This Week in Science

Science  30 Jan 2009:
Vol. 323, Issue 5914, pp. 559
  1. Duplications, Differences, and Divergence


    We know little about how genetic incompatibilities arise between individuals, populations, and species. One hypothesized path is by reciprocal loss of duplicated genes, but concrete evidence for this has been lacking. Now Bikard et al. (p. 623) describe a case where divergence between recently duplicated genes that have acquired divergent functions (paralogs) in Arabidopsis results in an incompatibility within the species, illustrating how reproductive isolation of strains could emerge due to gene duplication.

  2. Dealing with a Quick Quantum Death

    The states of quantum systems can be used to store information and carry out quantum logical operations, the power of which, if enlarged, is potentially much greater than that of classical computers for certain tasks. Quantum mechanical entanglement of the states lies at the core of such work. While it is known that the “programmed” quantum states decay as they interact with their surroundings, recent work has shown that entanglement can decay much more rapidly—essentially ending abruptly. Yu and Eberly (p. 598) review this effect, describing the influence it may have on quantum systems, as well as suggesting potential approaches for dealing with such a sudden loss.

  3. Strength and Size Matters

    For polycrystalline metals, as the crystal grain size becomes smaller the material becomes stronger, because it becomes increasingly more difficult for crystal defects to move and propagate—a phenomenon caused by the Hall-Petch relationship. Below a certain grain size, the strengthening effect ceases and some materials even become weaker with further reduction in grain size, in the range of tens or hundreds of nanometers. Lu et al. (p. 607) show how the deformation mechanisms change with grain size in a nanocrystalline copper, and specifically the role that twin boundaries play in controlling the strength of the copper.

  4. To Swarm or Not to Swarm?

    In desert locusts, the radical transformation from a harmless “solitarious” form to the swarm-forming “gregarious” phase is one of the most extraordinary and iconic examples of density-dependent phenotypic plasticity in nature, as well as one of the most economically devastating. Now Anstey et al. (p. 627; see the Perspective by Stevenson; see the cover) reveal a neurochemical mechanism linking the changed behavior of individuals to profound changes in population structure, which, in this instance, lead to swarming and mass migration. The key to the phenotypic change is the ubiquitous neurotransmitter serotonin, which is synthesized in response to multiple sensory cues that gauge locust population density, switching locusts from mutual aversion to mutual attraction—the first and essential stage in establishing swarms.

  5. Adding Hydrogen to Graphene


    Like its parent material, graphite, the atomically thin sheets of graphene do not readily undergo chemical reactions. Elias et al. (p. 610; see the Perspective by Savchenko) show that graphene will undergo hydrogenation reactions when exposed to atomic hydrogen, and this semi-metallic material becomes an electronic insulator. The properties of graphene can almost be fully restored by annealing the material. Although the material formed is still crystalline, it undergoes an unexpected compression and a decrease in the lattice constant of the C-C bond network.

  6. Modeling Chemical Communication

    In populations of single-celled organisms such as yeast, transitions can occur in their coordinated behavior that depend on their density in solution and that are believed to be triggered through chemical signaling. Taylor et al. (p. 614) modeled these transitions by studying large populations of chemical oscillators (about 100,000). Small porous particles that contained the catalyst that drives the oscillating Belousov-Zhabotinsky (BZ) reaction were studied suspended in solution with its reactants. A synchronization transition and a sudden dynamical quorum sensing transition were both observed as a function of oscillator density and the rate of exchange of signaling species through the surrounding solution.

  7. Slow Creep and Other Movements

    Recently, slow creep, along with a series of small shudders or tremors has been recognized in several major subduction zones, including in Japan and Cascadia in the Pacific Northwest of North America, which produce the largest earthquakes. The origin and effect of these tremors on earthquake hazards has not been clear, and accurately locating them is critical for inferring mechanism. La Rocca et al. (p. 620) describe a technique for accurately locating these micro-quakes based on a comparison of arrivals times of different wave types from the tremors. Using this approach, the Cascadia quakes were shown to cluster along the main subduction fault.

  8. Regulating Hypoxic Sensitivity

    The oxygen requirements of an organism and its cells vary widely. To identify genetic determinants of hypoxic sensitivity, Anderson et al. (p. 630) performed a screen of Caenorhabditis elegans for mutants that recover from severe hypoxia. A reduction-of-function mutant in an arginyl-transfer RNA (tRNA) synthetase, a component of the translational machinery, was isolated. Other aminoacyl-tRNA synthetases were similarly found to regulate hypoxic sensitivity. The level of translational suppression correlated with hypoxia resistance. An intact cellular response to unfolded proteins is required for the high-level hypoxia resistance of the mutant, suggesting that translational suppression functions cooperatively with the unfolded protein response to mitigate hypoxic cellular injury.

  9. Feeling Tense

    Transduction of force between the extracellular matrix and the cytoskeleton is important for cellular function (see the Perspective by Schwartz). Though there are models for how talin, a protein that links membrane integrins with the cytoskeleton, might transduce force, experimental evidence has been lacking. Now del Rio et al. (p. 638) show that stretching of talin at physiologically relevant forces, exposes cryptic binding sites for vinculin, a focal adhesion protein that is activated by talin binding, leading to cytoskeletal reorganization. Dynamic adhesion of cells to their microenvironment via cell surface bound integrins is important for cell spreading and migration. Friedland et al. (p. 642) found that integrins can be activated by tension generated between the internal cytoskeleton and a fibronectin coated surface. Tension regulates the integrin-fibronectin interaction, which switches between a relaxed and tensioned state and activates an intracellular signaling pathway.

  10. A Golden Route to Platinum Nanocrystals


    Precious metals in catalysts are often used in the form of nanoparticles, which increases the number of exposed surface atoms relative to the bulk material. Nanocrystals with well-defined surface planes and distinct edges can exhibit different or greater reactivity than less crystalline forms, but can be difficult to synthesize at sizes below 100 nanometers, and very small nanoparticles (below about 5 nanometers in diameter) are often rounded and less reactive. Schrinner et al. (p. 617) describe a route for converting nanoparticle alloys of gold and platinum, between 2 and 3 nanometers in diameter, into Pt nanocrystals. The particles were embedded in a polymer network on the surface of a latex particle, and a leaching reaction that removes gold proceeded slowly (over the course of hours) so that the particles formed single crystals. The network prevented the nanocrystals from agglomerating and was used as a support for the nanocrystals in hydrogenation reactions.

  11. Asymmetric Division in Plant Development

    In plants, as in other multicellular eukaryotes, asymmetric divisions are used to generate pattern and diversity during development. Asymmetric divisions must be oriented properly within their developmental context, but very little is known about how this is accomplished in plants. Plants lack proteins universally employed for polarization of asymmetric cell divisions in animals and pathways involved in orienting animal cell polarity. About 50 years ago, it was proposed, based on observations of cell division patterns in developing grass leaves, that the asymmetric divisions of subsidiary mother cells, which give rise to subsidiary cells of the stomatal complex, are polarized by the adjacent guard mother cell, the precursor of stomatal guard cells. Cartwright et al. (p. 649; see the Perspective by Sack and Chen) implicate a receptor-like protein, PAN1, in transmission of a guard mother cell-derived signal that polarizes subsidiary mother cells in preparation for their asymmetric division. Thus, asymmetric cell division can be polarized in response to an extracellular cue in plants.

  12. Myelination Matters

    One of the critical features of the vertebrate nervous system is the fast transmission of action potentials. Fast transmission is accomplished by myelinated nerves in vertebrates—myelination increases transmission speeds up to 300-fold. Kao et al. (p. 651) describe a key signal transduction pathway involved in myelination of mammalian neurons. Neuregulin and ErbB receptors activate the protein phosphatase, calcineurin, and the transcription factor, NFAT (nuclear factor of activated T cells), to induce another transcription factor, Krox20, which in turn controls a host of myelination genes.

  13. A Critical Point for Superconductivity

    Understanding the behavior of the carriers of the normal state in the high-temperature superconducting cuprates has been argued to be the key in understanding these materials. Arguments have been made for quantum critical behavior, but the experimental evidence has not been compelling because the interesting regime (where the quantum critical point is thought to reside in the phase diagram) is masked by the superconducting region. Cooper et al. (p. 603, published online 11 December; see the Perspective by Boebinger) use high magnetic fields to strip away the superconducting region and gradually unveil a quantum critical point. The measurements provide a clearer picture to interpret what is going on in these materials, as well as a reference with which to compare the behavior with other quantum critical systems.

  14. Calmodulin in Action

    The ability to apply force to single molecules has allowed the controlled investigation of protein energy landscapes. However, limits in resolution have meant that studies have mainly been carried out at nonequilibrium so that only unfolding and unbinding reactions could be investigated. Now Junker et al. (p. 633; see the Perspective by Best and Hummer) have used a custom-built low-drift atomic force microscope to observe directly fluctuations of single calmodulin molecules under equilibrium conditions in the presence of Ca2+ and target peptides. The results show how ligand binding modulates the folding dynamics of calmodulin.

  15. End in Sight

    The ends of linear eukaryotic chromosomes must be capped with specialized DNA sequences known as telomeres, to avoid progressive shortening and loss of genetic information due to the end-replication problem. The enzyme that adds these specialized sequences is known as telomerase, which is often mis-expressed in cancers, and consists of three known components: a reverse transcriptase enzyme, an RNA molecule (TERC), and the protein dyskerin. Venteicher et al. (p. 644) identify a fourth, dyskerin-associated subunit, telomerase Cajal body protein 1 (TCAB1), as part of the core mammalian telomerase. TCAB1 is essential for the in vivo function of telomerase, recruiting TERC to Cajal bodies within the nucleus, thereby delivering TERC to telomeres during S phase of the cell cycle.

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