This Week in Science

Science  19 Oct 2001:
Vol. 294, Issue 5542, pp. 473
  1. Crustal Shortening and Extrusion

    The major continent-to-continent collision of India into Eurasia has created the high-standing Tibetan plateau, major fault zones, and a natural laboratory for a crustal deformation study. Wang et al. (p. 574) combine 10 years of geodetic data into a comprehensive kinematic model of crustal motion throughout China. They find that 90% of the deformation is taken up in crustal shortening to create the plateau and most of the remaining 10% is taken up by eastward extrusion and rotation of the crust into southeastern China.

  2. Ion-Gated Carrier Transport in DNA

    Many experimental and theoretical studies have been aimed at elucidating the mechanism for the transport of electrons and holes through DNA. Barnett et al. (p. 567) have now used molecular dynamics simulations and first-principles structure calculations to show that even the hydrated counterions that interact with phosphate groups can exert a large effect on the rate of hole transport. Inclusion of the hydrated ion favors correlated motions of the helix that facilitate transport. Experimental studies revealed that electron transport rates are reduced in DNA strands containing uncharged methylphosphonate bridges.

  3. Stratospheric Effects on Weather

    Vigorous circulation in the troposphere makes predicting the weather more than 1 week in advance difficult. The stratosphere, which lies immediately above the troposphere, is far more stable but is not often considered to have much effect on surface weather patterns. Baldwin and Dunkerton (p. 581, see the news story by Kerr) present evidence that strong variations in stratospheric circulation at altitudes above about 50 kilometers can descend into the troposphere and affect the weather there. These events may be followed by months during which patterns of surface pressure are systematically altered, as reflected in large-scale atmospheric pressure patterns called the Arctic Oscillation and the North Atlantic Oscillation. This effect may allow storminess and storm tracks to be predicted more accurately.

  4. Lemur Origins

    Small, nocturnal lemurs are divided into two extant orders, the Lemuriformes, isolated in Madagascar, and the Lorisiformes, found in Africa and Asia. The fossil and molecular record of how and when these orders may have evolved is poorly sampled and contradictory. Marivaux et al. (p. 587) have discovered a lemuriform fossil, Bugtilemur mathesoni, in Oligocene sand deposits in Pakistan. Diversification of lemuriforms outside of Madagascar thus occurred at least 30 million years ago, which is more than 50 million years after the continental breakup of Madagascar from India. Hence, the lemuriforms either found land bridges from Madagascar to Asia or they evolved in Asia before the continental breakup.

  5. Making Better Contacts

    Studies of single-molecule conductivity yield a wide range of values, in part because of the difficulties of making reliable contacts to individual molecules. Cui et al. (p. 571; see the Perspective by Hipps) now describe a method for covalently attaching gold nanoparticles to alkane thiol molecules self-assembled on a gold surface. Current-voltage curves are observed that can be attributed to making contact with one to five molecules, and these scale as multiples of the single contact curve. The results are with a factor of 6 of the theoretically expected conductivity.

  6. Out of the Clouds

    Tropical deforestation has obvious primary costs, such as reduced carbon sequestration, loss of habitat, decreased biodiversity, and increased erosion, but what are the secondary costs? Lawton et al. (p. 584) use satellite imagery of clouds and regional atmospheric modeling to show that clearing lowland forests in Costa Rica alters the surface energy budget enough to diminish dry season cloudiness, which in turn deprives downwind montane cloud forests of the moisture that they need to survive.

  7. Starting Subduction

    How does a tectonic plate bend and break to initiate a subduction zone? Regenauer-Lieb et al. (p. 578) developed a nonlinear elasto-visco-plastic finite element model to examine the initiation of subduction. In their model, a sedimentary load was piled onto the lithosphere (similar to a passive continent-ocean margin), and the water content of the lithosphere was varied. The addition of water promoted the development of a narrow shear zone through the lithosphere that started the subduction process.

    CREDIT: REGENAUER-LIEB ET AL.
  8. Sex and Fitness

    The experimental evaluation of the adaptive significance of sexual recombination has been hampered by inconsistent experimental results. Rice and Chippindale (p. 555; see the Perspective by Lensky) present a set of Drosophila experiments that directly compared nonrecombining and freely recombination replicates. In the nonrecombining population, the variability among lines was very large, as expected, but the average increase in the favored allele was slight and seemed to saturate after about eight generations. In the recombining strain, the fitness continued to increase throughout the experiment.

  9. Blood Vessels Originating Organs

    Blood not only sustains and oxygenates organs—two reports show how blood vessel endothelium plays important roles in organ development (see the Perspective by Bahary and Zon and the 28 September news story by Seydel). Lammert et al. (p. 564; see the cover) found that the endothelium supplies signals that are necessary for pancreatic differentiation and insulin expression. Removal of the dorsal aorta in Xenopus embryos led to a failure in insulin expression, whereas vascularization in transgenic mice in the posterior foregut led to ectopic islet formation and insulin expression. Matsumoto et al. (p. 559) used flk-1 mutant mice, an embryo tissue explant system, and an angiogenesis inhibitor to show that endothelial cells promote liver development prior to vascular blood flow.

  10. Worming Secrets Out of p53

    The p53 tumor suppressor gene is among the most frequently mutated genes in human cancer. Although much has been learned about p53 from studies of mammalian cells, progress in understanding its function and regulation has been hampered by the lack of a genetically accessible system. Derry et al. (p. 591) have identified a p53 homolog in the nematode Caenorhabditis elegans, an organism previously thought to be devoid of p53. This gene, cep-1, functions in apoptosis and meiotic chromosome segregation in the germ line and mediates responses to environmental stress in somatic cells. The genetic potential offered by the C. elegans system is likely to lead to new insights into p53 and other cancer-related genes.

  11. Riding the Neural Crest

    Stitching together the circuitry that connects central and peripheral nervous systems requires a wiring diagram with complex assembly instructions. Begbie and Graham (p. 595) now show that, in the chick, neurons extending from peripheral epibranchial placodes toward the hindbrain find their way by following tracks defined by neural crest cells migrating in the opposite direction, from the hindbrain outward. These ganglionic connections later relay sensory information such as taste from the oral cavity to the brain.

  12. A Signal Assembly

    The signal recognition particle (SRP) is an RNA-protein complex that bridges active ribosomes and internal membranes and enables the coordinated synthesis and insertion of membrane proteins. Wild et al. (p. 598) have solved the structure of one of the protein components, SRP19, in complex with helix 6 of the SRP RNA. This complex, postulated to be one of the early intermediates in assembly of the SRP, reveals that recognition relies mainly on shape complementarity, meditated by a layer of water molecules, rather than direct nucleotide-amino acid contacts.

    CREDIT: WILD ET AL.
  13. Everybody Counts

    Organisms are discrete entities in space and time, but most ecological models simulate populations as a continuum. Lattice models can handle populations that have discrete numbers, and by this means Henson et al. (p. 602) have shown in laboratory populations of the flour beetle, Tribolium, that it does matter whether organisms are treated as points rather than smears. Accounting for lattice effects can dramatically alter the predictions of ecological models, especially those with complex dynamics used in conservation biology and wildlife management.

  14. γd T Cells Get Under the Skin

    T cells bearing the γd T cell receptor reside in large numbers within the epidermis of the skin. The growing evidence that γd T cells contribute to dermal integrity is supported by the study by Girardi et al. (p. 605; see the Perspective by Pardoll), who show these lymphocytes can protect against cutaneous malignancy. Thus, in the absence of γd T cells, mice coped poorly with experimentally induced forms of skin carcinoma. Induction of malignancy correlated with the up-regulation of the protein Rae-1, which contributed to the killing of carcinoma cells by a γd T cell line in vitro. Binding studies revealed that Rae-1 interacts with the NKG2d receptor on γd T cells, which suggests that this ligand may be a functional homolog of tumor-related MICA/B proteins in humans.

  15. The Past Molds the Future

    Trypanosomes possess a characteristic flagellum that winds helically around the cell. How is this complex structure passed on to the resulting daughter cells during cell division? Moreira-Leite et al. (p. 610) examined the process and found that the new and the old flagella are physically linked during division such that the structural form is passed directly from mother to progeny. This type of morphogenetic inheritance is known as cytotaxis.

  16. Variation in Food Supply, Time of Breeding, and Energy Expenditure in Birds

    One effect of global warming is to hasten the availability of food for birds each year, and many species have responded by advancing their breeding dates. By measuring metabolic rates and life-spans in bird populations with variable breeding times, Thomas et al. (Reports, 30 March 2001, p. 2598) concluded that the excessive parental metabolic effort resulting from mismatching of breeding time to peak food supply lowers reproductive efforts in adult birds. In a comment, Verhulst and Tinbergen protest these conclusions, suggesting that the Thomas et al. report suffers from “the problem of interpretation” in comparing persistence of birds in populations and that “only experiments can demonstrate a causal relationship between timing of breeding” and “any other parameter.” In response, Thomas et al. clarify their original presentation of “a unique natural experiment,” and state that their data were “consistent with theory predicting tradeoffs between current effort and future reproductive prospects.” The full text of these comments can be seen at

    www.sciencemag.org/cgi/content/full/294/5542/471a

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