This Week in Science

Science  05 Oct 2007:
Vol. 318, Issue 5847, pp. 13
  1. Rethinking Coral Composition


    Modern coral reefs are built primarily by scleractinian corals, which arose in the Triassic after the Permian extinction. Today, all of these corals form skeletons of aragonite, and this composition has been thought to be typical of fossil scleractinians as well. Stolarski et al. (p. 92) now have identified a Cretaceous scleractinian coral with a primary calcite skeleton. The fine preservation of internal structures and the Mg and Sr chemistry show that the calcite is primary, not diagenetic. This result tightens the evolutionary connection between these corals and rugose corals, which formed calcite skeletons but were eliminated in the Permian extinction. These results suggest that corals may be able to alter their biochemistry in response to changes in seawater chemistry.

  2. Intimate Contact

    Composite materials commonly consist of strong or stiff particles or fibers surrounded by a matrix material. Often the best properties are observed when the contact between the reinforcing materials and the matrix are maximized, but in many cases, poor distribution and adhesion of the reinforcing material limit dispersion. Podsiadlo et al. (p. 80) used a layer-by-layer deposition technique to distribute clay platelets into a polymer matrix, and obtain nearly the idealized properties for a series of thin, transparent films. The nanometer-scale clay platelets formed ordered sheets that allow very strong hydrogen bonding with the polymer matrix, which ensured efficient load transfer between the polymer and clay.

  3. The Importance of Neutrality

    Atmospheric aerosol particles, which have diameters between 3 and 10 nanometers (nm), are formed continually in all parts of the troposphere. These particles play key roles in climate because they can absorb or reflect sunlight and affect cloud formation and atmospheric chemical reactions. However, how and in what quantities aerosols form are poorly understood, largely because of the difficulty in observing the smaller particles from which they grow. Kulmala et al. (p. 89, published online 30 August) investigated the distribution of particles smaller than 3 nm in diameter and found that an abundant pool of neutral clusters is present at almost all times. These clusters dominate the process of atmospheric aerosol formation, at least over boreal forests. These findings dispel the suggestion that aerosol production is driven mainly by processes involving ion clusters.

  4. Check the Substrate Before You Grow


    Organic thin-film transistors (TFTs) can be fabricated by depositing an organic semiconductor layer on a polymeric insulator such as polystyrene (PS) or polymethyl-methacrylate that coats the gate electrode. Kim et al. (p. 76) show that these thin insulating layers have glass transition temperatures, Tg, well below that of the corresponding bulk material and in the range of temperatures normally used for deposition of the semiconductor layer. Pentacene layers grown at temperatures below the surface-depressed Tg exhibited much higher carrier mobilities and grain sizes than those layers grown at higher temperatures, where the material is rubbery and has greater polymer chain motion. Surface treatments that cross-link the chains of the insulating layer, such as oxygen-plasma treatment of PS films, increased the surface Tg and restored higher mobility.

  5. Missing Jigsaw Piece

    The Pacific tectonic plate apparently underwent a shift about 50 million years ago, as evidenced in the changing of the track of the Hawaiian-Emperor chain of seamounts. Why this happened has not been clear. Whittaker et al. (p. 83) show that additional plate movement between Australia and Antarctica around this time can be gleaned from magnetic and satellite gravity data, which would indicate that a major plate reorganization occurred between 50 and 53 million years ago. Revised Pacific Ocean-floor reconstructions suggest that subduction of the Izanagi spreading ridge and subsequent Marianas/Tonga-Kermadec subduction initiation may have been the ultimate causes of these events.

  6. Learning, Autism, and the Synapse

    A small number of individuals with autism harbor mutations in genes encoding neuroligins and neurexins, cell adhesion proteins that facilitate neuronal communication across synapses. Tabuchi et al. (p. 71, published online 6 September; see the Perspective by Crawley) studied the functional consequences of one of these mutations, an R451C substitution in neuroligin-3, by introducing the mutant protein into mice. The mice displayed enhanced spatial learning skills but impaired social interactions, and these behavioral changes were accompanied by a selective increase in inhibitory synaptic transmission. Thus, alterations in the balance of excitatory and inhibitory synapses can affect learning and such alterations may be a contributing factor in the pathogenesis of autism.

  7. Neuronal Roadmap

    As the neural system develops, a distinctive network of interneuron connections is created. Colón-Ramos et al. (p. 103) now find that, in the nematode worm Caenorhabditis elegans, the supporting glial cells provide the requisite road map for making these connections. Particular glial cells express netrin, a signaling molecule, which tells the postsynaptic neuron where to find its connection and tells the presynaptic neuron where to build the substructures required for the connection. Localization of the netrin expression in the glial cells serves to focus the neuronal synapse-building capacity in the right spot.

  8. Captive Breeding Reduces Reproductive Fitness


    Captive breeding programs to prevent extinction are now in place for restoring many endangered wild populations and species, although the impact of such programs remains largely untested. Araki et al. (p. 100) evaluated the reproductive success of captive-bred fish when they breed in natural environments. Captive-reared individuals with captive-reared parents have half the reproductive success of captive-reared fish with wild parents. The rate of fitness decline can be ∼40% per captive-reared generation, which suggests that breeding programs need further evaluation of their impact when used to restore declining wild populations.

  9. Fair's Fair...or Not

    The experimental benchmark for demonstrating that humans have developed a sense of fairness is their behavior when playing the ultimatum game. If the division of spoils proposed by the first player is not generous enough (roughly 40 to 50% of the total), the second player will usually refuse to accept the proposal (giving up any hope of a gain), which has the consequence of depriving the first player of any payout as well. Jensen et al. (p. 107) have now implemented a trimmed-down version of the ultimatum game in chimpanzees. When in the role of player 2, our nearest relatives, unlike human subjects, will accept any number of raisins, and, perhaps as a consequence, chimps show little propensity to make fair offers.

  10. Plant Protector Identified

    Plants that survive an initial pathogen attack often develop enhanced resistance to subsequent infections. For example, prior infection of tobacco plants by tobacco mosaic virus (TMV) exhibit enhanced resistance elsewhere in the plant to subsequent challenge by TMV or other pathogens, which is termed systemic acquired resistance (SAR). The development of SAR requires the movement of a signal made in the primary infected tissue through the phloem to the distal systemic tissue. Park et al. (p. 113; see the news story by Leslie) show that the mobile signal for SAR is a biologically inactive form of salicylic acid, methyl salicylate (MeSA), a key hormone for activating host defenses to many plant pathogens.

  11. Anatomy of an Immune Response

    Intravital imaging techniques allow experimentally induced immune responses to be traced in real time. Nevertheless, the techniques have often relied on the transfer of nonphysiological numbers of artificially labeled immune cells into animals. Khanna et al. (p. 116) report the use of in situ confocal microscopy of the spleen with a sufficient level of resolution to detect fine features of an immune response to a bacterial infection. Endogenous primary and secondary (memory) T cell responses could be compared, revealing unexpected relocalization within the spleen, as T cells underwent activation, expansion, and then migration out to peripheral anatomical sites.

  12. Cooling North But Not South

    Around 13,000 years ago, during the last transition from glacial to warm conditions, a severe cooling event called the Younger Dryas plunged the North Atlantic region into 1500 years of low temperatures. Whether the Younger Dryas also was expressed in the Southern Hemisphere has been a topic of considerable debate. Barrows et al. (p. 86) present a pair of records that show an absence of Younger Dryas cooling in the Southern Hemisphere in and near New Zealand. By dating a moraine in South Island, New Zealand, and by constructing a sea-surface temperature record of nearby Pacific Ocean waters, the authors show that this region remained warm during the Younger Dryas interval. The moraine that the authors dated is one that was previously thought to have recorded Younger Dryas cooling.

  13. Birds of a Feather?

    When populations meet and interbreed, hybrids often experience low fitness. Natural selection against hybridization may then reinforce any tendency for assortative mating. How can traits and preferences for hybrids stay associated within each nascent species, despite recombination resulting from hybridization? S'ther et al. (p. 95; see the Perspective by Ritchie) used female hybrids and cross-fostering experiments to show that assortative mating preferences in flycatcher hybrid zones are located on the avian paternally inherited Z chromosome. Z-linked genes are also responsible for male plumage traits and reduced hybrid fitness. Because the Z chromosome does not recombine between these flycatchers, speciation may thus proceed unconstrained by recombination.

  14. Deep Diversity

    Pyrosequencing technology can be used in high-throughput sequencing of amplicons of a small region of small subunit ribosomal RNA and allows high-resolution estimates of microbial diversity. Such resolution allows the data to be interrogated in ways that can shed light on the ecology of the communities sampled. Huber et al. (p. 97) have sampled the microflora associated with deep-sea hydrothermal vents in the Pacific Ocean and discovered that two vents with different chemistries have distinct microflora. Archaea tended to be less diverse than Bacteria and could be sampled more completely, but the general pattern was for a few distinct populations to dominate, with thousands of low-abundance types accounting for most of the novel diversity.

  15. The Silence of the Flowering Genes

    Transcription of genes is controlled by specific factors at each promoter and by a general state of activation or silencing in the surrounding chromatin. In the course of studying chromatin silencing in the plant Arabidopsis, Bäurle et al. (p. 109) noticed a defect in flowering. It seems that some of the genes already known to control flowering time, including FCA and FPA, also function to modify chromatin involved in other processes outside of flowering regulation. Haploid female gametophytic development and embryonic development seem to be particularly susceptible to failure of the chromatin-modification functions of FCA and FPA. Hence, FCA and FPA proteins are localized in the nucleus and are required for efficient silencing of a number of loci by mechanisms that seem to vary with different loci.

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