This Week in Science

Science  21 Aug 1998:
Vol. 281, Issue 5380, pp. 1109
  1. Marine Diversity: Local to Global

    Long and extensive records of global marine faunal diversity since the Cambrian (about 550 million years ago) have led to two different interpretations of the factors that affect diversity. One is that mass extinctions reset diversity trends and allow less dominant families to radiate where once dominant families lived, and the other is that long-term diversification, linked to biotic interactions, is unaffected over the long run by mass extinctions. Miller (p. 1157) reviews the development of these interpretations based on the marine record and concludes that there is actually a third interpretation that is gaining proponents through recent detailed regional studies of marine biota: Diversification is fueled by relatively rapid local or regional changes in the environment with mass extinctions being the extreme case. Diversity would come quickly in one region and then be transferred to a global scale during a longer time period, thus leading to the gradual increase in diversity seen in the global marine fauna.

  2. Starry Gases in Diamond Dust

    Diamonds found in meteorites are thought to represent solid grains created before the formation of the solar system and thus are tracers of presolar conditions. Verchovsky et al. (p. 1165) used ultracentrifugation to separate diamond grains collected from the Efremovka meteorite into four grain size fractions. The measured abundances of noble gases increased with increasing grain size, while those of nitrogen and carbon did not vary. The noble gases present were implanted as ions into the grains when they were in the interstellar medium, a process that was more efficient for larger grains.

  3. A Step in Tibet

    The Tibetan Plateau, created by the collision of India into Eurasia, is the largest and highest platform of thickened, folded, and faulted crust. Although the towering Himalayas provide some indication of the extreme crustal deformation produced by this collision, it is unclear how the Moho, the boundary between the crust and the uppermost mantle, may have been affected by this tectonism. Zhu and Helmberger (p. 1170) noticed a peculiar pair of pulses in some compressional teleseismic waveforms recorded at one seismic station on the northern boundary of the Tibetan Plateau. Their model implies that the jump in the Moho is caused by a thickened, weaker crust beneath the plateau that is juxtaposed against a vertical wall of thinner stronger crust and mantle north of the plateau. This modeled jump in the Moho may provide a barrier for lateral deformation of the crust and may cause the thickening of the plateau by vertical deformation.

  4. Laser-Shocked Metallic Deuterium

    Understanding the behavior of hydrogen under high pressure is fundamental for determining the structure and evolution of the jovian planets, extrasolar giant planets, brown dwarfs, and low-mass stars that are composed predominantly of hydrogen under high pressure. Collins et al. (p. 1178; see the news story by Kerr) used a recently developed technique to shock liquid deuterium to pressures between 22 and 340 gigapascals with an intense laser beam. Deuterium, an isotope of hydrogen, serves as a proxy for the hydrogen equation of state because its properties can be scaled to those of hydrogen by a correction for their density differences. The transition of deuterium from an insulating molecular fluid to a reflective metallic fluid occurred at about 100 gigapascals. Their work indicates that hydrogen is about 50 percent more compressible than had been previously predicted by theory.

  5. Identifying Fossils Organically

    Dinoflagellates, an important group of plankton, are widely recognized back to the Triassic (about 240 million years ago), although some data have indicated a much earlier origin. In a search for earlier evidence of dinoflagellates, Moldowan and Talyzina (p. 1168) isolated specific biochemical markers of dinoflagellates and examined enigmatic fossils and organic remains of Cambrian sedimentary rocks. The data imply that dinoflagellates were abundant by the Early Cambrian (540 million years ago).

  6. From Life to Clay

    The Burgess Shale, formed during the great metazoan diversity in the Cambrian, contains some of the most exquisitely preserved fossils and possibly some organic matter from these early animals. The manner in which these fossils were preserved has been debated. Orr et al. (p. 1173) used elemental mapping of some of these fossils and their adjacent matrix to demonstrate that clay minerals copied and replaced some of the soft tissue components. This previously unrecognized mechanism of preservation should improve our understanding of the history of the fossils and also the life of the metazoans because this preservation method can potentially distinguish between different soft tissue components.

  7. Squeeze at the Plate

    Subduction zones are collisional margins of plate boundaries where one plate slides below the other. A sloped line of earthquakes, the Benioff zone, traces the boundary between these plates, and Bilek and Lay (p. 1175) have used the earthquakes along the subducting oceanic crust beneath the island of Japan to characterize the properties of the downgoing crust as it is squeezed and heated with depth. A trend of longer rupture duration or greater stress drop for deeper earthquakes suggests that the plate boundary is becoming more rigid with increasing depth as the shallow looser sediments give way to deeper, more rigid basalt.

  8. Electrons on the Loose

    When electrons are confined to systems of reduced dimensions, such as planes or chains, the conditions required for forming a metallic state are not clear. Vescoli et al. (p. 1181; see the Perspective by Bourbonnais and Jérome) have studied the optical properties of a variety of organic linear chain conductors (Bechgaard salts) that can exhibit either insulating or metallic properties and compared these results with their magnetic behavior. The crossover from insulating to metallic behavior correlates with an increased probability for electrons to hop between chains and be deconfined. The metallic state appears to be anomalous; its properties are inconsistent with those of a standard Fermi liquid theory and are better described as a Luttinger liquid with strong correlations. They also find that while there is an energy gap for charge excitations in these metals, there is no gap for spin excitations.

  9. Motor Polarity

    The kinesin family of molecular motors moves intracellular cargo along microtubules. Most kinesin family members move specifically toward the unstable or plus end of microtubules—the end that would be directed away from the center of the cell toward the periphery. However, one kinesin, known as Ncd, moves with the opposite polarity, toward the minus end of microtubules. Endow and Waligora (p. 1200) have examined the molecular features of the motor that specify motor polarity. They defined a small region within the motor that is critical in maintaining the unusual polarity of Ncd.

  10. Nitrogen Delivery

    The symbiotic interaction between leguminous plants and their nodulating bacteria offers the advantage to the plant of a ready supply of fixed nitrogen. Kaiser et al. (p. 1202) have now cloned the transport protein that likely delivers the fixed nitrogen from the bacteroid to the plant. The protein is located in nodules and contains a single putative transmembrane domain.

  11. Memory Makers

    Why do we remember some things but not others? Brewer et al. (p. 1185) and Wagner et al. (p. 1188) show that photographs and words that elicited greater activation in prefrontal and parahippocampal regions of the brain during the initial exposure were more likely to be remembered and that forgotten experiences correlated with lower activities in these regions (see the Perspective by Rugg).

  12. Assessing Genetic Differences

    The ability to identify variations between genomes rapidly and efficiently will be central to understanding the molecular basis for individual differences between microbes, plants, or humans and will form the basis of identifying genes involved in multigenic or quantitative normal traits or diseases. Winzeler et al. (p. 1194; see the cover and the news story by Service) used high-density oligonucleotide arrays to scan for alternative alleles between two strains of yeast. The procedure did not require amplification steps and did not require knowledge beforehand of the specific nature of the variation.

  13. Heme Chaperone

    Many enzymes contain iron associated with heme as an essential cofactor for activity, but the mechanism of heme addition and heme targeting has been unclear. Schulz et al. (p. 1197) discovered a protein important for heme targeting in the periplasm of bacteria; CcmE appears to act as a chaperone, first binding heme and then releasing it to target enzymes.

  14. 'Green Rust' in the Lab and in the Soil

    S. C. B. Myneni et al. (Reports, 7 Nov., p. 1106) studied “abiotic selenium redox transformations” in the presence of iron oxides that are “called green rust.” Myneni et al. stated that they “provide direct evidence for the formation of reduced Se species in anoxic sediments.

    R. S. Oremland et al. comment that experiments in the report “done under highly specialized laboratory conditions” and the “agreement of rate constant data” provide “only circumstantial evidence” that green rust observed in soils is abiotic and not “directly mediated by bacteria.”

    In response, Myneni et al. agree that green rust can be produced by “many species of microorganisms,” but state that current research methods “do not rule out other mechanisms” of green rust formation in sediments. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/281/5380/1111a

  15. Tricking Mother

    Grafts and transplants from donors are rejected by the host unless the immune system is suppressed because the host's T cells react strongly to the foreign major histocompatibility antigens (MHC) on the graft. Why is a fetus, whose MHC is half from the father and, therefore, foreign to the mother, not instantly rejected? Munn et al. (p. 1191; see the news story by Gura) found that the tissues of the mouse fetus itself deprived the maternal T cells of a critical factor for their activation—tryptophan. A fetal enzyme (indoleamine 2,3-dioxygenase or IDO) catabolizes tryptophan. Inhibition of IDO disarmed the fetus and allowed the maternal T cells to reject the conceptus.

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