Meaning of YEAR IN REVIEW 2000: LIFE-SCIENCES in English


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. 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 permeatesenters and leavescells, 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 fibresa 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. 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 million360 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 million70 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 tailssauropods such as Apatosaurus, Diplodocus, and Brachiosaurusas 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 million86 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 199798 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 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 thermoclinethe 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 199395, 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 fungusa chytridiomycetecould 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 100350 m (3301,300 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

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