This Week in Science

Science  08 Nov 2002:
Vol. 298, Issue 5596, pp. 1131
  1. In Brevia

    The juvenile stages of eukaryote seaweeds take advantage of signaling molecules (N-acylhomoserine lactones) produced by prokaryotic marine biofilms, according to Joint et al. (p. 1207).

  2. Detached and Reversed in Tibet

    About 50 million years ago, India collided with Eurasia, and the oceanic slab associated with India was subducted beneath Eurasia along a north-dipping thrust fault. The collision created a much thicker continental crust, a large plateau, and the highest mountain chain on Earth, the Himalayas. Kind et al. (p. 1219) provide high-resolution seismic images of the Tibetan plateau in the crust and upper mantle. Their work shows that the subducted oceanic slab has become detached from the Indian continental crust subduction. The subduction may in fact have stalled and reversed, so that the Eurasian continent is now being underthrust beneath India along a south-dipping thrust fault.

  3. Superconducting Lithium

    At ambient pressure, lithium can be well described as a simple metal and has been shown to be so at ambient pressures down to 4 millikelvin. Recent theoretical work, however, has suggested that the structural changes induced under pressure may result in superconductivity. Using a diamond anvil technique modified to handle this chemically reactive material, Struzhkin et al. (p. 1213) present resistivity and magnetic susceptibility measurements, which confirm that superconductivity can be induced. However, the experimental transition temperatures (reaching ∼16 kelvin by ∼35 gigapascals) are appreciably lower than those that were predicted, which suggests that more sophisticated theoretical treatments are needed even for these putatively simple metals.

  4. Proxy Problems

    The determination of paleo sea surface temperatures (SSTs) usually requires geoechemical proxies, and two reports reassess assumptions that underlie two different methods. Alkenones produced by certain types of haptophyte algae have found wide application as an SST proxy because their degree of chemical unsaturation depends on temperature. Unfortunately, it appears that the ages of alkenones found in marine sediments are not necessarily the same as those of the planktonic foraminifera (forams) with which they were deposited. Ohkouchi et al. (p. 1224; see the Perspective by McCave) compared radiocarbon ages of alkenones and those of forams isolated from Late Quaternary sediments from the Bermuda Rise (northwestern subtropical Atlantic). On average, the alkenones are 4200 years older than the forams coexisting in their sediment cores. The alkenones in these cores are likely mixtures of local and transported products, and the authors conclude that alkenones may not accurately reflect SSTs at any single site. The reliability of the magnesium-calcium (Mg/Ca) ratio of seawater as a proxy for sea surface temperatures depends on a knowledge of any changes in this ratio over geological time. Dickson (p. 1222; see the news story by Kerr) measured the Mg/Ca ratio of fossil crinoids, which belong to the same phylum as starfish and sand dollars, in samples that have been protected from diagenesis by the formation of a protective mineral envelope of calcite and dolomite that developed during alteration. The Mg/Ca ratio of seawater has varied between its present value of 5.2 and less than 2 during the past 500 million years. These findings confirm the controversial claim that large changes in the major ion composition of the oceans have occurred over the course of the fossil record.

  5. Cells in 3D

    Cryoelectron tomography (cryo-ET) can provide three-dimensional images of unfixed and unstained cellular structures embedded in vitreous ice at resolutions of a few nanometers. Now Medalia et al. (p. 1209; see the news story by Goldman) have applied cryo-ET to intact eukaryotic cells. Looking at the highly motile cells of Dictyostelium discoideum at 5- to 6-nanometer resolution, they visualize the ribosome-decorated endoplasmic reticulum, identify the 26S proteasome, and directly visualize branches and cross-links in the cytoplasmic actin network. As higher resolution is achieved, cryo-ET has the potential to bridge cellular and molecular structural studies.

  6. Stressed Out

    If stressed, brittle materials should either fail if sufficient force is applied, or retain their original strength, independent of the number of stress cycles the material undergoes. Why, then, does polycrystalline silicon, a brittle material, suffer from cyclic fatigue? Kahn et al. (p. 1215) fabricated samples with intrinsic residual stresses and found that environmental effects do not enhance the static failure. After a low number of cycles, the failure depends critically on the ratio of tensile to compressive stresses and occurs through subcritical crack growth during each cycle. Environmental factors become important for high-cycle fatigue.

  7. The Long and the Short of Evolution

    Sperm morphology is one of the most rapidly diverging traits as species differentiate. The extreme variation in sperm length across the genus Drosophila make it an ideal system for studying how this evolution occurs. Miller and Pitnick (p. 1230) show that sperm morphology and female reproductive tract morphology interact competitively in Drosophila melanogaster to determine differential male fertilization success. An experimental study shows that sperm length evolution is driven by the evolution of the female reproductive tract.

  8. Anticipating Outcomes

    The use of theoretical ecological models to help prioritize the outcomes of conservation efforts is the subject of two reports (see the Perspective by Côté and Reynolds). Kolar and Lodge (p. 1233) analyzed fish species that have invaded the North American Great Lakes in order to identify characteristics common to successful alien invasion species. They develop a general quantitative approach to species-specific ecological risk assessments and make specific predictions as to the future threat posed by species now native to the Ponto-Caspian basin in Eurasia in terms of their establishment, spread, and impact in the Great Lakes. Lens et al. (p. 1236) examine the relative effects of habitat fragmentation and deterioration on rainforest birds in Kenya. They report correlations between the tendency for species to breed in habitat fragments and those same species' abilities to colonize patches and tolerate degraded patches. They combine these findings with historical museum data to address the effects of declines in habitat quality through measurements of morphological fluctuating asymmetry, a measure of the effects of stress. The authors conclude that the characteristics of individual species can be used to predict habitat occupancy with a high degree of confidence.

  9. Gleanings About the Grain

    Plant meristems function as undifferentiated sources for cells that go on to form shoots, inflorescence, and flowers. In maize, the grain develops out of a series of increasingly defined meristems. Chuck et al. (p. 1238) analyzed a maize mutation known as branched silkless 1 (bd1) and show that it affects the identity of spikelet meristems and causes indeterminate branches to form instead. Similarities between the maize bd1 gene and genes in other grasses suggest early evolution of the distinctive grain inflorescence architecture.

  10. Picking a Signaling Pathway

    How is specificity gained when numerous genes are regulated by a single transcription factor in response to diverse stimuli? Hoffmann et al. (p. 1241; see the Perspective by Ting and Endy) examined how a transcription factor called nuclear factor-κB (NF-κB) is differentially controlled by three different isoforms of an inhibitor protein called IκB. They combined computational modeling with biochemical data that they obtained from genetically engineered cells that express only one IκB isoform. During the cell's exposure to a stimulus, a bimodal and temporal signal processing mechanism determines which IκB-NF-κB pathway and downstream target genes get activated.

  11. Plasticity in the Adult Brain

    In the brain, chondroitin sulfate proteoglycans (CSPGs) are important elements of the extracellular matrix. In the adult, they form perineuronal nets, particularly concentrated around inhibitory interneurons. These nets are thought to play an important part in the inhibitory mechanism that ends the critical period during development. Pizzorusso et al. (p. 1248; see the Perspective by Fox and Caterson) showed a correlation between the development of perineuronal nets and the end of the critical period. They also demonstrated that the extension of the critical period by dark rearing is accompanied by delayed maturation of perineuronal nets. Finally, enzymatic degradation of CSPGs in adult animals restored plasticity. These findings directly confirm that the mature extracellular matrix is a crucial factor in the regulation of experience-dependent plasticity.

  12. Vibrations Get Oriented

    Vibrational spectra become more complex for larger molecules and more difficult to interpret. Dong and Miller (p. 1227) show how the application of electric fields can be used to pull out additional information from vibrational spectra for molecules solvated in droplets of liquid helium. A strong dc field can orient a molecule along its dipole moment, and this orientation can enhance or diminish a mode depending on the angle between the molecule's dipole moment and the various vibrational transition moments. The authors use this method to show that the NH2 group of adenine is tilted out of plane, as well as to resolve the three tautomers of cytosine.

  13. Controlling Neuron Myelination

    Myelination during development of the peripheral nervous system depends on signaling through neurotrophins and their receptors. Cosgaya et al. (p. 1245; see the Perspective by Hempstead and Salzer), in analyzing the effects of particular signaling pathways, find that some neurotrophins keep myelination at bay, and other neurotrophins encourage myelination. The shift from glial cell development to axonal myelination is effected by a shift from signaling by neurotrophin-3 and its tyrosine kinase receptor TrkC to signaling by brain-derived neurotrophic factor and the receptor p75NTR. Insight into how myelination is controlled during normal development may lead to greater abilities to manage myelination in the context of neuronal injury.

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