This Week in Science

Science  24 Sep 2004:
Vol. 305, Issue 5692, pp. 1869
  1. Dislocation Dynamics


    Atomic dislocations in crystals are responsible for many key material properties, such as strength and toughness. Dislocation can be observed by transmission electron microscopy, but this method is not well suited for studies of dislocation dynamics, which occur over a wide range of length scales. Schall et al. (p. 1944) used laser diffraction and confocal microscopy to watch dislocation movements within films of much larger colloidal particles. The defect behavior in these systems is similar to their atomic counterparts, and the critical parameters associated with the defects were determined.

  2. Solar Wind Scavenging of Mars

    The Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) onboard the Mars Express spacecraft measured an unexpectedly deep penetration of the solar wind into the martian ionosphere and uppermost atmosphere from January to March 2004. Lundin et al. (p. 1933) report that the shape and composition of Mars' ionosphere may be irregular over time scales as short as a few minutes and that the peak thermal pressure of the ionosphere is not always strong enough to hold off the solar wind. The radiation causes the escape of hydrogen and oxygen ions and ionized molecular species, such as CO2+, from the top of Mars' atmosphere. The measurements will help in understanding how the hydrosphere and atmosphere of Mars evolved over time.

  3. A Superfluid Solid

    Theoretical work has predicted that solids can, under the right conditions, behave as superfluids and flow without dissipation. The requirement is that the solid must condense into the same quantum state—a solid version of the Bose-Einstein condensates that have been demonstrated in the gas and liquid phases. Kim and Chan (p. 1941, published online 2 September 2004; see the Perspective by Leggett) present experimental evidence that solid helium can be teased into such a state. As the temperature of the solid He is reduced below 250 millikelvin, rotational studies indicate that the solid He decouples from its confining environment and exhibits superfluid-like motion. The result indicates that all three states of matter can undergo Bose-Einstein condensation.

  4. Swords or Plowshares


    In metazoa, germ cells and gonadal somatic cells often form separately, and the two cell types must come together through cell migration. Extracellular lipid phosphates have been implicated in the migration of germ cells. The lipid phosphate phosphohydrolases Wunen and Wunen2 are expressed in the soma of the Drosophila embryo and repel migrating germ cells. Renault et al. (p. 1963, published online 26 August 2004) now show that Wunen2 also plays a complementary role within the germ cells: Wunen2 prevents germ cell death and allows for their proper migration. Somatic cells and germ cells compete for the same lipid substrate such that hydrolysis and uptake of the lipid in the soma provides a repellent effect but mediates cell survival and migration in germ cells.

  5. Sulfide-Driven Segregation

    The primitive mantle contained trace amounts of platinum group elements, and their current distribution provides useful data for estimating mantle evolution. Whereas Ru, Os, and Ir tended to be retained in the residual depleted mantle, Pd, Re, and Pt segregated into the silicate melt that separated into the enriched crust. Bockrath et al. (p. 1951) find that sulfide phases drive this segregation. A crystalline monosulfide enriched in Ru, Os, and Ir stays in the mantle, and a sulfide melt enriched in Pd, Re, and Pt rises along with the silicate melt.

  6. Changing Forces in Midstream

    Studies of the West Antarctic Ice Sheet, which contains enough ice to cause a 6-meter rise in sea level were it to melt completely, suggest that it may be susceptible to rapid collapse. Knowing how it responded to past climate change may help in predicting its dynamics. Siegert et al. (p. 1948; see the news story by Kerr) have discovered an internal layer fold in the ice sheet that cannot be explained in the current ice flow regime. They contend that the fold axis marks a paleo flow line that developed when the ice sheet changed from a former flow configuration to the present one, and that the activation of the southern tributary of Ice Stream D a few thousand years ago can explain the change in flow direction.

  7. Predicting Catastrophic Ecological Change

    Some ecosystems undergo gradual change in response to changing environmental conditions, but others can undergo abrupt, sometimes irreversible changes. Ecosystems that are vulnerable to catastrophic shifts typically lack early warning signals of massive change because their behavior is by definition nonlinear. Rietkerk et al. (p. 1926) review recent research on such ecosystems and suggest imminent catastrophic shifts can, in some cases, be predicted on the basis of self-organized patchiness caused by a resource-concentration mechanism.

  8. Mitigating Sun-Seeking Behavior

    The moment the growing seedling emerges, the Sun acts as both power and poison. Precursors of chlorophyll lie ready to be converted to functioning chlorophyll, but the precursor form can do more harm than good if exposed to light. Huq et al. (p. 1937) have now identified a transcription factor, PIF1, that interacts with light-sensing phytochromes and regulates chlorophyll biosynthesis. PIF1 negotiates the balance between chlorophyll synthesis and the sensing of light such that emergent seedlings become photoautotrophic before they would otherwise burn up.

  9. Advancing HIV Prevention in Developing Countries

    The challenge of transferring information to service providers is especially critical for AIDS, where the greatest need is in developing countries. Kelly et al. (p. 1953) conducted a randomized trial to evaluate the use of advanced distance communication and Web-based technology to train AIDS service providers from nongovernmental organizations (NGOs) in 78 developing and resource-poor countries in implementing an HIV-prevention intervention. NGOs developed new programs or adopted elements of the interventions roughly twice as frequently as controls, and also passed their new knowledge to large numbers of additional providers in their countries.

  10. Sport Fishing Threat

    For decades, it has been assumed that recreational fishing has a minimal impact on most species of fish. However, a rigorous reanalysis of multiple fisheries records for the United States shows that this is not always the case. For some threatened species, Coleman et al. (p. 1958, published online 26 August 2004; see the cover and the 27 August news story by Grimm) show that recreational fishing poses more of a threat than commercial fishing. Thus, new fishing regulations that target recreational fishers are needed to complement existing catch restrictions on the commercial sector.

  11. Species Richness Down the Amazon

    The influence of tributaries on the ecological communities of rivers is an issue of growing interest. Fernandes et al. (p. 1960; see the Perspective by Gascon and Smith) took three sets of deepwater trawl samples near the mouth of each of the 13 main tributaries of the Amazon: One set upstream in the tributary itself, one above the tributary's mouth in the Amazon mainstream, and the third in each set below the mouth of the tributary. Richness of electric fish species in the Amazon was higher below each tributary than above, and the fauna from the tributary matched the downstream sample better than the match between the upstream versus downstream Amazon samples. The effect was greater than would be predicted by simple mixing of the tributary and mainstream faunas.

  12. What Is NO Good for in Plants?


    Nitric oxide (NO) is an important signaling molecule in animal systems, but its role in plants is poorly understood. He et al. (p. 1968) report that NO controls flowering in the plant Arabidopsis thaliana. Exposure to exogenous NO delayed flowering, and mutation of a gene called nox1—a putative phosphoenolpyruvate/phosphate translocator—resulted in delayed flowering and increased NO production. The mutant also exhibited altered expression of the target genes of two separate pathways that control the vegetative-to-reproductive phase transition in response to either external or internal cues. Thus, NO may integrate and fine tune the flowering response to both environmental and internal stimuli.

  13. Membrane Recycling--Memory's Key?

    During long-term potentiation (LTP), the neurophysiological correlate of memory, synaptic efficacy is enhanced by the physical insertion of AMPA receptors at the postsynaptic membrane. Park et al. (p. 1972) show that recycling endosomes are the source of AMPA receptors for LTP. Transport through recycling endosomes maintains AMPA receptors at excitatory synapses and is required to “awaken” silent synapses. Blocking transport out of recycling endosomes traps AMPA receptors in neuronal dendrites, prevents stimulus-dependent AMPA receptor insertion, and completely abolishes LTP at CA1 hippocampal synapses. Moreover, stimuli that trigger LTP accelerate the recycling of AMPA receptors and also general endocytic recycling in hippocampal neurons.

  14. Exposing the Opportunist

    Legionella pneumophila is a Gram-negative pathogen of freshwater amoebae that can cause opportunistic infections of humans by replicating in macrophages in the lung. Chien et al. (p. 1966) report the complete genome sequence of L. pneumophila. The genome reveals genes found so far only in L. pneumophila and the closely related opportunistic pathogen Coxiella burnettii, nine genes that are typical of intracellular pathogens, as well as established and apparently novel virulence factors.

  15. Examining Insular Avian Extinctions

    Hundreds of bird species on oceanic islands have become extinct since the arrival of humans, because of local causes that include hunting, habitat destruction, competition, and predation by introduced species. Variability in the likelihood that bird species have been driven extinct has been ascribed to variation in the intrinsic characteristics of the islands themselves, such as their area or their degree of isolation from the nearest continental landmass. However, islands also vary in the richness of their introduced predator communities. From data compiled from over 200 oceanic islands, Blackburn et al. (p. 1955) show that birds living on islands with more predators have been more likely to go extinct, that the influence of predators is greater on endemic bird species, and that each predator introduction increases extinction probability.