This Week in Science

Science  18 Jan 2008:
Vol. 319, Issue 5861, pp. 257
  1. Shockingly Hot

    CREDIT: M. GÜDEL AND K. BRIGGS/ESA/NASA

    Four extremely bright and massive stars, called the Trapezium, primarily illuminate the Orion Nebula. Güdel et al. (p. 309, published online 29 November; see the Perspective by O'Dell and Townsley) show that the Orion Nebula glows in x-rays because it is flooded with very hot gas in excess of 1 million kelvin. This heating is likely the result of shocks from powerful outflows from one bright star in the Trapezium. The majority of the stars in our Galaxy are found in regions similar to the Orion Nebula, so this phenomenon should be widespread throughout the galactic plane.

  2. Evolution in Soay Sheep

    Quantitative trait loci (QTLs) are genetic loci correlated with a typically additive phenotypic trait such as height or color. In Soay sheep, dark coat color is associated with large body size, which is heritable and positively correlated with fitness. Through mapping of the region associated with the loci determining coat color, Gratten et al. (p. 318) detail the linkage of several causal mutations within a small genomic region within a free-living sheep population. A strong correlation was found between coat color and weight, but these relations showed antagonistic effects between body size and lifetime fitness. This study provides empirical support for the role of a negative genetic correlation in the evolutionary dynamics of a natural population.

  3. Organizing Brain Development

    In early development, the cortex of the mammalian brain is built layer by layer, from the inside out. But before that occurs, the neural epithelium consists of only a single layer of stem cells. Mangale et al. (p. 304; see the Perspective by Grove) have now determined that in mice the gene Lhx2 determines cortical identity for cells at these earliest stages. Lhx2 establishes whether cells give rise to the cortical hem, which is now seen to function as an organizer of hippocampal development.

  4. Describing Disorder

    Insights into the structure of amorphous materials usually come from theoretical modeling or from microscopy studies of larger colloidal particles. Although scanning tunneling microscopy (STM) methods can achieve atomic resolution, molecular overlayers on metal substrates either to form bulk-like islands or crystallize into ordered films. Otero et al. (p. 312, published online 13 December) report STM images of cytosine molecules adsorbed on gold surfaces which show that the molecules are highly mobile at room temperature. Upon cooling to below 150 kelvin, disordered hydrogen-bonded networks form that can be characterized as being built up from three types of subunits—zigzag filaments and five- and six-membered rings.

  5. The Not-So-Old Mill Streams

    Our understanding of natural stream development has been greatly influenced by many studies in the eastern United States. Walter and Merritts (p. 299, cover; see the Perspective by Montgomery) now show that mill dams were pervasive—there were more than 65,000 mills in the 19th century—and that these structures thoroughly changed the natural character of the floodplains and streams. Mapping of about 20 streams in detail showed that these dams, collectively spanning the full length of many streams, along with widespread deforestation could cause up to 5 meters of sediment to accumulate in floodplains. Streams were changed from branching channels across broad wetlands to confined meandering channels.

  6. Vitamin A in Corn

    Vitamin A deficiency affects the vision of millions of children each year, a problem that could be addressed with dietary adjustments. Harjes et al. (p. 330) show that natural maize variants offer a wide range of the precursors to vitamin A. The cyclase gene lcyE is critical in assigning carotenoid variants to different metabolic pathways. A selective breeding program, rather than complicated transgenic methods, may be able to deliver more nutritious maize.

  7. Heads or Tails

    CREDIT: P. PETERSEN AND W. REDDIEN

    Planarians—tubular flatworms—can regenerate all of their body parts and entire organ systems after amputation. However, the mechanism by which the animal “knows” how to generate a head after head removal or a tail after tail removal, a property called regeneration polarity, is unknown. Gurley et al. (p. 323, published online 6 December) and Petersen and Reddien (p. 327, published online 6 December) now find that a conserved factor within the Wnt signaling pathway is used to distinguish head from tail. Decreased Wnt signaling through β-catenin causes the regeneration of heads, whereas activation of Wnt signaling induces tails.

  8. Cellular Origin of Leukemia

    Little is known about preleukemic cells, in which deleterious mutations first arise and function, because such cells are “clinically silent.” Hong et al. (p. 336) studied a pair of identical twins, one of whom has childhood acute lymphoblastic leukemia. Both twins possessed a cell population that contained the disease- causing chromosomal translocation but was not yet malignant. Subsequent modeling experiments in mice revealed that these preleukemic cells could go on to generate the self-renewing cells that propagate leukemia.

  9. Superexchange in an Optical Lattice

    Superexchange interactions in superlattice systems are effective spin-spin interactions between neighboring particles, are mediated by virtual hopping processes, and form the basis of most quantum-statistical spin models that are used for the explanation of quantum magnetism. In most condensed-matter systems, the sign and magnitude of the interactions are fixed. Atoms trapped in optical lattices offer the possibility of a tunable parameter space in which the sign and size of the interactions can be varied. Trotzky et al. (p. 295, published online 20 December; see the Perspective by Lewenstein and Sanpera) report on the direct detection and control of the underlying superexchange spin interactions between neighboring cold atoms trapped at sites of optical lattice, thus illustrating the possibilities for investigating quantum spin systems with ultracold atoms.

  10. Subduction and Mantle Mineral Alignment

    In subduction zones, a wedge of mantle material is caught above the subducting plate and beneath the overlying crust. Flow within the mantle also produces flow in the wedge that can align mantle minerals. Such alignment should cause seismic waves to propagate faster in one direction, but systematic patterns have been difficult to discern. A global survey and analysis of such data by Long and Silver (p. 315) shows that the confusing patterns may reflect the migration of trenches forward (toward the direction of the incoming plate) and backward. This flow tends to induce flow parallel to the trench both in the mantle beneath the subducting slab and in the wedge that scales with the rate of trench migration.

  11. Towards Ecosystem-Based Management

    The rapid global decline in coastal ecosystems has led to calls for “ecosystem-based management” (EBM), which could reconcile declining coastal ecosystem services with human development pressures. Barbier et al. (p. 321; see the Perspective by Valiela and Fox) argue that such a strategy is likely to fail unless a key assumption underlying the assessment of coastal management decisions is amended. Economic valuation often assumes that ecosystem services are linearly related to changing habitat size or area, which often leads to an “all or none” choice of either preserving or converting habitat to human use. A case study of mangroves in Thailand shows that by incorporating nonlinear wave attenuation in estimating coastal protection values of mangroves, the optimal land use option is not a stark “all or none” choice but instead an integration of development and conservation uses consistent with EBM goals.

  12. Tumor Suppressor Yin-Yang

    The Wnt signal transduction pathway is a critical player in the development of animals. However, inappropriate activation of this pathway triggers tumorigenesis; for example, disruption of the adenomatous polyposis coli (APC) tumor suppressor, a negative regulator of Wnt signaling, results in colorectal carcinoma development. Although APC is well known to function in inhibiting gene expression, Takacs et al. (p. 333) now report that fruit fly homologs of APC have an opposing job in the activation of Wnt signaling. Separable domains in APC not only restrained inappropriate signaling when the Wnt signal was absent but also promoted signaling upon Wnt stimulation.

  13. Noise, Life, and Death

    There is increasing interest in understanding noise in biological systems with the goal of gaining insight into underlying mechanisms. Gene expression has been particularly investigated with many experiments measuring how the variation in single-cell protein levels depends on transcription and translation rates. Pedraza and Paulsson (p. 339) present a generalized theory for stochastic gene expression, that shows that molecular mechanisms that affect the periods between protein “births” and “deaths” can modulate noise without affecting transcription or translation rates. Because of this situation, standard single-molecule measurements can identify what components contribute to fluctuations, but not how they contribute. Analyses of the mechanisms underlying fluctuations will require a time series where births and deaths can be observed directly.

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