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This month in Science Roundup:
Evolution in Action
http://www.sciencemag.org/sciext/btoy2005/
Each year, the editors and news staff of Science look back at the big science stories of the past 12 months, and dub one of them the Breakthrough of the Year. In 2005, the top prize went to "evolution in action" -- a host of genetic studies and field observations that have shed light on how living creatures evolve. Notable strides included sequencing of the chimpanzee genome, reconstruction of the 1918 flu virus, and insight into the driving forces of speciation in stickleback fish and small birds called European blackcaps. A special section of the 23 Dec 2005 Science highlighted these accomplishments, and other advances worthy of runners-up commendation, including a host of successful planetary missions, genetic clues to brain disorders such as schizophrenia and Tourette syndrome, insights into the turbulent lives of neutron stars, and the best look yet at the molecular structure of voltage-gated potassium channel. Not all the news was good, unfortunately. Budget woes brought U.S. particle physics research to a halt, thus affording it the onerous distinction of "Breakdown of the Year." But more exciting science is sure to follow in the coming year: influenza studies, gravitational waves, and RNAi-based treatments are among the fields chosen by our editors as areas to watch in 2006.
Science Online offered array of multimedia enhancements to round out its year-end tribute. A streaming-video presentation ( http://www.biocompare.com/science/btoy2005 ) highlighted the year's top breakthrough; a podcast ( http://www.sciencemag.org/about/podcast.dtl ) featured discussions about the runners-up; and each article was accompanied by a set of links to selected papers and relevant Web resources. Finally, ScienceCareers.org profiled some of the researchers who are applying new technology to the advancement of evolutionary science.
Genetics of Skin Color
The chief determinant of skin color is the pigment melanin, which protects against ultraviolet rays, but little is known about the specific genes that contribute to the striking variations in human skin tone. Now, an exciting clue has emerged from an unlikely source: the zebrafish. In a Research Article in the 16 Dec 2005 Science, Lamason et al. ( http://www.sciencemag.org/cgi/content/short/310/5755/1782 ) reported the identification of a zebrafish pigmentation gene, the human counterpart of which appears to play a key role in determining skin color. The team found that zebrafish golden mutants, which have lighter colored stripes than wild-type fish, arise from a mutation in the slc24a5 gene, which is thought to be involved in ion exchange across cellular membranes. The gene is highly conserved in vertebrates and expression of the human version in the golden zebrafish restored the wild-type coloring. A search for genetic variants among humans revealed that European populations carry a slightly different version of the gene than African and East Asian populations. The variants differ by only one amino acid, but seem to account for a major part of the difference between African and European skin tones. An accompanying News story by M. Balter ( http://www.sciencemag.org/cgi/content/short/310/5755/1754a ) highlighted the study.
Making Uncertain Decisions
Research shows that people are more comfortable making choices with defined risks than they are making choices when the risks are ambiguous. In a Report in the 9 Dec 2005 Science, Hsu et al. ( http://www.sciencemag.org/cgi/content/short/310/5754/1680 ) investigated the neural distinctions between these two types of decision-making. The researchers used functional magnetic resonance imaging (fMRI) to monitor the brains of subjects during a gambling task that required making decisions based on varying levels of uncertainty. The results show that distinct areas of the brain, including the amygdala and the orbitofrontal cortex (OFC), are more active during ambiguous than risky decision making. These areas have both been implicated as emotion-processing centers. On the other hand, risk was found to activate a brain region called the dorsal striatum, which has previously been implicated in predicting reward. Interestingly, another component of the study revealed that patients bearing lesions in the OFC do not distinguish between degrees of uncertainty and show no aversion to ambiguity. As noted in an accompanying Perspective by A. Rustichini ( http://www.sciencemag.org/cgi/content/short/310/5754/1624 ), "[e]lucidating the neural processes underlying decision-making may help us understand important economic differences between ambiguity and risk."
MicroRNA Marvels
MicroRNAs (miRNAs) are small 20- to 25-nucleotide RNA molecules that have been found to control many aspects of plant and animal development by sequence-specific interaction with other RNAs and subsequent repression of protein-coding genes. Two Reports in Science this month provide new insight into the scope and magnitude of miRNA's influence.
--Farh et al. (16 Dec 2005, published online 24 Nov; http://www.sciencemag.org/cgi/content/short/310/5755/1817 ) showed that microRNAs influence the expression or evolution of the majority of mammalian genes. Thousands of protein-encoding messengers RNAs (mRNAs) contain short sequences or "seeds" that are evolutionarily conserved and that match portions of miRNAs. The team found that many of the genes with conserved sites are highly expressed at developmental stages and that their levels tend of fall as miRNA begins to accumulate. This mechanism ensures that key genes are only turned on at the right time and/or place. Interestingly, the team also found that mRNAs with nonconserved seed sequences are also easy targets of miRNAs -- but only in the test tube. In real cells, mRNAs with nonconserved sites were generally absent in cells with corresponding microRNAs. This suggests that over the course of evolution many mRNAs, in order to maintain their function and possibly ensure fitness of the organism, have evolved to avoid sites that readily pair up with miRNAs.
--Boehm and Slack ( 23 Dec 2005; http://www.sciencemag.org/cgi/content/short/310/5756/1954 ) found that in the worm C. elegans, the microRNA lin-4, which is a key regulator of the timing of larval development, also regulates adult longevity. Reducing the activity of lin-4 shortened life span and accelerated tissue aging, whereas overexpressing lin-4 or reducing the activity of its target lin-14 (a putative transcription factor) extended life span. Experiments indicate that the miRNA and its target exert their effects by regulating the insulin/IGF-1 signaling pathway, previously shown to be a key modulator of worm life span. An accompanying Perspective by A. Antebi ( http://www.sciencemag.org/cgi/content/short/310/5756/1911 ) highlighted the Report.
Crossing Membranes
http://www.sciencemag.org/sciext/membranes/
How cells generate and maintain their internal structures and integrity depends in large part on the effectiveness of the cell membrane, a semipermeable lipid bilayer, in keeping the inside in and the outside out. In a special collection of articles published with the 2 Dec 2005 issue, Science and its online companion the Signal Transduction Knowledge Environment ( STKE; http://stke.sciencemag.org/ ) explored the mechanisms used by cells to transport ions and macromolecules across the membrane. Review articles in Science described the translocation of proteins through membrane-bound cellular compartments, the ins and outs of DNA transfer in bacteria; and the principles of selective ion transport in channels and pumps. STKE looked at how information is transmitted across cell membranes to allow cells to communicate with each other and to respond to signals in their environments. Insight into all of these membrane-related processes is fundamental to our understanding of all other aspects of cellular and organismal physiology.
Predicting Antibody Efficacy
Different classes of antibody (also known as immunoglobulins) perform diverse functions in the immune system. In mammals, IgG -- the major immunoglobulin class in serum -- exists as four structurally distinct subclasses and although all of them can recognize antigens on foreign cells, each subclass has different abilities to trigger immune defenses. In a Report in the 2 Dec 2005 Science, Nimmerjahn and Ravetch ( http://www.sciencemag.org/cgi/content/short/310/5753/1510 ) showed that different IgG behaviors are determined by the binding affinities of a portion of the antibody called the constant region, for different inhibitory and activating receptors on an immune cell's surface. In essence, the receptors dictate how efficiently an antibody will recognize a foreign cell, bind to it, and destroy it. The team further demonstrated that in mice, the ratios of activating-to-inhibitory receptor binding can be used to predict the ability of natural and synthetic antibodies to elicit different immune responses. As noted in an accompanying Perspective by J. M. Woof ( http://www.sciencemag.org/cgi/content/short/310/5753/1442 ), if the same method works in humans, it may offer a new way to predict antibody performance for clinical applications like cancer therapeutics and vaccine development.
Lizards and Island Biogeography
Biogeography involves the study of patterns of species distribution in space and time. It is a science that has become central to conservation biology because effective conservation strategies depend on accurate biodiversity metrics. Unfortunately, the large spatial and temporal scales of evolutionary and biogeographical processes make large-scale field studies of population ecology problematic. But large-scale patterns are mediated by small-scale population-level processes such as competition, immigration, and habitat usage, and islands have proven valuable model systems in which to study them. In a Report in the 16 Dec 2005 Science, Schoener et al. ( http://www.sciencemag.org/cgi/content/short/310/5755/1807 ) described a field experiment conducted on small islands in the Bahamas, designed to directly test the relation between a population process -- survival of anoles lizards -- and vegetation type, a key feature of island biogeography. The team found that on islands where they introduced a ground-hunting predatory lizard, anoles survival was highest in the tallest vegetation, indicating that the lizards shifted to higher perches to escape their predators. However, on islands without the introduced predator, anoles survived better in habitats with shorter vegetation. This is most likely because taller vegetation tends to attract more birds, which are natural anoles predators. An accompanying Perspective by R. S. Thorpe ( http://www.sciencemag.org/cgi/content/short/310/5755/1778 ) highlighted the study and hinted at the possibility of using similar studies to investigate the adaptation of species and communities to climate change.
First Bird Revisited
Archaeopteryx -- a magpie-sized creature with sharp teeth, broad wings, and a long bony tail -- is broadly recognized as the first known bird. It has been represented by nine skeletal specimens dating to about 150 million years ago (during the Late Jurassic period), but most of them are fragmentary or poorly preserved. In a Report in the 2 Dec 2005 Science, Mayr et al. ( http://www.sciencemag.org/cgi/content/short/310/5753/1483 ) described the tenth and best-preserved Archaeopteryx specimen yet. The new specimen provides important details about the feet and skull of this ancient bird that link it to its close dinosaur relatives. Unlike modern birds, the first toe of Archaeopteryx is not fully reversed, suggesting that the bird did not have a perching foot. But like some classes of dinosaurs, including the large-clawed Velociraptor, its foot does show a hyperextendible second toe. Wing and tail feather impressions are also well preserved. An accompanying News story by E. Stokstad ( http://www.sciencemag.org/cgi/content/short/310/5753/1418 ) discussed the high-profile specimen and its current low-key home: a small, privately owned dinosaur museum in Thermopolis, Wyoming.
Land Use and Climate Change
The presence of greenhouse gases in the atmosphere is the best known impact of human activity on climate change, but other anthropogenic factors like land use also play a role. Clearing forests for agriculture, for example, affects the reflective properties of Earth's surface, which in turn alters the absorption rate of solar radiation, heat transport dynamics, and even the spatial patterning of thunderstorms. In a study reported in the 9 Dec 2005 Science, Feddema et al. ( http://www.sciencemag.org/cgi/content/short/310/5754/1674 ) investigated the effect of changes in land cover on global climate in simulations for the next century. They found that although land-cover changes are unlikely to affect global average temperature (because regional variations tend to cancel each other out), they can significantly alter regional climate metrics including surface temperature and precipitation, particularly in mid-latitude and tropical areas. As noted in an accompanying Perspective by R. A. Pielke Sr. ( http://www.sciencemag.org/cgi/content/short/310/5754/1625 ) the results underscore the importance of considering the large and complex range of effects of anthropogenic disturbances in order to better understand the collective human influence on climate.
Zircon's Tale of Early Earth
Although the Earth is estimated to have formed about 4.6 billion years ago (Ga), the oldest preserved continental crust dates to about 4 Ga . So when did the growth of the Earth's crust begin, and how long did it take? The long-favored paradigm is that the first crust formed after 4 billion years ago and grew slowly until the present day. Now new isotopic data from minerals called zircons -- common components of crustal rocks -- suggest that the crust formed much earlier. In a Report in the 23 Dec 2005 Science (published online 17 Nov), Harrison et al. ( http://www.sciencemag.org/cgi/content/short/310/5756/1947 ) presented measurements of the radioactive decay of lutetium-176 to hafnium-176 (half-life 37 billion years) from more than 100 of the oldest known zircons. The data imply that significant continental crust must have formed on Earth perhaps as early as 4.5 billion years ago. The results also support the view that this original crust was largely recycled back into the mantle. As noted in an accompanying Perspective by Y. Amelin ( http://www.sciencemag.org/cgi/content/short/310/5756/1914 ), the new data push back our knowledge of Earth's history to less than 100 million years after planetary accretion and provide important insights about how Earth formed and evolved.
How Metals Soften
Adding impurities to a metal often makes it harder and stronger -- a phenomenon that humans have exploited since the dawn of the Bronze Age some 5000 years ago. Nevertheless, some alloys have the puzzling property of being softer than their starting materials. In a Report in the 9 Dec 2005 Science, Trinkle and Woodward ( http://www.sciencemag.org/cgi/content/short/310/5754/1665 ) helped explain why. The team used a series of computer models to study the softening of molybdenum alloyed with platinum and rhenium. Calculations indicate that the softening effect has to do with changes in the mobility of dislocations -- defects in a crystal structure that can cause it to shift or deform under stress. Adding some solutes lowers the energy needed to move the metal's dislocations -- and the more mobile the dislocations, the more plastic the alloy becomes. The team's model further explains how a given solute concentration can soften an alloy at one temperature and harden it at another, and how at a given temperature, increasing solute concentration can first soften and then harden the alloy. Understanding the fundamentals of metal softening could aid the development of materials tailored for specific applications such as structural components for turbine engines or nuclear power plants that can withstand large changes in temperature. An accompanying Perspective by D.C. Chrzan ( http://www.sciencemag.org/cgi/content/short/310/5754/1623 ) highlighted the Report.
Unbalanced Superfluidity
Similar to a superconductor, in which electrical current moves without resistance, a superfluid is a phase of matter characterized by flow without friction. A number of elements and compounds including isotopes of helium have been found to undergo the superfluid phase transition at ultracold temperatures and theorists believe that the superdense cores of neutron stars may be superfluid as well. According to standard theory, a superfluid cannot exist when one type of particle has more energy than the other. In the case of atoms, for example, the numbers of opposite-spinning atoms need to be equal. Now two Reports published online in Science Express on 22 Dec 2005 show that cold clouds of atoms with unequal populations of atomic spins can maintain a surprisingly robust superfluid state ( see the News story by A. Cho in the 23 Dec issue; http://www.sciencemag.org/cgi/content/short/310/5756/1892a ). Both studies involved creating an ultracold gas of the isotope lithium-6 and trapping it with a laser beam for analysis. Zwierlein et al. ( http://www.sciencemag.org/cgi/content/abstract/1122318 ) found that superfluidity persists when atoms spinning one way outnumber those spinning the other way by as much as 70%. The imbalanced gas mimics the dense soup of subatomic "quarks" at the center of a neutron star, as some types of quarks outnumber others. Meanwhile, Partridge et al. ( http://www.sciencemag.org/cgi/content/abstract/1122876 ) looked at the spatial structure of the mixed spin system and found that when the imbalance exceeds 9%, the gas separates into a phase that is consistent with a superfluid paired core surrounded by a shell of normal unpaired atoms. The new work may help scientists better understand similar imbalances, like those found in magnetized superconductors.
Science in Tunisia
Science concluded its yearlong Global Voices of Science essay series with an essay by Tunisian physicist Zohra Ben Lakhdar ( http://www.sciencemag.org/cgi/content/short/310/5753/1435 ) in the 2 Dec 2005 issue. When Lakhdar she was growing up in Tunisia in the 1950s and 1960s, opportunities for women in science were virtually nonexistent. Her essay chronicles how she overcame personal, political, social, and cultural obstacles to become a professor of physics at the University of Tunis, El Manar, in Tunisia in 1978. She was the first woman in her country to achieve that status. Her work now includes theoretical studies of optical phenomena and the development of spectroscopic methods to study the influence of pollutants on the quality of air, water, and plants and one day she hopes to develop technologies that would make it possible to convert deserts into arable land and transform sea water in to potable water. She is devoted to widening the pathway to science for more Tunisian and African women and hopes to establish an international scientific center of optics and photonics where African researchers can study and train. An online slideshow highlighted the essay ( http://www.sciencemag.org/sciext/globalvoices/ ).
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