This Week in Science

Science  10 Dec 2010:
Vol. 330, Issue 6010, pp. 1451
  1. Assessing Biodiversity Declines


      Understanding human impact on biodiversity depends on sound quantitative projection. Pereira et al. (p. 1496, published online 26 October) review quantitative scenarios that have been developed for four main areas of concern: species extinctions, species abundances and community structure, habitat loss and degradation, and shifts in the distribution of species and biomes. Declines in biodiversity are projected for the whole of the 21st century in all scenarios, but with a wide range of variation. Hoffmann et al. (p. 1503, published online 26 October) draw on the results of five decades' worth of data collection, managed by the International Union for Conservation of Nature Species Survival Commission. A comprehensive synthesis of the conservation status of the world's vertebrates, based on an analysis of 25,780 species (approximately half of total vertebrate diversity), is presented: Approximately 20% of all vertebrate species are at risk of extinction in the wild, and 11% of threatened birds and 17% of threatened mammals have moved closer to extinction over time. Despite these trends, overall declines would have been significantly worse in the absence of conservation actions.

    1. Making a Point with Metamaterials

        A long-predicted electromagnetic excitation, the toroidal moment (or anapole), is associated with toroidal shape and current flow within a structure and has been implicated in nuclear and particle physics. This distinct family of electromagnetic excitations has not been observed directly because they are masked by much stronger electric and magnetic multipoles. Kaelberer et al. (p. 1510, published online 4 November) have developed a metamaterial structure based on stacked loops of inverted split-ring resonators (“metamolecules”) whose response under excitation is consistent with the existence of a toroidal moment. The metamaterial is designed so that both the electric and magnetic dipole moments induced by an incident electromagnetic wave are suppressed, while the toroidal response is isolated and resonantly enhanced to a detectable level.

      1. Fragile Tin Oxide Electrodes

          While tin oxide has a high energy density, and would thus make an attractive anode material for a Li-ion battery, it undergoes significant volume changes when Li is intercalated. The large strains cause cracking, pulverization, and a resultant loss of electrical conduction. Huang et al. (p. 1515; see the Perspective by Chiang) used in situ transmission electron microscopy on a single tin oxide nanowire to identify the physical changes that occur during intercalation and observed a moving cloud of dislocations that separated the reacted and unreacted sections. Upon completion of the electrochemical charging, the nanowire showed up to 90% elongation and a 35% increase in diameter.

        1. For the Love of Iron

            Iron-loving elements such as Re, Os, Ir, Pt, Rh, Pd, and Au must have been delivered to the upper mantle of Earth, Mars, and the Moon after formation of the planetary cores, because, before that, these elements tended to bond with the core's metallic iron, stripping them from the planetary upper layers. Using Monte Carlo models, Bottke et al. (p. 1527) show that the relative abundances of iron-loving elements on Earth, Mars, and the Moon can be explained if most of the impacting planetesimals that delivered the elements had sizes extending up to several thousand kilometers. In these circumstances, most of the iron-loving elements would arrive in a small number of random impacts, the most massive of which hit Earth but not the Moon. Some of these impacts may also have altered Earth's obliquity, produced the Moon's orbital inclination, and delivered water to the Moon's mantle.

          1. Mechanical Transparency

              In atomic gases and other solid-state systems with appropriate energy levels, manipulation of the optical properties can be induced with a control pulse, allowing the system to transmit light of specific wavelengths that would otherwise have been absorbed. Weis et al. (p. 1520, published online 11 November) now report electromagnetically induced transparency in an optomechanical system whereby the coupling of a cavity to a light pulse is used to control the transmission of light through the cavity. This approach may help to allow the engineering of light storage and routing on an optical chip.

            1. Positive Message


                Climate warming affects both cloud number and cloud properties, which in turn affect warming itself, creating a cloud-climate feedback that complicates predictions of the amount of warming caused by increasing concentrations of atmospheric carbon dioxide. This feedback has generally been considered to be positive, but so far we have only a qualitative idea of the effect. Dessler (p. 1523; see the news story by Kerr) estimated the magnitude of the feedback by analyzing 10 years of satellite data on the flux of radiation through the top of the atmosphere. As expected, the feedback is positive and within the canonical range of estimates of how much warming will occur for a doubling of atmospheric CO2: 2°C to 4.5°C.

              1. All in the Mind

                  Pavlov's experiments, in which dogs salivate in anticipation of food, mirror our own imagined experience; that is, thinking about the future consumption of chocolate enhances our desire for it and our motivation to obtain it. After several bites, however, our appetite usually wanes and the offer of a second bar is less appealing than the first. Morewedge et al. (p. 1530) show that the decrease in hedonic response can also be induced by having imagined eating the first bar of chocolate. In comparisons of subjects asked to imagine the repetitive consumption of candy or cheese, they observed a specific drop in the amount consumed when subjects were actually offered the previously imagined foods to eat.

                1. Sex Triangle

                    CREDIT: GARETH BLOOMFIELD

                    The model organism Dictyostelium discoideum is a social amoeba that has three sexes, or mating types, that do not resemble those in any other eukaryote studied so far. Any two sexes can form a diploid zygote, which will recruit other haploid cells to form a macrocyst. Bloomfield et al. (p. 1533; see the Perspective by Kessin) found that sex in this amoeba is determined by several genes at a locus on chromosome 5, with each mating type represented by a different version of the locus. Not all of the genes were directly essential for successful mating, and the master regulators appeared to be two small soluble intracellular proteins, which control mating types I and III, and a combination of homologs of these two proteins that control mating type II.

                  1. Muscle Building

                      The signaling mechanisms involved in actin filament formation for myofibril formation, which is required for growth factor-induced muscle maturation and hypertrophy, remain unclear. Takano et al. (p. 1536; see the Perspective by Gautel and Ehler) now show that the mechanism involves the interaction of nebulin and N-WASP. N-WASP is an activator of the Arp2/3 complex, which induces branched actin filaments in nonmuscle cells. The nebulin–N-WASP complex formed in muscle, however, causes nucleation of unbranched actin filaments within myofibrils without the Arp2/3 complex. Nebulin–N-WASP–mediated myofibrillar actin filament formation is required for muscle hypertrophy and might explain a congenital hereditary neuromuscular disorder caused by nebulin gene mutation: nemaline myopathy.

                    1. From Blight to Powdery Mildew

                        Pathogenic effects of microbes on plants have widespread consequences. Witness, for example, the cultural upheavals driven by potato blight in the 1800s. A variety of microbial pathogens continue to afflict crop plants today, driving both loss of yield and incurring the increased costs of control mechanisms. Now, four reports analyze microbial genomes in order to understand better how plant pathogens function (see the Perspective by Dodds). Raffaele et al. (p. 1540) describe how the genome of the potato blight pathogen accommodates transfer to different hosts. Spanu et al. (p. 1543) analyze what it takes to be an obligate biotroph in barley powdery mildew, and Baxter et al. (p. 1549) ask a similar question for a natural pathogen of Arabidopsis. Schirawski et al. (p. 1546) compared genomes of maize pathogens to identify virulence determinants. Better knowledge of what in a genome makes a pathogen efficient and deadly is likely to be useful for improving agricultural crop management and breeding.

                      1. Getting HIV Under Control

                          Approximately 1 in 300 people infected with HIV are HIV “controllers” who are able to maintain long-term control of the virus without medication and who do not progress to AIDS. Uncovering the genetic basis for this ability is of great interest. Pereyra et al. (p. 1551, published online 4 November; see the Perspective by McMichael and Jones) now present genome-wide association results from patients enrolled in the International HIV Controllers Study. The analysis compared HIV controllers of European, African-American, and Hispanic descent with HIV progressors and found >300 variants that reached genome-wide significance, all of which were in the major histocompatibility class I (HLA) region on chromosome 6. Analysis of the effects of individual amino acids within classical HLA proteins revealed six independently significant residues, five of which lined the peptide-binding groove. Thus, differences in binding to viral peptide antigens by HLA may be the major factors underlying genetic differences between HIV controllers and progressors.

                        1. Supersolidity in a Spin

                            Observing superfluid flow in a solid is a counterintuitive finding that has been accomplished by freezing 4He inside a torsional oscillator and monitoring the oscillating period as the temperature is lowered: A reduction in the oscillating period will be observed at the supersolid transition when the mass of the superfluid decouples from the oscillator and the remaining normal component of the solid. However, extraneous classical effects can also cause this reduction, and so, to confirm supersolid formation, Choi et al. (p. 1512, published online 18 November) performed a slightly different measurement. Rotation was superimposed onto the oscillating motion, and the period and the shear modulus of the system were measured simultaneously. These two quantities exhibited very different responses to the rotation speed, suggesting that supersolidity (rather than classical effects that would also affect the shear modulus) is indeed at the root of the previously observed change in the oscillating period.

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