This month in Science Roundup:
- Coral Reef Complexities
- E-Mail and Evolving Social Networks
- Geometry without Words
- Feline Family Tree
- Virus Tracks Cougar Recovery
- Viral Network Interactions
- The Soil Antibiotic Resistome
- A Biofuels Future?
- Cloud Formation on Titan
- Stretchable Silicon
- Twisted Magnetism
- Measuring Mountain Uplift
Coral reefs, among the world's most complex and fragile ecosystems, are facing threats ranging from overexploitation to water pollution. Although there is no single cure for all of their ills, marine protected areas have emerged as a potentially powerful means for managing reefs. Nevertheless, the effects of these reserves on fish populations remain poorly understood. In a Report in the 6 Jan 2006 Science, Mumby et al. ( http://www.sciencemag.org/cgi/content/short/311/5757/98) examined the impacts of a large and long-established coral reef reserve in the
E-Mail and Evolving Social Networks
Social networks are dynamic processes that evolve over time depending on the shared characteristics, activities, and affiliations of their members. In a Report in the 6 Jan 2006 Science Kossinets and Watts ( http://www.sciencemag.org/cgi/content/short/311/5757/88) offered an empirical analysis of social network evolution in a large university community. Over the course of one academic year, the team compiled a registry of e-mail interactions between more than 43,000 students, faculty, and staff. In combination with encrypted information about personal attributes, as well as lists of classes taught and attended, the team assembled a quantitative picture of how the strength of personal interactions depends on similarities between the individuals and how the interactions change with time. The results show that at least in this particular environment, people were more likely to form ties with others when they had a shared "focus" such as a class that brought them together or a mutual acquaintance, but were less likely to interact solely on the basis of shared characteristics such as age or gender. Comparative studies of corporate or military networks could shed light on which features of network evolution are generic and which are specific to a given cultural, organizational, or institutional context.
Are the conceptual principles of math and science inherent in the human mind, irrespective of language or schooling? In a Report in the 20 Jan 2006 Science, Dehaene et al. ( http://www.sciencemag.org/cgi/content/short/311/5759/381) presented an intriguing study of an indigenous Amazonian group called the Mundurukú that sheds light on basis of human geometric knowledge. Most of the Mundurukú have had little or no schooling, their language has few words dedicated to mathematical, geometrical, or spatial concepts, and maps are not part of their culture. Nevertheless, the researchers found that both Mundurukú children and adults proved competent at grasping basic geometric concepts, such as parallelism, symmetry, and congruence, performing just as well as American children in a series of tests. Furthermore, the Mundurukú were also able to use geometric relationships diagrammed on paper to locate hidden objects, and again performed as well as American children but not as well as American adults. Thus, the new study offers evidence for geometrical intuition in the absence of schooling, experience with graphic symbols or maps, or a vocabulary of geometrical terms. An accompanying News story by C. Holden ( http://www.sciencemag.org/cgi/content/short/311/5759/317a) highlighted the Report.
Modern cats are one of the world's most successful families of carnivores, but piecing together their evolutionary history has been difficult in part because of incomplete fossil records and the lack of distinguishing skeletal features among ancient cats. In a Report in the 6 Jan 2006 Science, Johnson et al. ( http://www.sciencemag.org/cgi/content/short/311/5757/73) presented a new cat family tree based on analyses of X-chromosome, Y-chromosome, and mitochrondrial DNA sequences from all 37 living cat species. Modern cats appear to have originated in Asia during the Late Miocene (<11 million years ago, Ma) and to have undertaken a series of 10 intercontinental migrations that correlate with major changes in sea level. Beginning with divergence of the Panthera lineage from which all the big cats including lions, jaguars, leopards, and tigers emerged, the major feline lineages were established during a short evolutionary time period (10.8 to 6.2 Ma). During this time, low sea levels exposed land bridges between continents and facilitated cat migration throughout the globe. Sea levels then rose, confining species to their respective continents, but then fell again about 3 million years ago allowing for a second round of migration and speciation. The domestic cat lineage probably diverged between 6.7 and 6.2 million years ago in Asia and Africa, either from ancestors that had never left Asia or from North American cats that had traveled back across the Bering land bridge.
Because viruses evolve so quickly, they hold great promise as genetics tags to record changes in the demography of their host populations, which tend to evolve much more slowly. In a proof-of-principle study reported in the 27 Jan 2006 Science, Biek et al. ( http://www.sciencemag.org/cgi/content/short/311/5760/538) used a fast -evolving RNA virus called feline immunodeficiency virus (FIV), to track cougar populations in western North America, which underwent drastic declines in the early 20th century due to heavy hunting pressure, but have recently begun to recover. Cougars are commonly infected by their own type of FIV, but do not suffer any major consequences, thus offering a unique opportunity for researchers to trace their regrowth via changes in the spatial and temporal distribution of the virus. The team analyzed DNA samples from 352 cougars in Montana, Wyoming, British Columbia and Alberta, more than one quarter of which were infected with FIV. Their analysis suggests that eight main lineages of the virus began to evolve separately only within the last 20 to 80 years, probably corresponding to several drastically shrunken and geographically separated cougar groups. The data also suggest that concurrent with the recovery of separated populations, surviving lineages have consistently expanded their range -- good news for cougar fans.
Virus infection triggers dramatic changes in the host and in the infecting virus, many of them involving interactions between host and viral proteins. In a Report in the 13 Jan 2006 Science, Uetz et al. ( published online 8 Dec 2005; http://www.sciencemag.org/cgi/content/short/311/5758/239) provided new insight into these interactions from a network perspective. The team first mapped the protein interactions for two herpesviruses, Kaposi sarcoma-associated virus and varicella-zoster virus, which causes chickenpox and shingles. According to their analysis, each viral network looks like a cohesive "module" arising from high connectivity between all the proteins in the network. This is very different from the structure of cellular protein networks, which show a clustered organization of smaller subnodules that often correspond to individual protein complexes or pathways. How then does a pathogen integrate with a host network? To answer this question, the team used computational modeling to "dock" the viral network into a prototypical map of human protein interactions. The simulations reveal that rather than targeting a single module in the human network, the virus melds into multiple, weakly connected human modules, thereby adopting features of a cellular network. As noted in an accompanying Perspective by J. S. Bader and J. Chant ( http://www.sciencemag.org/cgi/content/short/311/5758/187), understanding host and pathogen networks and how they combine during infection should aid the development of new strategies for controlling human disease.
The soil beneath our feet is a reservoir rich with antimicrobial agents. Indeed, most clinically relevant antibiotics, including erythromycin, tetracycline, and vancomycin, originate from soil-dwelling bacteria. Now a Report in the 20 Jan. 2006 Science demonstrates that the same microbes that have been key to our success in controlling bacterial disease also represent a vast reservoir of antibiotic resistance. In a study of 480 spore-forming bacteria collected from diverse soil sources (urban, agricultural, and forest), D'Costa et al. ( http://www.sciencemag.org/cgi/content/short/311/5759/374) found that each strain was resistant to at least seven or eight different antibiotics and some to as many as 20. The antibiotics tested comprised natural products, their semisynthetic derivatives, and fully synthetic molecules; and included both well-established and newly developed drugs. Further analysis of the soil "resistome" revealed a number of previously uncharacterized antibiotic resistance mechanisms, some involving inactivating enzymes and others involving biochemical detoxifying reactions or unfamiliar gene mutations. As noted in an accompanying Perspective by A. Tomasz ( http://www.sciencemag.org/cgi/content/short/311/5759/342), these protective mechanisms may offer clues for generating a new arsenal of therapeutic drugs.
Driven by surging global oil demand, dwindling supply, and growing concerns about greenhouse gas concentrations, the world is now seriously considering biofuels -- renewable energy sources derived from plant materials -- as alternatives to fossil fuel. Ethanol is a renewable resource already in use as a liquid fuel, but its production from corn and cellulose is energy intensive, and some analyses have found that the overall process uses more energy than it creates. Now, a Report by Farrell et al. ( http://www.sciencemag.org/cgi/content/short/311/5760/506) in the 27 Jan 2006 Science shows that the studies reporting negative net energy values are flawed and that fuel ethanol can yield more energy than is required for its synthesis. But although current corn ethanol technologies are much less petroleum-intensive than gasoline, they have greenhouse gas emissions similar to those of gasoline. New production methods are therefore needed to boost ethanol's environmental performance.
In a related Review article, Ragauskas et al. ( http://www.sciencemag.org/cgi/content/short/311/5760/484) highlighted progress in biofuels research in areas ranging from plant genetics (to increase crop yield) to enzymatic and other catalytic methods for converting biomass into valuable fuels and products. The integration of these technologies offers the potential for the development of sustainable biopower that can enhance energy security, reduce emissions and provide economic transport ( see also the Editorial by S. E. Koonin; http://www.sciencemag.org/cgi/content/short/311/5760/435)
Recent telescope observations and images from the Cassini spacecraft have provided vivid glimpses of clouds in the hazy atmosphere of Titan, Saturn's largest moon. In a Report in the 13 Jan 2006 Science, Rannou et al. ( http://www.sciencemag.org/cgi/content/short/311/5758/201) proposed a general circulation model that incorporates cloud microphysics to explain how these clouds form. The model allows prediction of the shapes, locations, altitudes, opacities and lifetimes of the clouds, which are likely the result of a dynamically controlled methane cycle similar to that of water on Earth. It mimics the two types of clouds currently seen on Titan -- large long-lasting clouds near the south pole, and smaller transient clouds at mid-southern latitudes -- but also predicts clouds in other places like the north pole, where they have not (or not yet) been observed. The model does have its limitations. It assumes that the clouds are homogenous across longitudes even though previous observations indicate that clouds are actually concentrated at certain longitudes. Future details about Titan's gravitational tides and about the moon's surface topography and geology may help explain these differences. An accompanying Perspective by E. Lellouch ( http://www.sciencemag.org/cgi/content/short/311/5758/186) highlighted the study.
Stretchable silicon could be the next wave in high-performance electronics. From flexible sensors to paperlike displays, one can envision an array of uses for supple materials with good electronic properties. Now, an achievement reported in the 13 Jan 2006 Science has opened the door to these possibilities. Khang et al. ( http://www.sciencemag.org/cgi/content/short/311/5758/208) described synthesis of a stretchable form of single-crystal silicon with microscale, wavelike geometries. To create the material, the researchers first fabricated ultrathin silicon ribbons (about 100 nanometers thick) on a wafer using procedures similar to those used in conventional electronics. Then, a flat rubber substrate was stretched and placed on top of the ribbons. Peeling the rubber away lifts the ribbons off the wafer and leaves them adhered to the rubber surface. Releasing the stress in the rubber causes the silicon ribbons to buckle, thus forming a well-defined wavy structure that is amenable to either stretching or compression. The researchers used the new material to build diodes and transistors and showed that the wavy devices performed as well as traditional rigid devices and could be repeatedly stretched and compressed without damage or alteration of their electrical properties.
Magnetic materials are in essence, assemblies of billions of tiny electron spins. Within a given material, spins are typically oriented in parallel or antiparallel, but they can also take on more complex arrangements. For example, helical spin order -- in which spin direction rotates by a constant angle around the axis of a helix -- has been observed in some rare-earth metals and alloys. In a Report in the 20 Jan 2006 Science, Uchida et al. ( http://www.sciencemag.org/cgi/content/short/311/5759/359) offered an unprecedented look at the details of this type of magnetic structure. Using a special imaging technique called Lorentz microscopy, the researchers showed that helical spin structure is more complicated and dynamic than predicted from past structural analysis. Specifically, helical spin order exhibits a variety of magnetic defects at the boundaries between spin orientations -- similar to the atomic dislocations that occur in crystal lattices -- in response to changes in temperature and magnetic field. Because the relative orientation of spin moments between magnetic planes affects the flow of electrical current, the new details could have important implications for the development of spintronic devices that rely on helical spin order. An accompanying Perspective by F. Nori and A. Tonomura ( http://www.sciencemag.org/cgi/content/short/311/5759/344) highlighted the study.
Determining the elevation history of Earth's mountains and plateaus, a field known as paleoaltimetry, is important to understanding tectonic processes as well as ancient climate change. However, this has been notoriously difficult because of a lack of direct elevation proxies in the geological record. Mountain ranges cause air masses to rise and lose moisture, thereby creating an oxygen isotope composition gradient in the precipitation that is quantitatively related to elevation of the land surface, and indirectly related to temperature. Although this gradient is preserved in ancient oxygen-containing minerals, factors such as climate change or the seasonality of precipitation can confound elevation reconstructions based solely on this isotopic data. In a Report in the 27 Jan 2006 Science, Ghosh et al. ( http://www.sciencemag.org/cgi/content/short/311/5760/511) described a new and improved method of assessing surface uplift based on the temperature-dependent binding between rare carbon and oxygen isotopes in ancient carbonate minerals. The approach allows researchers to couple changes in mineral growth temperature through time with knowledge of the modern temperature change with elevation garnered from conventional isotope data. Using this technique, the team provided strong evidence indicating that the Altiplano plateau in the Bolivian Andes rose rapidly -- roughly 3700 meters between about 6.7 and 10.3 million years ago. As noted in an accompanying Perspective by M. A. Poage and C. P. Chamberlain ( http://www.sciencemag.org/cgi/content/short/311/5760/478), the new method "represents an exciting refinement in paleoaltimetry that will improve our understanding of the evolution of mountain ranges."