This Week in Science

Science  31 Jul 2009:
Vol. 325, Issue 5940, pp. 514
  1. Genesis of Binary Stars


      Numerical simulations of collapsing clouds of primordial gas indicate that the first luminous objects to form in the universe were isolated massive stars. Turk et al. (p. 601, published online 9 July) now show that it is possible for single primordial clouds to break up into two dense cores. Three-dimensional calculations, which follow the evolution of primordial gas (composed of hydrogen and helium, with traces of deuterium and lithium) and dark matter starting from realistic, cosmological initial conditions, suggest that these cores may evolve to form binary star systems.

    1. Fighting for Fisheries

        In the debate concerning the future of the world's fisheries, some have forecasted complete collapse but others have challenged this view. The protagonists in this debate have now joined forces to present a thorough quantitative review of current trends in world fisheries. Worm et al. (p. 578) evaluate the evidence for a global rebuilding of marine capture fisheries and their supporting ecosystems. Contrasting regions that have been managed for rebuilding with those that have not, reveals trajectories of decline and recovery from individual stocks to species, communities, and large marine ecosystems. The management solutions that have been most successful for rebuilding fisheries and ecosystems, include both large- and small-scale fisheries around the world.

      1. Perfectly Flat?

          Plasmonic devices, which exploit the interactions of light with surface electrons, show great promise for applications in sensing, communications, and energy conversion. A key hindrance is the deposition of patterned metals used for plasmonics, because, as deposited, the terminal surfaces are rough and not amenable to patterning by directional dry-etching techniques. Nagpal et al. (p. 594) use patterned silicon substrates on which they add gold, silver, or copper and then apply an epoxy layer to the deposited metal. When pulled apart, the metal separates from the silicon, where the adhesion is poorer, leaving an ultra-smooth surface. The resulting surface plasmon propagation lengths approach the theoretical values for perfectly flat films.

        1. Optimizing Molecular Sieve Production

            CREDIT: CHOI ET AL.

            Microporous membranes composed of aluminosilicate minerals are known as zeolites and are often called molecular sieves because of their ability to filter or separate small molecules. The separation performance is partly governed by the selectivity for one species over another, and this can be compromised by defects, which allow for easy diffusion pathways. To create the porosity, structure-directing agents are used, which need to be removed during a long thermal treatment that can generate defects. Choi et al. (p. 590) show that for the silicalite-1 system, a rapid thermal treatment significantly reduces the defect density, with corresponding improvement in the filtration of very similar species, such as xylene isomers.

          1. Tracing Mantle Oxidation

              The chemical composition of the Earth's mantle varies with tectonic setting. For example, basaltic melts near subduction zones are more oxidized than magma near divergent plate boundaries. Kelley and Cottrell (p. 605; see the Perspective by Hirschmann) examined melts formed in different tectonic environments, using highly sensitive synchrotron-based analytical methods. The oxidation state of Fe increased with water content and mobile trace elements concentrations. Thus, fluids released from wet subducting plates drive mantle oxidation above subduction zones, which may help to explain the spatial differences in oxygen fugacity of the mantle.

            1. Expanding Sulfonylurea Mechanisms

                Sulfonylureas are important drugs used for treatment of diabetes that act through adenosine triphosphate–sensitive potassium channels to promote secretion of insulin from the pancreas. Zhang et al. (p. 607) present another mechanism by which the beneficial effects of sulfonylureas may also be obtained. Sulfonylureas were identified in a screen for substances that modify the activity of Epac2, a guanine nucleotide exchange factor for the small guanosine triphosphatase Rap1. Mice lacking Epac2 were less responsive to sulfonylureas, which may suggest that Epac2 would be a useful target for development of drugs for treatment of diabetes.

              1. Young and Flexible

                  How an infant learns to understand and speak a language remains a deep scientific mystery even though millions of kids accomplish these feats each year. Furthermore, this learning capacity is apparently not even taxed to its utmost; children who grow up in bilingual families learn two languages just as rapidly as those who learn only one. Kovács and Mehler (p. 611, published online 9 July) have assessed the cognitive flexibility of preverbal 1-year-old children raised in monolingual versus bilingual households and find that the bilingual group displays an impressive facility in handling inconsistent, language-like, inputs. That is, the kids exposed to two distinct languages since birth were able to associate two distinct syllabic structures—AAB and ABA—with looking leftward and rightward, whereas the monolinguals could only do so only with the simpler AAB structure.

                1. Maintaining Mutual Ignorance

                    Our gut is colonized by trillions of bacteria that do not activate the immune system because of careful compartmentalization. Such compartmentalization means that our immune system is “ignorant” of these microbes and thus it has been proposed that loss of compartmentalization might result in an immune response to the colonizing bacteria. Microorganisms are sensed by cells that express pattern recognition receptors, such as Toll-like receptors, which recognize patterns specific to those microbes. Slack et al. (p. 617) show that Toll-like receptor–dependent signaling is required to maintain compartmentalization of bacteria to the gut of mice. In the absence of Toll-dependent signaling, intestinal bacteria disseminated throughout the body and the mice mounted a high-titer antibody response against them. This antibody response was of great functional importance because, despite the loss of systemic ignorance to intestinal microbes, the mice were tolerant of the bacteria. Thus, in the absence of innate immunity, the adaptive immune system can compensate so that host and bacterial mutualism can be maintained.

                  1. Mapping the Neuronal Map

                      CREDIT: IMAI ET AL.

                      In vertebrates, sensory information is topographically represented as a neural map in the brain. How is the neural map formed in the brain? Nearly a half-century ago, Roger Sperry proposed the “chemoaffinity” model, in which the positional cues on the target determine the axonal projection site, thereby establishing the topographic neural map. However, molecular mechanisms of topographic map formation remain controversial. Imai et al. (p. 585, published online 9 July; see the Perspective by Miyamichi and Luo) now report that the topographic map is formed by axon-axon interactions before the axons reach the target. In the mouse olfactory system, the topography of the map is determined by the relative expression levels of a guidance receptor, Neuropilin-1, and its repulsive ligand, Semaphorin-3A, expressed in axons. Topographic organization occurs even in the absence of the target, the olfactory bulb. These findings require that Sperry's model, which suggests that only the targets determine the topography of neural maps, needs to be reconsidered.

                    1. Brain Rewiring After Stress

                        Chronic stress, mainly through the release of corticosteroids, affects executive behavior through sequential structural modulation of brain networks. Stress-induced deficits in spatial reference, working memory, and behavioral flexibility are associated with synaptic and dendritic reorganization in both the hippocampus and the medial prefrontal cortex. However, the effects of chronic stress on action selection strategies are unclear. Dias-Ferreira et al. (p. 621) examined whether chronic stress affects the ability of animals to select the appropriate actions based on the consequences of their choice, and found that rats exposed to chronic unpredictable stress rapidly shift toward using habitual strategies. The shift in behavioral strategies observed in chronically stressed animals corresponded to dramatic and divergent changes in connectivity in the associative and sensorimotor corticostriatal circuits underlying these behaviors.

                      1. Gradual Unpacking

                          Eukaryotic DNA is packaged onto nucleosomes, which form the main constituent of chromatin. This packaging material presents a barrier to accessing the genome by the various machineries that need to deal with the DNA: replication, recombination, repair, and transcription complexes, for example. Hodges et al. (p. 626; see the Perspective by Otterstrom and van Oijen) use single-molecule techniques to analyze how a yeast RNA polymerase II ternary elongation complex copes when it encounters a single nucleosome directly in its path. The polymerase does not actively peel the DNA from the nucleosome's surface but, instead, waits patiently until the DNA fluctuates off the nucleosome and then advances, increment by increment, until the nucleosome is destabilized. Under certain conditions the destabilized nucleosome, rather than being lost entirely from the DNA, can be passed back to the DNA behind the polymerase.

                        1. Electron Breakdown

                            An electron possesses charge and spin. In general, these properties are confined to the electron. However, in strongly interacting many-body electronic systems, such as one-dimensional wires, it has long been theorized that the charge and spin should separate. There have been tantalizing glimpses of this separation experimentally, but questions remain. Jompol et al. (p. 597) looked at the tunneling current between an array of one-dimensional wires and a two-dimensional electron gas and argue that the results reveal a clear signature of spin-charge separation.

                          1. Monitoring Monocyte Reservoirs

                              Monocytes are cells of the immune system that are recruited to sites of tissue injury and inflammation where they help to resolve the infection and are important for tissue repair. The bone marrow and blood are believed to be the primary reservoirs from which monocytes are mobilized after injury. Swirski et al. (p. 612; see the Perspective by Jia and Pamer) now demonstrate that the spleen also serves as a critical reservoir of monocytes that are recruited during ischemic myocardial injury. Monocytes in the spleen are very similar in phenotype to blood-derived monocytes and are mobilized to the injured heart, where they represent a large fraction of the total monocytes that are recruited. The chemoattractant, angiotensin II, is required for optimal monocyte mobilization from the spleen and emigration into injured tissue.