This Week in Science

Science  04 Jan 2002:
Vol. 295, Issue 5552, pp. 9
  1. In Brevia

    The fluorescent glow of budgerigars' crown and cheek feathers are shown by Arnold et al. (p. 92) to be a sexual signal and not merely a by-product of plumage pigmentation.

  2. Electrically Excited Single Photons

    The development of practical quantum-information processing requires the ability to send single photons on demand. Several techniques have been developed that can provide single photons, but all have been based on systems that have been optically excited. The ideal single photon source would be electrically driven and operate at room temperature. Yuan et al. (p. 102) have constructed a device in which a quantum dot is embedded in a p-i-n diode. Although the device operates at temperatures below 200 millikelvin, subnanosecond voltage pulses deliver single photons on demand.

  3. Freeze-Thaw Cycles on Mars

    Liquid water is not stable under current martian surface conditions, but the presence of small gullies on the poleward-facing slopes of mid- to high-latitude martian surfaces suggests that erosion by liquid water might have occurred. Costard et al. (p. 110) developed a global climate model for Mars when it had a higher obliquity (about 300,000 years ago). Their model shows that more melting of the polar ice caps would have occurred and an increase in the surface pressure and temperature at mid- to high-latitudes in the summer would have allowed liquid water to flow. Similarly shaped gullies have been observed in periglacial debris flows from eastern Greenland. These observations, taken together with the modeling, suggest that the martian gullies formed by a freeze-thaw cycle of water on the surface, rather than through flow of an aquifer system.

  4. Magma Flow at Mid-Ocean Ridges

    The isotopic concentrations of uranium, thorium, and lead in dredged basalt samples have been used to trace magma flow to mid-ocean ridge spreading centers. Zou et al. (p. 107; see the Perspective by Elliott) measured anomalous 230Th/238U ratios in basalt samples taken 20 kilometers away from the axis of the East Pacific Rise that indicate the presence of an off-axis magma reservoir or of anomalous lateral flow of melt from the ridge axis. Either source will require revisions to current models of mid-ocean ridge dynamics.

  5. Uncertain Futures

    Estimates of uncertainty in predictions of long-term climate change usually are based in part on the informed but still subjective assessment of “experts,” and more objective means of determining uncertainty in such forecasts is desirable. Forest et al. (p. 113; see the news story by Kerr) use a two-dimensional statistical-dynamical model to evaluate what joint range of values for three of the most important properties that control climate system behavior—climate sensitivity, the rate of heat uptake by the deep ocean, and the strength of the net aerosol forcing—is consistent with 20th-century records of upper-air temperatures, surface temperatures, and ocean temperatures.

  6. Just Add Water?

    Noble metals resist air oxidation and corrosion by water. Indeed, structural studies of the initial stages of water adsorption on clean surfaces of metals such as ruthenium indicate that simple ice-like bilayers form. Other spectroscopic results appear to be inconsistent with such a structure. Feibelman (p. 99; see the Perspective by Menzel) presents density functional calculations which indicate that water molecules in the first adsorbed layer on the close-packed Ru(0001) surface actually dissociate. The calculated structures show how the oxygen atoms of the first layer can be coplanar and suggest more generally that dissociation may be necessary for water to wet a metal surface.

  7. A Start for Stars

    Was the first luminous object in the newly created universe a star or something more exotic? Simulations indicate that large density contrasts in a collapsing protogalactic halo would have fragmented any protostellar core and stifled star formation. Abel et al. (p. 93; see the Perspective by Rees and the special issue on star formation) have completed a three-dimensional hydrodynamic simulation of halo collapse from cosmological to stellar length scales, starting with a flat, cold dark matter cosmology. Three-body hydrogen formation could have proceeded without fragmentation long enough and fast enough to allow the development of a stable, fully molecular core. The molecular core then could stably accrete more mass to form a star. Thus, the first light may have been from a star. These simulations also show that metal-free stars can form in isolation from collapsing molecular clouds.

  8. Live Long and Avoid Oxygen Radicals

    Severely restricting the food intake of a rat or mouse will result in an extremely long-lived animal. Larsen et al. (p. 120; see the Perspective by Tatar and Rand) show that when fed a diet without coenzyme Q, the nematode Caenorhabditis elegans can also live a longer life. Coenzyme Q is a lipid electron acceptor necessary for proper function of the respiratory chain in mitochondria. There is a paradoxical similarity in these two cases—why does the lack of proper energy production lead to longer life-spans? The authors propose that less coenzyme Q in nematodes and less food in rodents may result in fewer damaging reactive oxygen species, which are released as a by-product of respiration. Alternatively, the imbalance of coenzyme Q may alter transcription of genes that influence the aging process.

  9. Mineralization by Bacteria

    Shewanella putrefaciens has a hitherto unrecognized capacity to generate iron oxide crystals, apparently within membrane-bounded compartments at the poles of the cells. Glasauer et al. (p. 117) show that when grown with hydrous ferric oxide, in conditions mimicking natural conditions in the soil, Shewanella co-precipitates a variety of fine-grained iron minerals within its cells. Iron may be recycled in these bacteria, and these distinct crystals could provide a useful fossil tracer of prokaryotic life.

  10. Clues from Close and Distant Relatives

    The DNA damage response (DDR) process in eukaryotes includes DNA damage-induced checkpoint control pathways and DNA repair processes. Boulton et al. (p. 127) describe a combination of protein-protein interaction mapping and high-throughput RNA-mediated interference to identify 23 genes in the Caenorhabditis elegans that are required for DDR, 11 of which had not been implicated previously. One of the genes identified is the ortholog of human BCL3, a gene frequently altered in chronic lymphocytic leukemia. A first-generation physical map of the chimpanzee genome has been generated that will provide a solid basis and resources for analysis of its genome. Fujiyama et al. (p. 131) have mapped more than 60,000 bacterial artificial chromosomes (BACs) from a chimpanzee genome library to the human genome by means of BAC-end sequences to search for relatively large rearrangements between both genomes. Two clusters containing large, nonrandom differences were found relative to human chromosome 21.

  11. Go Fourth and Recombine

    It has been reported for many years that unlike the other Drosophila chromosomes, the fourth chromosome undergoes no recombination. Wang et al. (p. 134) conducted a whole chromosome polymorphism survey of natural Drosophila populations from around the world and found an unexpected high level of variation and recombination. This chromosome can be divided into different regions of variation, which suggests that different regions in the fourth chromosome have different evolutionary histories.

  12. Uncapping Tumstatin's Effects

    Tumstatin belongs to a family of extracellular matrix-derived protein fragments that have attracted attention as potential anticancer agents because they are potent and nontoxic inhibitors of angiogenesis. Maeshima et al. (p. 140) found that tumstatin functions as an endothelial cell-specific inhibitor of protein synthesis that depends on the messenger RNA 5' cap structure. This activity requires interaction of tumstatin with αVβ3 integrin and results in endothelial cell apoptosis.

  13. Push Me, Pull You

    When Bacillus subtilis sporulates, its cells undergo an uneven division process during which the chromosomal DNA needs to be transported across a septum from a mother cell into a neighboring daughter cell. Sharp and Pogliano (p. 137) show that the SpoIIIE protein, known to help mediate this process, defines directionality by its preferential localization in the mother cell, from which it actively pumps chromosomal DNA into the forespore. When SpoIIE was forced to accumulate in the forespore, the direction of DNA transport could be reversed.

  14. Molecular Basis of Mossy Fiber LTP

    Long-term potentiation (LTP) at hippocampal mossy fiber synapses is distinct from most other forms of LTP. Mellor et al. (p. 143) found that the hyperpolarization-activated mixed cation current Ih unexpectedly plays an important role. Calcium entry into the hippocampal granule cells during repetitive stimulation activates a calcium/calmodulin sensitive adenylate cyclase. The subsequent rise in cyclic adenosine monophosphate activates protein kinase A, which leads to increases in Ih. The resulting depolarization causes an enhancement of mossy fiber synaptic transmission.

  15. Phenol Via Palladium

    Phenol, an important commodity chemical, is synthesized from benzene primarily through a three-step process (the cumene process) that is energy inefficient and produces unwanted by-products. Niwa et al. (p. 105) report a one-step process in which benzene is partially oxidized by oxygen and hydrogen in species generated at a supported palladium membrane. Hydrogen is fed to one side of the membrane, and oxygen on the other side of the membrane reacts with atomic hydrogen to produce a reactive oxygen species that then attacks the benzene ring. Although conversions are somewhat low (2 to 16% at 250°C), selectivities for phenol are high (80 to 97%).

  16. Leaving Too Soon

    Cell-surface glycosides are believed to function in cell interactions, and Akama et al. (p. 124; see the Perspective by Muramatsu) now show that a specific N-glycan participates in spermatogenesis. By examining a null mutation in mice, it is shown that α-mannosidase IIx (MX), an enzyme that forms intermediate N-linked carbohydrates, is necessary for the adhesion of germ cells to Sertoli cells. If MX function is eliminated in mice, the spermatogenic cells fail to adhere to Sertoli cells and results in the premature release of spermatogenic cells from the testis into the epididymis, yielding an infertile male. A better understanding of spermatogenesis in mice may shed light on similar events that occur in male infertility in humans.

  17. A Cellulose Primer

    Cellulose, a ubiquitous and complex polymeric carbohydrate, is synthesized one block at a time. However, getting the polymerization started in the first place relies on biochemical mechanisms that differ from those that continue the polymerization. Peng et al. (p. 147; see the Perspective by Read and Bacic) now propose that the initialization of a cellulose polymer begins with formation of a sterol-glucoside, from which the glucan chain is subsequently elongated. The sterol is later removed from the initial primer unit by a membrane-associated cellulase, encoded by the gene Korrigan.

  18. The Isomap Algorithm and Topological Stability

    Tenenbaum et al. (Reports, 22 December 2000, p. 2319) presented a computational algorithm, called Isomap, that offered an approach to nonlinear dimensionality reduction, the problem of “finding meaningful low-dimensional structures hidden in … high-dimensional observations.” Balasubramanian and Schwartz comment that “the basic idea of Isomap has long been known,” and that the implementation by Tenenbaum is “topologically unstable,” requiring “a priori information about the global geometry of the high-dimensional data manifold” to allow selection of an appropriate computational neighborhood size. Tenenbaum et al. respond that their algorithm is substantially simpler, more efficient, and more broadly applicable than previous approaches. They also suggest a method for selecting an optimal neighborhood size without a priori information about the data geometry.

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