This Week in Science

Science  11 Aug 2006:
Vol. 313, Issue 5788, pp. 729
  1. Fishing for Change


    Bottom-feeding fish in the family Prochilodontidae are the most important component of the commercial and subsistence freshwater fishery in South America, but are declining because of overharvesting, dams, and land-use changes. Taylor et al. (p. 833) show experimentally how the loss of a single Prochilodontid species will change a fundamental ecosystem-level process, the synthesis and degradation of carbon, in this species-rich tropical ecosystem. The high abundance and diversity of consumers at lower trophic levels is no “insurance” against changes in ecosystem functioning: None of the more than 100 other fish species compensated for the functional role performed by the single Prochilodontid species that was removed.

  2. As the Rain Falls

    Around 9500 years ago, the Eastern Sahara entered a much wetter phase that made it suitable for widespread human settlement. Kuper and Kröpelin (p. 803, published online 20 July) combine nearly 500 of their own radiocarbon dates from 150 archaeologic sites with ones previously reported to develop a detailed chronicle of habitation shifting with precipitation patterns in this region during much of the past 10,000 years. Settlements bloomed throughout the region when rainfall abruptly increased and disappeared as aridity spread from north to south until ∼5000 years ago.

  3. Puzzling X-ray Pulses

    Neutron stars, the remnants of supernovae explosions, can spin on time scales of minutes or faster. However, in the center of the gas shells of the supernova remnant RCW103, which exploded just 2000 years ago, De Luca et al. (p. 814) found an unusual x-ray source pulsing with a much longer period of 6.67 hours that showed no faster variations. This object could be an x-ray binary system consisting of a compact object and low-mass star in eccentric orbit. If the object is instead a single neutron star, it could be a rare magnetar that is being slowed down, perhaps by a supernova debris disk.

  4. When a Star Is Born

    When stars condense out of gas clouds, forces other than gravity can impede their collapse. Girart et al. (p. 812; see the Perspective by Crutcher) show that magnetic forces can be strong enough to slow the collapse by identifying an hourglass shape in the material around a forming star. In their images taken with the Sub-millimeter Array, they see aligned polarization vectors pinched inward at the waist near the central star. This hourglass shape mirrors expectations from star-formation theory in which gravity eventually overcomes other forces. The polarization pattern shows that magnetism is more important in this case than turbulence in supporting the gas cloud.

  5. Staying Even


    Climate models have suggested that the amount of snow falling on the interior of Antarctica should increase as the world warms because warmer air can hold more moisture and produce more snow. Some studies that have used satellite observations or reanalyzed previous climatological data have suggested that there has been a net accumulation of snow, but a study by Monaghan et al. (p. 827) shows that no significant buildup has occurred during the past 50 years. By combining field observational data with model simulations, they provide a 5-decade-long picture of regional variability of Antarctic snowfall. Interdecadal variability of snow accumulation for the 16 regions examined was observed, but no net overall trend has resulted. Annual variability and decadal trends can be as large as, or larger than, inferred long-term trends. This finding also argues against net increases of snowfall in the interior of Antarctica mitigating global sea-level rise.

  6. Caught in the Act

    Mount Etna is one of the most active volcanoes in the world, with ongoing magma intrusions and a recent increase in explosive eruptions. In late 2002, a particularly violent outburst occurred, with fire fountains and tephra fallout. Patanè et al. (p. 821; see the Perspective by Foulger) caught the 2002 Etna eruption in seismic data from a dense network of receivers and were able to map changes in three-dimensional shear and pressure-wave velocity during the pre-eruptive and eruptive periods. Anomalous low-velocity zones appeared just before the eruption that were indicative of rising gas-rich magma within the volcano.

  7. Nailing Networks

    Do the networks between individuals affect how they perform as a group? Kearns et al. (p. 824) approached this question through a graph-coloring problem. Individuals had to select a color so that their choice would not overlap any of their network neighbors. Network structure had a dramatic effect on performance and, depending on the structure, providing participants with more information could decrease or increase the times individuals or groups needed to reach a solution.

  8. Tracking Smallpox

    Before its eradication in 1980, smallpox was endemic around the world. Esposito et al. (p. 807, published online 27 July) sequenced 45 isolates of smallpox taken before eradication and found little variation. However, phylogenetic analysis revealed three distinct clades dividing into West African, Asian, and South American groups. These clades evolved by recombination and genome reduction, and the findings have implications for virulence. In any potential outbreak, it should be possible to trace the source.

  9. Musseling Up Defenses

    Invasive species not only alter the composition and balance of ecological communities; they can also act as selective forces. Freeman and Byers (p. 831; see the news story by Stokstad) present evidence for the rapid evolution of an inducible morphological defense in the common Atlantic mussel, Mytilus edulis, in response to the invasive Asian shore crab, Hemigrapsus sanguineus, within only 15 years of the crab's introduction. This phenomenon—a thickening of the shell when exposed to waterborne cues indicating the predator's presence—is consistent in laboratory and field experiments.

  10. Sunrise, Sunset

    To avoid navigational errors when cue availability changes because of weather conditions or time of day, the compass systems of migrating birds must be calibrated with respect to a common reference system. Muheim et al. (p. 837) provide experimental evidence in Savannah sparrows that the magnetic compass is recalibrated with respect to polarized light cues at both sunrise and sunset. In addition, recalibration of the magnetic compass occurs both before and during migration, and a view of the polarization patterns down to the horizon is required for recalibration of the magnetic compass.

  11. Actin and Coronin in Immune Cells

    The actin cytoskeleton regulates many aspects of cellular and organismal biology. Coronins have been implicated in the regulation of cytoskeletal dynamics. Föger et al. (p. 839; see the Perspective by Dustin) focused on understanding the in vivo functions of the actin-binding protein coronin 1. Coronin 1 was required for chemokine-mediated migration of immune cells and for organizing cytoskeletal changes.

  12. Peptide Regulators of Plant Development


    Cell-cell communication is essential for organized tissue formation. Recently, a role for peptides in plant development has been established. In the shoot apical meristem of Arabidopsis, cell fate determination involves the CLAVATA3 gene, which encodes a putative peptide ligand, and the CLAVATA1 gene, which encodes a leucine-rich repeat receptor-like kinase. However, the precise identity of the individual signaling peptide has remained elusive (see the Perspective by Simon and Stahl). Two independent groups, Ito et al. (p. 842) and Kondo et al. (p. 845), have now isolated specific CLAVATA-derived 12-amino acid peptides involved in the regulation of plant meristem development.

  13. Bacterial Sneak Attack

    A functional nonribosomal peptide polyketide synthase (pks) gene cluster on a genomic island has been discovered in Escherichia coli that induces DNA double-strand breaks in the infected host-cell DNA and in turn causes a block in mitosis. Nougayrède et al. (p. 848; see the Perspective by Hayashi) found that the pks island is widely distributed in commensal E. coli strains and is even found in a strain used as a probiotic agent. The genotoxic effect may be exploited by the bacteria to slow the rate of renewal of the intestinal epithelium by blocking the cell cycle. Thus, the relation between pathogenicity and commensalism may be more complicated than has been assumed. These findings may provide clues about the role of microorganisms in the development of colonic cancers.

  14. Mesoporous Germanium

    A number of routes exist for creating materials that are structured on the nanometer scale, and similarly for creating materials that are mesoporous, but merging the two properties into a single material is quite a challenge. Armatas and Kanatzidis (p. 817, published online 20 July) have now achieved this goal for germanium, an important semiconductor material of technological interest. They link together negatively charged Ge Zintl ions with tetravalent Ge in the presence of a liquid crystalline surfactant that templates the framework. The surfactant was removed with an ion-exchange process, and the resulting mesoporous germanium showed hexagonal pore ordering and size-dependent optical properties and photoluminescence.

  15. Fruit Flies and Mental Retardation

    Neurotrypsin is a multidomain neuronal trypsin-like serine protease predominantly expressed in the developing and adult nervous system. A mutation in the human neurotrypsin gene is involved in non-syndromic mental retardation. To gain further insight into the pathophysiology of mental retardation associated with neurotrypsin mutations, Didelot et al. (p. 851) investigated the Drosophila neurotrypsin ortholog, Tequila (Teq). They found that Teq mutants have a specific impairment of long-term memory and that teq messenger RNA is up-regulated during a short time window after spaced training. Tequila is specifically required in adult mushroom bodies, the insect memory centers, and impairment of long-term memory capacity after transient teq silencing is reversible.

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