphere. Stomata, found on the underside of leaves, are openings through which gases such as carbon dioxide and oxygen can enter and leave a plant. Experiments with living plants had shown that such characteristics of stomata as their density on the leaf surface are influenced by the atmospheric concentration of carbon dioxide. Consequently, by charting the changes observed in the stomata of fossil leaves through time, researchers were attempting to build a picture of changing carbon dioxide levels over millions of years.

      Invertebrate paleontologists, while still pursuing mass extinctions, were coming to recognize the existence of evolutionary stasis between extinctions. In other words, following a mass extinction and the subsequent few million years of recovery, which are marked by rapid evolutionary change and reorganization of living communities, ecological patterns stabilize for tens or hundreds of millions of years until the next mass extinction.

      Researchers also began shifting their attention to the sudden, very rapid origination of animal species and higher groups in the Cambrian Period—the so-called Cambrian explosion or big bang of evolution that took place more than 500 million years ago. As observed in the Cambrian fossil record, animals emerged fully developed in a geologically "sudden" time as short as 5 million to 10 million years in duration. Unusually well-preserved and abundant fossil localities are windows to past life. Such windows were being reconstructed and interpreted: the Middle Cambrian Canadian Burgess Shale by the English paleontologists Matthew A. Wills and Derek E.G. Briggs of the University of Bristol, England, and Richard A. Fortey of the British Natural History Museum, London; the Swedish Upper Cambrian Orsten by the German scientist Dieter Walossek of the Rhenish Friedrich Wilhelm University, Bonn, Germany; and other Early Paleozoic localities by Jerzy Dzik of the Polish Academy of Sciences, Warsaw. Uranium-lead isotope dating of volcanic rocks from Siberia allowed Samuel A. Bowring and his co-workers from the Massachusetts Institute of Technology, Harvard University, and Yakutian Geoscience Institute, Yakutsk, Russia, to place more exactly the beginning of the Cambrian at 544 million years ago, compared with the 570 million-year figure previously accepted. Many hypotheses for the sudden appearance of animals in the Cambrian were offered, among the more popular of which were those involving the oxygen level of the sea, climate, sea-level changes, biological "arms races," complexities regarding body forms and structures, and even sampling errors (i.e., the Cambrian explosion is not real but an artifact of the way fossils have been collected and classified).

      Adolf Seilacher of the University of Tübingen, Germany, and Yale University was the first paleontologist to receive the Crafoord Prize—the equivalent of the Nobel Prize for fields not covered by the traditional Nobels—from the Royal Swedish Academy of Sciences. A few years earlier Seilacher had proposed a fifth kingdom, Vendobionta, for the Precambrian Ediacaran fossils that had been classified originally among existing phyla and fitted into the general plan of such living animals as jellyfish, sea pens, worms, and certain problematic creatures. More recently he interpreted the Ediacaran fauna as an extinct animal phylum.

      Among highlights in vertebrate paleontology, Paul C. Sereno of the University of Chicago reported on previously unknown species of carnivorous and herbivorous dinosaurs from the Early Cretaceous (about 130 million years ago) that he and colleagues discovered in the southern Sahara Desert. Strong similarities between the African dinosaurs and North American forms led Sereno to question accepted ideas about the way the supercontinent of Pangaea began fragmenting about 150 million years ago into the continents of today and to suggest that the land bridge that linked the two landmasses which would become the present-day northern and southern continents was maintained far longer than previously thought. (For information on a new species of crested theropod dinosaur, see .)In November Scott R. Woodward of Brigham Young University, Provo, Utah, reported that he and his co-workers had extracted DNA from 80 million-year-old fossil bone fragments, found in an underground coal mine, that he believed came from a dinosaur. Until other researchers could reproduce his results, however, both the ancientness of the DNA that he isolated and the proposed identity of its source would be regarded skeptically. (MATTHEW H. NITECKI)

      This updates the articles evolution, theory of (evolution); geochronology.

▪ 1994

Introduction

ZOOLOGY

      Cannibalistic salamanders, social structures of frogs and pilot whales, and warm-blooded fish were all involved in zoological advances in 1993. In addition, studies of the fossil record challenged traditional theories regarding the origin of avian flight and the ancestry of humans.

      Studies on cannibalism in salamanders and on the social structure of whales provided support for theories of kin selection, the tendency to favour genetic relatives over unrelated individuals, by revealing situations in which animals modify their behaviour when they belong to a genetically related family unit. The larval young in some populations of tiger salamanders (Ambystoma tigrinum) are known to become cannibalistic, feeding on other tiger salamanders. The cannibals grow larger than noncannibalistic larvae and develop specialized structures in the mouth that aid in eating other salamanders. Cannibalism occurs most frequently when larvae develop under crowded conditions. David W. Pfennig of Cornell University, Ithaca, N.Y., and James P. Collins of Arizona State University discovered that tiger salamanders reared in genetically unrelated groups are more likely to develop into cannibals than are salamanders raised in groups of siblings. They conducted experiments in which similar-sized larvae were placed in various groups, some being all siblings and some being unrelated. All larvae were of similar size so that variation in body size could not be used by the larvae as a cue to whether individuals were related. The investigators hypothesized that larval salamanders release chemical cues that can be used to distinguish close kin, which have a similar "smell," from unrelated larvae.

      Kin-selection theory was also supported by evidence that individual organisms can increase their own genetic success by curtailing breeding and possibly helping their relatives. In a study of the biology of long-finned pilot whales in the Faeroe Islands, southeast of Iceland, Bill Amos of the University of Cambridge and colleagues Christian Schlötterer and Diethard Tautz of the University of Munich, Germany, found that males exhibit atypical behaviour. Pilot whales form large social groups called pods. The investigators used molecular techniques to establish that pod members were closely related, forming an extended family. A pod normally has more adult females than males and may number more than 100 individuals. In most mammalian species in which females live in groups, genetic inbreeding is avoided when male offspring disperse from their homesite before they become breeding adults. Pilot whale males remain with their family pod, yet genetic studies revealed that males in a pod rarely or never breed with the females, who might be their mothers or sisters. Mating is presumably carried out when different pods encounter each other in the ocean. Whether or how the nonbreeding males contribute to the welfare of their relatives in a pod remained to be learned, but defense from marine predators or assistance in a communal feeding effort was suggested. Ironically, the cohesive family structure of long-finned pilot whales makes them prey to human whale hunters. In the early 1990s about 1,700 of the whales were killed each year because pods could easily be herded into coastal areas.

      Evidence of the way in which mating systems can develop in the best interest of an individual but not necessarily of the species was presented by Godfrey R. Bourne of Florida Atlantic University in studies of the mating system of a tropical frog, Sinax rubra, in Guyana. If given a choice, females, regardless of body length, select smaller male mating partners, usually about 80% of their own size. Experiments revealed that this size ratio of female to male produced the highest rate of fertilization of a female's eggs. During mating, a male frog clasps a female and releases sperm while she deposits eggs in the water. A male frog that is larger than the female is not in the proper position for the sperm to reach all of the eggs; thus, he fertilizes significantly fewer. Males smaller than the optimal size do not have enough sperm to fertilize all the eggs. Therefore, to maximize egg fertilization and have the highest reproductive success, a female needs a mate of the proper size and so chooses one accordingly.

      A larger male frog, however, often displaces the smaller one that is chosen by the female and ends up mating with her instead. Competition between a small male that is preferred by the female and a large male intruder can reduce female reproductive success because fewer eggs are fertilized. On the other hand, the breeding success of the larger male is enhanced. Smaller males sometimes successfully mate by remaining quiet but alert for approaching females. When a female passes by on her way to check out a calling male, a small, silent "satellite" male may intercept her and mate. This competition among males can reduce the reproductive success of a particular female but ensure the propagation of a particular male's genes.

      Most of the world's fish species, along with reptiles and amphibians, are ectotherms, or cold-blooded animals, having body temperatures corresponding to that of their surroundings. Endothermic, or warm-blooded, animals have the ability to elevate body temperature internally. The trait is characteristic of mammals and birds as well as some sharks and certain marine fish, including mackerels, tunas, and billfishes (e.g., marlins and swordfish). Two contrasting theories exist to explain what selection pressures were influential in the evolution from ectothermy to endothermy. One theory proposes that endothermy arose following selection for a capability to maintain stable body temperatures across a broad range of environmental temperatures, permitting exploitation of varying thermal conditions. The other proposes that endothermy evolved in response to selection for an increase in aerobic capacity (ability to use oxygen) associated with higher metabolism and a more active lifestyle.

      Barbara A. Block and colleagues of the University of Chicago used techniques of molecular genetics to establish the phylogenetic relationships among ectothermic fish species and the three groups of endothermic fishes. They found mackerels, tunas, and billfishes each to be more closely related to ectothermic species than to each other, documenting that endothermy evolved independently in the three different groups. In some species, such as the butterfly mackerel and swordfish, warming is restricted to the central nervous system and retina. The phylogenetic distribution and variable expression of endothermy among the fish groups led the researchers to conclude that endothermy in fishes evolved in response to the advantages of expanding into habitats of varied temperatures, not to a requirement of increased aerobic capacity.

      A new living species of large mammal, the first such in more then 50 years, was identified from a physical examination and molecular analysis of skulls, teeth, and skins collected from a largely unexplored rain forest in Vietnam's mountainous central neck. The animal itself, however, had yet to be seen alive by the scientists involved at the time their findings were published. John MacKinnon of the Asian Bureau for Conservation, Hong Kong, and colleagues of the Vietnamese Ministry of Forestry placed the animal in the Bovidae family, which includes cattle, goats, sheep, and antelopes, and described it as weighing about 100 kg (220 lb) as an adult and having a rich brown coat with white and black markings and sharp, straight horns up to 52 cm (20 in) in length. The new bovid, called the "forest goat" or "spindle horn" by local Vietnamese hunters, was given the name Pseudoryx nghetinhensis.

      Two major theories have been proposed for the evolution of flight in birds. One is that flight evolved in ground-dwelling animals that were preadapted for flight. The other is that flight originated in tree-dwelling species. On the basis of fossils from the Late Jurassic (150 million years ago), Archaeopteryx is the generally accepted predecessor of flying birds and the focus of most theoretical discussions on the origin of avian flight. During the year Alan Feduccia of the University of North Carolina lent credence to the origin of flight from tree-dwelling forms by means of a study that compared the claw geometry of Archaeopteryx with that of modern birds. The claw curvature of the ancient bird was shown to be similar to that of modern birds that perch in trees or climb tree trunks rather than to that of ground-dwelling birds, suggesting that this earliest known feathered ancestor of birds was arboreal. (See Ornithology, below.)

      Among the most controversial evolutionary interpretations from the fossil record are those surrounding the relationships between humans and other primates. David R. Begun of the University of Toronto examined fossil hominids from Hungary estimated to be 10 million years old. He concluded that they may be the closest known relatives of chimpanzees, gorillas, and humans. In addition, his findings supported the view that humans are more closely related evolutionarily to chimpanzees than either are to gorillas, a position held by many molecular biologists.

      (J. WHITFIELD GIBBONS)

Entomology.

      The fossil record of Insecta, the most diverse class of living animals, has received less attention in the English-language scientific literature than many other major animal groups. Part of the reason lies with the perception that insect fossils are rare. Refuting that notion, a study by Conrad C. Labandeira of the Smithsonian Institution, Washington, D.C., and J. John Sepkoski, Jr., of the University of Chicago revealed that insect diversity has exceeded that of four-limbed vertebrates since Carboniferous times (about 325 million years ago). The investigators compiled geochronological records for 1,263 insect families, relying on extensive fossil records reported in German, Russian, and Chinese literature. Objectives of the study were to determine the fossil diversity and rates of evolution of insects and to relate these data to the worldwide development of angiosperms (flowering plants) that originated in the Cretaceous Period (about 125 million years ago). One conclusion was that the high diversity and radiation of modern insect families began nearly 100 million years before flowering plants first appeared, rather than after and in response to their appearance. The researchers also concluded that the increasing diversity of insect families has persisted over geologic time because of low extinction rates rather than because of high rates of evolution during particular periods.

      The merit in using fossilized material to interpret evolutionary relationships is often controversial. A study reported during the year, however, demonstrated the utility of using extinct insects to resolve a dilemma regarding the relationships between major groups of organisms. An Australian termite, Mastotermes darwiniensis (order Isoptera), had long been considered the most primitive isopteran and the "missing link" between cockroaches and termites. To establish the relationship between termites and roaches, Rob DeSalle, Ward Wheeler, and David Grimaldi of the American Museum of Natural History, New York City, and John Gatesy of Yale University used molecular techniques to examine and compare DNA sequences from the genes of M. darwiniensis and other living species of insects as well as an extinct termite (M. electrodominicus) from the Dominican Republic. The fossil, preserved in amber 25 million-30 million years old, yielded what was at the time the oldest DNA extracted from a fossil. (In mid-1993 scientists reported recovering DNA from a weevil encased in amber 120 million-135 million years old.) The investigators concluded that termites, including the genus Mastotermes, are a monophyletic group (all derived from the same common ancestor) that evolved independently from the roaches.

      Most flies (order Diptera) emit and hear low-frequency (100-500 Hz) sounds that travel short distances, whereas crickets emit high-frequency (usually above three kilohertz) sounds audible at much greater distances. Daniel Robert and Ronald R. Hoy of Cornell University and John Amoroso of the University of Florida reported the discovery of a parasitoid fly (genus Ormia) having an ear capable of detecting high-frequency sounds made by crickets (genus Gryllus). Male field crickets produce far-reaching high-frequency sounds to attract females; however, female parasitoid flies are also attracted to the calling males, on or near which they deposit larvae that burrow into the host cricket. The cricket dies within 10 days, by which time the larvae have developed into pupae that emerge. The newly discovered hearing organ (tympanic ear) in the fly is anatomically and functionally characteristic of a cricket's and represents an instance of convergent evolution that allows the fly to exploit the mating behaviour of its host.

      James T. Cronin and Donald R. Strong of the University of California at Davis conducted experiments to examine egg-laying patterns of a parasitoid, the fairyfly wasp (Anagrus delicatus), in relationship to its plant hopper host, Prokelisia marginata. Plant hoppers, the most abundant herbivorous insects in the Atlantic and Gulf coastal marshes of North America, both feed and lay eggs on salt marsh cord grass (Spartina alterniflora). The female wasp seeks out and lays its eggs only in the eggs of plant hoppers. The investigators measured the time the wasps took to search grass leaves for plant hopper eggs and then to deposit their eggs. They discovered that the wasps spent more than an hour on a plant once plant hopper eggs had been located. Although other plant hopper eggs were available to parasitize, a female wasp laid only a few of her eggs before leaving to search other plants, thus distributing her eggs among different leaves of grass. Such behaviour stood in contrast to the traditional view that parasitoids minimize the time invested in egg-laying activity. The researchers found that 20-30% of the cord grass leaves in the habitat aged and died during the approximately 24 days required for parasitoid larval development, resulting in deaths of the eggs of both species of insects. One conclusion was that, although wasp egg-laying rates are lower than can be achieved, the strategy of spreading eggs among several grass patches increases the probability that at least some offspring survive.

      (ANNE R. GIBBONS)

      This updates the article insect.

Ornithology.

      The ability of captive African gray parrots (Psittacus erithacus) to mimic human speech and other sounds is well known, but observations of wild populations in West Africa had not indicated that they practice vocal mimicry naturally; that is, of the kind commonly seen in such birds as mockingbirds and starlings. However, analysis of a sound recording of a gray parrot in Zaire revealed the unmistakable reproduction of sounds from nine bird species and one kind of bat, the first evidence of sound copying by gray parrots in the wild. Furthermore, additional tapes of wild gray parrots in Gabon and Côte d'Ivoire suggested that such impressionism may be widespread.

      That American blue jays eat large numbers of acorns in autumn and bury many more for winter consumption has long interested ornithologists, for although these nuts contain unpalatable tannins known to upset the digestive enzymes of other animals that consume them, blue jays appear to suffer no harm. Carter Johnson of South Dakota State University discovered that jays eating acorns that had been invaded by acorn weevil larvae suffered no weight loss provided that each bird consumed with the nuts roughly 100 larvae a day. By comparison, other jays that ate only pristine, uninfested nuts did lose weight.

      An individual Clark's nutcracker, another hoarder, may hide 30,000 conifer seeds in 6,000 separate holes in the forest floor. The birds successfully retrieve many of the seeds, displaying an excellent spatial memory, but Alan Kamil of the University of Massachusetts at Amherst and J.P. Balda of Northern Arizona University found in experimenting with some birds' orienting ability that the birds relocated seeds whether or not they approached each cache from the same direction as when burying the seeds. Thus, instead of relying on direction as an aid to memory, the nutcrackers may generate a kind of "cognitive map."

      In the U.K., where the breeding biology of common bird species probably has been more widely studied than in other countries, egg-laying dates for 33 species of the 82 studied showed a trend, over the 30 years to 1990, toward earlier laying. Among the species the advance varied from one to 22 days, with a mean of 8 days. One contributory cause could be global warming.

      The dunnock, or hedge sparrow, a small dun-coloured European perching bird (passerine), was the subject of a 10-year study by N.B. Davies of the University of Cambridge published as Dunnock Behaviour and Social Evolution. Within a population of dunnocks, nearly every conceivable mating system can be found. Some males monopolize the sexual favours of two females, while others have but one mate and still others share either one or two females with another male. Why then, when most birds are monogamous or nearly so, do dunnocks have such a variable mating system?

      In broad outline Davies' finding is that the dunnock's mating system is a product both of a variable ecology and of conflict between individuals. A female will defend a territory large enough to satisfy her nutritional requirements. Males then defend territories that enclose female territories and, in so doing, control the reproductive opportunities of the females. Some territories are so large, however, as to require two males to defend the one or two occupant females. Thus commences one of the many conflicts. A female prefers both males to mate with her so that both stay and feed the offspring. In contrast, each male prefers to monopolize the female. Thus, the dominant male attempts to guard the female and keep his weaker rival at bay. The female, preferring the attention of both males, attempts to find the weaker male, who will be enticing her at a distance from a bush. It is the female's task to elude the dominant male. Once she has done so, he will flit about frantically looking for the pair so as to break them up.

      Knowledge of Archaeopteryx, the most well-known ancient fossil bird, comes from a half-dozen specimens found in Bavarian rocks about 150 million years old. A somewhat younger fossil bird, Sinornis satensis, which dates from about 135 million years ago, had been known from only two specimens, one from Spain and one from Mongolia, until a third, more complete specimen, found in China, was described in the early 1990s. It was the only one of the three to be found with intact hand bones, which reveal the transition from reptilian forelimb to avian flight wing. Furthermore, the specimen displays a grooved wrist bone, which would have enabled this early bird to fold its wing back as modern birds can. On the other hand, Sinornis also shows a short, toothed reptilian snout and a lizardlike pelvis. In 1993 a still younger fossil bird, 75 million years old, was described from two partial skeletons unearthed in Mongolia. The species was flightless, having had stubby arms ending in a large single claw, and may have evolved from an earlier flying form, as did rheas, emus, and ostriches. Named Mononychus olecranus, meaning "one claw, elbow head," the fossil bird appeared more closely related to modern birds than to Archaeopteryx. (See Zoology, above.)

      An average of two to three fully scientifically defensible discoveries of new bird species are made each year, adding to the approximately 9,250 living species known. In the past two years new discoveries included two warblers from China: the Chinese leaf warbler (Phylloscopus sichuanensis), distinguished from its closest relative, P. chloronotus, by its very different song and calls, and the Hainan leaf warbler (P. hainanus), a distinctively deep-yellow species.

      A summary of the results of an exceptionally long-term study (more than 40 years) of the fulmar, a seagoing petrel, revealed that males most commonly do not first breed until they are 10 years old and females, 12 years. The fulmar's mean adult life span appeared to be about 34 years, and the oldest known individual died at about age 46.

      (JEFFERY BOSWALL)

      This updates the article bird.

MARINE BIOLOGY

      Against the background of a prediction by the Intergovernmental Panel on Climate Change in 1990 that the global sea level is set to rise at the rate of 50-90 cm per 100 years, a Bermudian study in 1993 revealed that coastal areas of mangrove were being lost even at the current lower rates of 28 cm per 100 years. (A centimetre is about 0.4 in.) Mangrove fringes were shown to have kept up, by peat accumulation, only with mean sea level rises of 9-19 cm per 100 years. From 1983 to 1990 salt marshes in the Mississippi River delta were lost to the sea by coastal submergence at the rate of 50 sq km (19.3 sq mi) per year. In response, U.S. scientists investigated the potential for creating new salt marsh habitats on dredged material on which smooth cordgrass (Spartina alterniflora) had been transplanted. Initially, the transplanted marshes had lower sediment concentrations, fewer crustaceans, and greater Spartina densities than those of natural marshes but, given time, transplanted marshes could function as natural marshes.

      Waters of the Antarctic (or Southern) Ocean generally exhibit a low production of phytoplankton (the plant and plantlike component of plankton) and a low standing phytoplankton crop despite uniquely high nutrient content. South African studies of this so-called Antarctic Paradox demonstrated locally enhanced primary productivity associated with water stabilization by ice-melt water around Bouvet Island and the South Sandwich Islands in the far South Atlantic Ocean. Joint U.S. and U.K. studies showed that numbers of Antarctic fur seals (Arctocephalus gazella) and macaroni penguins (Eudyptes chrysolophus) correlated positively with the density of Antarctic krill (Euphausia superba), posing important new questions as to how swimming (and flying) predators locate and aggregate near concentrations of marine prey.

      U.S. researchers showed that both natural assemblages and cultures of phagotrophic nanoflagellates (the tiniest flagellates that ingest nutrients in the form of particles) consume and digest a variety of marine viruses, necessitating changes in current concepts of microbial processes in the sea. A Norwegian study concluded that decline of some blooms (rapidly formed dense populations) of the coccolithophorid microalga Emilian huxleyi was attributable to infection by viruses and consequent lysis (disintegration) of the algal cells. The same workers reported from Norwegian and Danish waters unusual viruslike particles with tails. The heads measure 340-400 nanometres (billionths of a metre), six to seven times larger than most marine viruses, and the tails are 2.2-2.8 micrometres (millionths of a metre) long. They may be new giant viruses whose host is unknown. Very large single-celled organisms, first discovered in the mid-1980s in the gut of a surgeonfish (Acanthurus nigrofuscus) and assumed to be protozoans, were shown by U.S. researchers using RNA analysis to be giant bacteria, the largest known to date. Measuring a half millimetre (0.02 in) in length, the reclassified organisms challenged scientists to explain how bacterial-cell architecture and nutrient-transport systems can support cells so large.

      A U.K. experiment conducted from the RRS Discovery from April to August 1989 as part of the Joint Global Ocean Flux Study (JGOFS) observed the south-to-north development of the spring phytoplankton bloom in the North Atlantic. As recently reported by investigators, the start of the bloom was correlated with the onset of water stratification, and seasonal succession commenced with diatoms, followed by coccolithophores, flagellates, and dinoflagellates. German studies detailed the distribution of zooplankton (the animal and animal-like component of plankton) at two sites in the temperate northeast Atlantic from the surface down to 4,500 m (14,800 ft). Downward from about 2,000 m (6,600 ft) above the seafloor, the depth-related decline in numbers of organisms and biomass was arrested. This characteristic was partly attributed to an upward flux of organic material, which was now recognized as a general feature in the deep ocean but the intensity and constancy of which was still poorly understood.

      Trilobite larvae (so called for their resemblance to the extinct trilobites) of the horseshoe crab Limulus polyphemus were found overwintering in densities of 1,000-10,000 individuals per square metre (about 11 sq ft) at depths greater than 15 cm in the intertidal sands of Delaware Bay on the U.S. east coast. Hitherto it had been assumed that all such larvae emerge in summer. This previously unrecorded life-history phenomenon might indicate a physiological tolerance that has contributed to the success of this ancient species over geologic time. Larval behaviour of scleratinian corals (Manicina areolata) off Panama and of fish species on Caribbean reefs was shown to exhibit remarkable lunar periodicity associated particularly with the timing of new moons. Synchrony of behaviour has advantages, but the adaptive significance of new moon timing remained to be explained. (ERNEST NAYLOR)

      This updates the articles crustacean; fish; mollusk.

BOTANY

      Every four or six years, scientists assemble at an International Botanical Congress. The purpose of the gathering is to exchange research information and to pass resolutions that will guide research efforts in the future. In 1993 the 15th such meeting took place in Yokohama, Japan, the first ever to be held in Asia; both Crown Prince Naruhito and Princess Masako (see BIOGRAPHIES (Naruhito, Crown Prince, and Princess Masako )) of Japan attended the opening ceremonies. The formal sessions were preceded by meetings focusing on plant nomenclature, and field trips were offered both before and after the meeting. The more than 3,000 scientists who attended heard symposium talks from botanists representing more than 30 nations on a range of topics, from the evolution of maize (corn) and pattern formation in flowers and shoots to global ecology and forestry.

      The majority of the earliest botanical books that still exist, either in museums or rare-book libraries, are the result of the intensive study of plants by those who have since been labeled herbalists. These botanist-physicians collected plants, made drawings, and described each plant by its "virtues"; that is, by its usefulness to humans for treating diseases and disorders. Their writings and illustrations appeared in collected works called herbals, which date back to the Middle Ages. Interest in medicinal and other uses of plants eventually developed into the present subdiscipline called economic botany and more recently into ethnobotany, which is the study of plant uses by indigenous peoples such as those who exist today in parts of Africa, South America, and the South Pacific. As a result of the work of the herbalists of yesterday and the ethnobotanists of today, many medicinal properties of plant extracts have been discovered. One of the more recent is taxol, a compound made by evergreens of the genus Taxus, which has been shown to be active against several kinds of cancer.

      The biological activity of taxol was first investigated in the late 1960s and early 1970s, when the compound was shown to disrupt the cell-division cycle (mitosis). Because the hallmark of cancer is uncontrolled cell proliferation, the compound appeared promising as an agent for slowing or halting tumour growth, and the desirability of producing it in quantity for medical research stirred the interest of both botanists and chemists. Taxol was first isolated from the inner bark of the Pacific yew tree (Taxus brevifolia). Unfortunately, the chemical is present in the bark in very low concentrations, and stripping the bark kills the tree, a limited resource in old-growth forests of the northwestern U.S. and Canada.

      Recently a close chemical relative of taxol, deacetylbaccatin III, was isolated from leaves of the European yew tree (Taxus baccata). The discovery was important because it provided chemists with a chemical that could be converted to an active substance similar to taxol; furthermore, because the leaves regrow on the plant, the trees do not die following harvest. Of perhaps even greater significance was a report in 1993 that taxol is produced by a fungus found growing as a parasite on the bark of a species of yew tree in Montana. The finding suggested the possibility of producing taxol in large fermentation tanks similar to the way penicillin is produced from the fungus Penicillium notatum. Meanwhile, other laboratories were engaged in devising chemical analogues of taxol that might prove as good as or better than the original compound in clinical trials—another sign of the growing enthusiasm for this family of drugs, first discovered in plants.

      The range of studies that used Arabidopsis thaliana as the experimental organism of choice continued to expand during the year. The small plant, which until recently had been known only as an inconspicuous weed, was fast becoming an invaluable tool for research in plant genetics, plant physiology, plant developmental biology, and plant molecular biology. Arabidopsis belongs to the mustard family, which includes such important crops as cabbage, broccoli, cauliflower, rape seed, and bok choy. The information explosion centring on Arabidopsis partially explained why this organism was chosen for a multinational genome research project, similar in direction to the much more publicized human genome effort.

      Because the plant is small, up to 30 cm (12 in) in height, it can be grown in large numbers in small spaces. Its diminutive seeds can be germinated in quantity in a single petri dish, making it easy to screen for plants having genetic mutations. By 1993 mutant plants had been isolated for a long list of characters. The small genome (total genetic endowment) for Arabidopsis was estimated to be about 100 million nucleotide bases, which are the molecular building blocks of DNA, which carries the genetic code. Compared with the human genome (estimated to be about three billion bases), this organism presents a much simpler model and allows for the analysis of defective as well as normal genes, using all of the power of modern biotechnology. Many of the mutations so far discovered are in so-called homeotic genes, resulting in disturbed patterns of development such that flower parts appear in incorrect locations. For example, flower petals become stamens (pollen-producing male organs), or stamens become carpels (ovule-bearing female structures). Using such developmental mutants, scientists were achieving a deeper understanding of the ways in which genes are regulated (switched on and off) at appropriate times. (PHILIP D. REID)

MOLECULAR BIOLOGY

      Red blood cells, or erythrocytes, are specialists in carrying molecular oxygen (O₂) from the lungs to the tissues of the body and for carrying carbon dioxide (CO₂) in the opposite direction. Hemoglobin, which is responsible for the red colour of blood, is the oxygen-carrying protein in erythrocytes. Carbonic anhydrase is the enzyme that, by catalyzing the conversion of carbon dioxide to another chemical species, allows the blood to take up carbon dioxide rapidly from the tissues and release it rapidly in the lungs. Hemoglobin uses atoms of iron for reversibly binding oxygen, whereas carbonic anhydrase uses atoms of zinc at its catalytic centre.

      All of the carbonic anhydrase in blood is found in the erythrocytes. It is significant that there is none of the enzyme in the blood plasma, the liquid portion of the blood. Indeed, in 1992 it was discovered by Eric D. Rousch and Carol A. Fierke of the Duke University Medical Center, Durham, N.C., that blood plasma contains a protein that strongly inhibits carbonic anhydrase. The inhibitor ensures that any carbonic anhydrase that might leak from the erythrocytes into the plasma will be rapidly inactivated. Why must carbonic anhydrase activity be restricted to the erythrocytes?

      Answering this question requires an understanding of the structure and function of hemoglobin. This protein is a tetramer, composed of four iron-containing, oxygen-binding subunits (called hemes) chemically bonded to a large protein unit (globin). Each subunit is 500 times larger than the molecule of oxygen that it carries. The reasons why hemoglobin must be a tetramer and as large as it is reveal an intricate choreography of chemical events that ensure that, whereas hemoglobin meets the body's need for oxygen, it simultaneously assists in eliminating carbon dioxide. They also reveal how much complexity underlies even seemingly simple physiological processes and how perfection of a function can be approached by stepwise refinements of imperfect mechanisms.

      The efficient transport of oxygen and of carbon dioxide depends on the modulation of the affinity of hemoglobin for oxygen by five different factors. Their roles will be discussed separately and then the individual strands woven together.

      One modulating factor is the cooperative interaction among hemoglobin's subunits in binding oxygen. The affinity of the tetrameric hemoglobin for oxygen is less than would be expected for a comparable monomeric protein; i.e., one containing a single heme subunit. For example, compared with myoglobin, a protein found in red muscle fibre, hemoglobin has only 1/26 the affinity for oxygen. Myoglobin functions well in its roles of storing oxygen in red muscle and increasing the rate of oxygen diffusion, but its affinity for oxygen is so great that it would be useless as a carrier of oxygen in the blood, for it would not release oxygen to the tissues. On the other hand, although the amount of oxygen bound by myoglobin increases in direct proportion to the concentration of oxygen (to the limit of one bound O₂ molecule per monomeric molecule of myoglobin), the amount of oxygen bound by hemoglobin increases exponentially as the 2.8th power of the concentration of oxygen (to the limit of four O₂ molecules per tetrameric molecule of hemoglobin). Hence, at low concentrations of oxygen, doubling its concentration would only double the amount bound by myoglobin but would increase the amount bound by hemoglobin 5.6-fold.

      It is the cooperativeness among hemoglobin's subunits that accounts for its exponential response to changes in oxygen concentration. The essence of the cooperativeness is that binding of a molecule of oxygen to one subunit makes it easier for a second molecule of oxygen to bind to a neighbouring subunit; the binding to the second causes a further increase in affinity for O₂ at the third subunit; and so on. This cooperativeness depends on a change in the shape of the subunit upon binding of oxygen. Because the subunits are tightly packed together in the hemoglobin tetramer, a change in shape of one subunit induces a comparable change in shape of its neighbours and thus an increase in their affinity for oxygen.

      The second modulating factor is acidity, or the concentration of protons (hydrogen ions, or H⁺). When a subunit of hemoglobin binds oxygen, it not only changes shape but also becomes a stronger acid and releases a proton. The oxygenation of one subunit of hemoglobin (HHb⁺) to form oxyhemoglobin (HbO₂) can be expressed by the following equilibrium:

      (1) HHb⁺ + O₂ ↔ HbO₂ + H⁺.

      The balance of this reaction can be shifted forward or in reverse by a change in the concentrations of either reactants or products. Raising the concentration of O₂ favours the forward direction and the binding of O₂, whereas raising the concentration of H⁺ (increasing the acidity) favours the reverse direction and the release of O₂.

      The effect of acidity on the binding of oxygen to hemoglobin was first reported by the Danish physiologist Christian Bohr in 1904 and is now called the Bohr effect. Bohr knew that working muscles become acidified and so understood that his discovery was physiologically significant. One source of acidification is lactic acid, a metabolic product made by muscle cells in extracting energy from glycogen. The other is carbon dioxide, which is hydrated (combined with a molecule of water [H₂O]) under the catalytic influence of carbonic anhydrase to make the bicarbonate ion (HCO₃^-), accompanied by the release of a proton. This reaction can be expressed by the following equilibrium:

      (2) CO₂ + H₂O ↔ HCO₃^- + H⁺.

      As the erythrocytes pick up carbon dioxide from the tissues, the hydration of CO₂ via carbonic anhydrase generates acid (H⁺). The increase in H⁺, in turn, drives reaction (1) in reverse, thus favouring the release of O₂. Once the erythrocytes reach the lungs, their release of CO₂ via the reverse of reaction (2) diminishes H⁺ and so drives reaction (1) forward, favouring the uptake of O₂. That the release of carbon dioxide in the lungs facilitates the binding of oxygen to hemoglobin was appreciated by the British physiologist J.S. Haldane in 1914.

      There is another important aspect to the effect of acidity on the oxygenation of hemoglobin via reaction (1), one having to do with buffering, or minimizing changes in the acidity of the blood. As shown in reaction (2), carbon dioxide entering the blood from the tissues is hydrated by carbonic anhydrase in the erythrocytes with the release of protons. The protons could seriously acidify the blood traversing the tissues were it not for the fact that they are at the same time being taken up by oxyhemoglobin as it releases oxygen—the reverse of reaction (1). Conversely, in the lungs the loss of carbon dioxide from the blood would seriously deplete H⁺ but for the fact that the hemoglobin present is releasing protons as it binds oxygen—reaction (2). Loss of carbon dioxide thus helps drive the oxygenation of hemoglobin in the lungs, while gain of carbon dioxide drives the release of oxygen from oxyhemoglobin in the tissues. The involvement of protons in both reactions (1) and (2) provides the basis for this synergism while simultaneously allowing the transport of large amounts of potentially dangerous acid without significant changes in the acidity of the blood.

      The third factor contributing to the modulation of the affinity of hemoglobin for oxygen is carbon dioxide. Not all of the carbon dioxide that enters the blood from the tissues is hydrated via reaction (2). Some of it reacts directly and reversibly with hemoglobin and in so doing diminishes hemoglobin's affinity for oxygen. This reaction provides another mechanism through which the release of oxygen is favoured in tissues, where carbon dioxide is high, and the binding of oxygen is favoured in the lungs, where carbon dioxide is low.

      The chloride ion (Cl^-) is the fourth modulating factor for hemoglobin. The hemoglobin molecule contains binding sites for chloride, and the binding of chloride decreases hemoglobin's affinity for oxygen. The significance of the chloride effect is enhanced by changes in chloride concentration within the erythrocyte during the respiratory cycle. As blood passes through the tissues, chloride rushes into the erythrocytes, facilitating the release of oxygen. When the blood enters the lungs, chloride leaves the erythrocytes, favouring the binding of oxygen. Carbon dioxide is the agent that drives these movements of chloride, and it does so in the following way. In the tissues carbon dioxide diffuses into the erythrocytes, where carbonic anhydrase converts it into bicarbonate while freeing a proton—reaction (1). Whereas the proton is taken up by the hemoglobin as it releases oxygen via reaction (2), the bicarbonate remains free in solution. As the concentration of bicarbonate rises, it diffuses from the erythrocyte by way of specialized channels in the cell membrane. Because electrical neutrality must be maintained, for each negatively charged bicarbonate that diffuses out of the erythrocyte, some other negatively charged ion must go the other way. That compensating ion is chloride, the most abundant negatively charged ion in blood plasma.

      This shift of bicarbonate out of the erythrocytes when they are in the tissues and into the erythrocytes when they are in the lungs, with chloride always moving in the opposite direction, has long been known as the chloride shift, or the isohydric shift. It was earlier understood as a necessary consequence of the confinement of carbonic anhydrase to the erythrocyte. It can now be seen as yet another adaptation that aids delivery of oxygen from hemoglobin to the tissues and uptake of oxygen by hemoglobin in the lungs.

      The final factor involved in the modulation of hemoglobin is a compound called 2,3-diphosphoglycerate (DPG). DPG has long been known to be required in catalytic amounts as a cofactor for the action of the enzyme phosphoglyceromutase (PGM). That enzyme is required for the metabolism of the sugar glucose, which occurs in erythrocytes. It had not been clear, however, why erythrocytes contain much higher concentrations of DPG than do other cells. This seeming anomaly was clarified in the early 1970s by Reinhold and Ruth Benesch of Columbia University, New York City, who showed that hemoglobin contains a binding site for DPG and that occupancy of that site markedly decreases the affinity of hemoglobin for oxygen.

      In the absence of DPG, hemoglobin would be a poor carrier of oxygen because it would hold oxygen so tightly as to prevent its significant release to the tissues. DPG, by binding to the oxygen-free form of hemoglobin but not to oxyhemoglobin, competes with oxygen in the erythrocytes for binding to hemoglobin. In so doing it decreases the affinity of hemoglobin for oxygen just enough to make it an effective carrier of oxygen from the lungs to the tissues. One of the adaptations of the human body to the modestly lower oxygen levels encountered at high altitudes is an increase in the concentration of DPG in erythrocytes. This increase provides more complete release of oxygen from hemoglobin in the tissues without significantly compromising the degree to which hemoglobin is oxygenated in the lungs.

      Given the foregoing background, one is now able to understand why carbonic anhydrase activity in the blood must be restricted to the erythrocytes and why an inhibitor of carbonic anhydrase is needed in the blood plasma. If carbonic anhydrase were present in the plasma, then protons and bicarbonate would be formed in the plasma from carbon dioxide as blood passed through the tissues. The bicarbonate would then diffuse into the cells, and chloride would have to move out to maintain electrical neutrality. Loss of chloride from the cells would decrease the binding of chloride to hemoglobin, which would increase hemoglobin's affinity for oxygen at the very time when a decrease in affinity would be desirable to assist the release of oxygen to the tissues. Conversely, in the lungs bicarbonate leaving the erythrocytes would exchange with chloride moving in; again, this exchange would decrease the affinity of hemoglobin for oxygen just when the opposite was desirable.

      It is thus clear that the binding of chloride to hemoglobin, with concomitant decrease in affinity of hemoglobin for oxygen, can have physiologically useful effects only when the hydration of carbon dioxide is restricted to the erythrocytes. The presence of carbonic anhydrase inside the erythrocytes, and of an inhibitor of carbonic anhydrase outside these cells, guarantees such an outcome.

      In the end, given all the things that hemoglobin accomplishes, one wonders not why this exquisite molecule needs to be so much bigger than the oxygen that it carries but rather how so small a molecule can do so much.

      (IRWIN FRIDOVICH)

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      This updates the articles agriculture, history of (agriculture, origins of); animal behaviour; biology; biosphere; cancer; conservation; disease; evolution, theory of (evolution); heredity; mammal; reproduction; sensory reception (senses).

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