This Week in Science

Science  26 Oct 2001:
Vol. 294, Issue 5543, pp. 741
  1. And in Brevia ...

    An analysis by Sagarin and Micheli (p. 811) of the data set provided by the Nenana Ice Classic betting competition in Alaska, where the moment that a wooden tripod falls through river ice in the spring is guessed, indicates that the ice now breaks up on average 5.5 days earlier than it did 84 years ago, when the competition started.

    CREDIT: SAGARIN AND MICHELI

    A study of 3352 men and women in the Ivory Coast by Levine et al. (p. 812) reveals that women work on average 2.75 hours longer than men, yet sleep for the same period of time, indicating that women spend less time at leisure and nonwork-related travel than men.

  2. Teamwork in Plants

    Are ecosystems with more plant species more productive? Debate has centered on whether productivity effects attributed to diversity were “sampling effects,” sometimes interpreted as experimental artifacts, or as “niche complementarity,” which is evidence for positive effects of diversity on productivity. Tilman et al. (p. 843) present the results of a 7-year field experiment in a Minnesota grassland system which show that sampling effects explained much of what happened during years 1 to 3 and that niche complementarity took over in years 5 to 7. High-diversity communities can be markedly more productive than the best-performing monoculture. The results have implications for the likely effects of biodiversity loss on ecosystem function, and hence for applied habitat management and conservation.

  3. Chemical Waves That Spiral In

    Spiral waves in reaction-diffusion systems, seen initially in the Belousov-Zhabotinsky (BZ) reaction, also occur in surface reactions and in many biological systems, such as Ca2+ release in cells. To date, all of the spirals propagate outward from the center. Vanag and Epstein (p. 835) show that when the BZ reaction is run in water droplets dispersed in a water-in-oil emulsion, the spirals can run backward and propagate inward. Spirals propagated inward when a critical droplet fraction of the microemulsion was exceeded.

  4. High Seas

    Sea level is rising, but how quickly and why? Tide gauge records indicate that sea level has been rising at 1 to 2 millimeters per year (mm/yr) for the past 100 years. However, the Intergovernmental Program on Climate Change's evaluation of expected climate- and human-related impacts suggests that this value should be closer to 0.7 mm/yr. Cabanes et al. (p. 840; see the Perspective by Church) report the results of satellite measurements of sea level between 1993 and 1998 that show an average rate of more than 3 mm/yr. They calculate that most of this rise is due to thermal expansion of the oceans, and that the residual amount, presumably due to global warming, is about 0.7 mm/yr. Estimates based solely on tide gauge values might overestimate the rate of sea level rise by a factor of 2.

  5. The Universal Quantum Hall Effect?

    One of the pursuits of theoretical physics is the unification of the three pillars of modern physics—quantum mechanics, special relativity, and general relativity. Unification of the first two principles has been done successfully with the development of relativistic quantum field theory, but unifying gravity and quantum mechanics has remained elusive. An ideal solution would be finding a quantum-mechanical wave function, or Hamiltonian, of a system from which relativity emerges. Zhang and Hu (p. 823) have taken the quantum Hall effect, a many-body effect involving electrons confined to a two-dimensional (2D) plane in a magnetic field, and generalized the mathematical description to a 4D space plus time. Upon examination of the low-energy states on the surface of this space, they find that certain elements of electromagnetism and gravity emerge from the mathematics. By no means a grand unification theory, the work does suggest that the symmetry properties of other systems may provide a route for further study.

  6. Toward Rational C60 Synthesis

    Although the vaporization of graphite to form C60 and C70 produces these compounds in quantity, it has been difficult to produce higher fullerenes by this route. An initial step toward the rational synthesis of higher fullerenes would be to accomplish this goal with C60 itself. Boorum et al. (p. 828) report the synthesis in nine steps of polycyclic aromatic hydrocarbon (PAH) precursor, C60H30, that loses hydrogen under laser irradiation to form C60. Control experiments with labeled compounds and larger and smaller PAHs confirm that the remaining bonds form directly during dehydrogenation and that the parent PAH does not merely fragment and recombine.

  7. Vent Fauna of the Indian Ocean

    Hydrothermal vents on mid-ocean ridges support a surprisingly diverse fauna of chemosynthetic organisms. Although some sparse sampling has been performed on the Pacific and Atlantic sea floor, little is known about the fauna from the Indian Ocean. Van Dover et al. (p. 818) sampled and analyzed fauna from two hydrothermal vents, the Kairei and Edmond, along the Central Indian Ridge. They found a potentially new family of scaly-foot gastropods, faunal affinities with some of the western Pacific vent fauna, the absence of some major Pacific fauna (such as tubeworms), and a dominant shrimp species that has only evolved in the past 500,000 years that has affinities to Atlantic vent shrimp. The Indian Ocean fauna from these two vents represent a new biogeographic province with affinities to, and differences from, other oceanic ridge provinces that are thousands of kilometers away, but that are still in evolutionary communication over geologic time scales.

    CREDIT: VAN DOVER ET AL.
  8. Predicting Foot-and-Mouth Outcomes

    The current foot-and-mouth epidemic in the United Kingdom has devastated the livestock industry there and has had substantial consequences both for farming and other sectors of the economy. Keeling et al. (p. 813; see the 5 October news story by Enserink) modeled the dynamics of the epidemic using an individual farm-based spatial model to study its spread. The spatial detail of the epidemiological data is much finer than for most other diseases and allows the exploration of the dynamics of disease at much greater resolution than is normally possible with livestock diseases. This aspect of the model will permit improved prediction of the likely course of the epidemic under various scenarios of disease control.

  9. Boron, Pectins, and Plant Cell Walls

    Plant cell walls are complex mixtures of complex carbohydrates as well as multiple other components. As the plant grows and develops, its cell walls must be altered in order to accommodate changes in cell shapes. O'Neill et al. (p. 846; see the Perspective by Höfte) now show, through biochemical analysis and study of mutant versions of Arabidopsis, that disruptions in the structure of rhamnogalacturonan pectins and their borate-mediated dimerization in the cell wall have striking effects on the plant's growth. Thus, the pectins, as well as the better understood cellulose fibrils, are both critical to cell wall function.

  10. Tiny RNA World Discovered

    Two small temporal RNAs (stRNAs), let-7 and lin-4, play an important role in the development of the nematode Caenorhabditis elegans. Let-7 is also highly conserved throughout bilateral animals, including Drosophila and humans. Are there other small regulatory RNAs? Three reports by Lagos-Quintana et al. (p. 853), Lau et al. (p. 858), and Lee et al. (p. 862) indicate that there are a very large number (>60) of these tiny ∼22-nucleotide microRNAs (miRNAs)and that they have molecular characteristics similar to the two known stRNAs. The miRNAs are developmentally and tissue-specifically expressed and are conserved between different organisms. Some of the RNAs are organized in an operon-like fashion and may be processed from a single precursor. In a Perspective, Ruvkun calls the RNAs the “biological equivalent of dark matter” and suggests that they may provide a potent means for regulating gene expression.

  11. A Sense of Direction

    Intracellular signaling events that regulate chemotaxis are thought to be restricted to the edge of the cell that faces the highest concentration of attractant. However, a chemotactic Dictyostelium amoeba displays a constant and uniform surface distribution of its chemotactic receptors. Ueda et al. (p. 864) have used single-molecule imaging on live Dictyostelium cells to determine that the ligand binding kinetics of chemotactic receptors at the leading edge is different from other locations on the cell surface. Thus, the mechanism for directional sensing relies on differences in the signaling state of the receptors.

    CREDIT: UEDA ET AL.
  12. Genetic Response to a Pathogen

    Different classes of pathogens infect and propagate themselves within their hosts by highly diverse means. Specialized dendritic cells (DCs) encode pathogen-specific molecular patterns and then direct the most appropriate immune response. Huang et al. (p. 870; see the Perspective by Modlin and Bloom) scrutinized genes expressed by DCs in response to viruses, bacteria, and fungi using microarray analysis. Along with a central cohort of genes that were expressed regardless of the pathogen encountered, large non-overlapping sets of genes were also induced, which suggests that highly tailored programs of expression occur.

  13. Testing the Limits

    The reactions of open-shell molecules with an unpaired electron, such as NO and the OH radical, are of interest because they allow one of the fundamental assumptions of chemical physics, the Born-Oppenheimer approximation, to be tested. This approximation allows the theorist to neglect coupling between the electronic structure (the bonding) and the nuclear motion (such as vibrations); however, open-shell molecules have a higher density of electronic states that actually promote such coupling. Kohguchi et al. (p. 832) performed experimental studies for the scattering of NO molecules with Ar atoms and were able to reproduce some but not all of the experimental features in quantum close-coupling calculations.

  14. The Plus Side of Negative Excitons

    An exciton is the bound state of an optically excited electron-hole pair within a semiconductor. Recombination of the exciton results in the emission of a photon of well-defined wavelength, so there is much interest in exploiting excitons for optoelectronics. However, the charge of the ground-state exciton is neutral, and so moving them around a semiconductor would be difficult. Although the existence of charged excitons would facilitate a route for manipulating charge and light in a semiconductor, direct proof of their existence has remained elusive. Sanvitto et al. (p. 837) now present experimental evidence clarifying their existence and show that they can have substantial mobilities, up to 6.5 × 104 centimeters squared per volt-second.

  15. Comparing Bacterial Genomes

    In a large-scale bacterial genome-sequencing project, Glaser et al. (p. 849) offer a comparative analysis of the complete bacterial genomes from two members of the genus Listeria, one pathogenic (L. monocytogenes), one nonpathogenic (L. innocua). Interestingly, the few hundred mutually distinct genes are scattered throughout the genomes in about 100 gene islets. The sequences reveal surprising synteny with the genomes of Bacillus subtilis and Staphylococcus aureus and an unexpectedly high number of gene orthologs. Among the Listeria genus-specific genes, surface and regulatory proteins are overrepresented, which indicates that lateral gene transfer and species diversification have occurred. Nevertheless, most of the virulence genes are on the L. monocytogenes “virulence locus,” although there seems to be a trace of acquisition and subsequent loss of a virulence gene cassette by L. innocua.

  16. Dealing with DNA Damage in Different Ways

    In a critical process known as the “DNA damage checkpoint,” cells monitor their DNA for damage and halt cell division until the damage has been repaired. Kondo et al. (p. 867) now show that two yeast proteins that function in signaling that DNA damage has occurred actually bind close to the site of damage, but do so in distinct ways. They used a chromatin immunoprecipitation method to detect proteins that bound to double-stranded break in the DNA resulting from continuous expression of the HO endonuclease. One of the proteins bound, Mec1, belongs to a family of protein kinases that includes the ATM protein mutated in the human disease ataxia telangiectasia. Another protein bound at the site of damage was Ddc1, itself thought to be a DNA-binding protein that may recruit other proteins to a complex formed at the damage site. Recruitment of Ddc1, but not that of Mec1, required the action of another participant in the checkpoint pathway, called Rad24. The results help clarify how cells recognize damaged DNA and then initiate signals that produce the appropriate physiological response.

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