This Week in Science

Science  20 Aug 2010:
Vol. 329, Issue 5994, pp. 882
  1. Through a Lens Darkly

    According to recent measurements, 72% of the energy content in the universe is in the form of dark energy, a gravitationally repulsive constituent that is powering the accelerating expansion of the universe but whose nature is unknown. Now, Jullo et al. (p. 924) show how observations of systems of multiple images produced by the strong gravitational lensing effect of a single mass distribution can be used to constrain the properties of dark energy. Applied to the cluster Abel 1689—a galaxy cluster that is known for its lensing properties—and combined with the results of other techniques, this method brings down the overall error in the equation of state parameter of dark energy by 30%.

  2. When Black Holes Collide

    When galaxies merge, their central black holes interact, initially coming together in a binary system and eventually coalescing into one single black hole. The dynamics of orbiting binary black holes in vacuum is well understood; however, when black holes merge, their accretion disks are expected to combine into a circumbinary disk anchoring a magnetic field. Numerical simulations from Palenzuela et al. (p. 927; see the Perspective by Yunes) now show that when this surrounding environment is taken into account, the black holes effectively stir the plasma that surrounds them, generating collimated beams of electromagnetic radiation that then transition to a single jet as the black holes merge due to the emission of gravitational waves.

  3. Lunar Lobate Scarps Revealed

    CREDIT: NASA/GSFC/ARIZONA STATE UNIVERSITY

    Lunar lobate scarps are relatively small-scale landforms that are thought to be formed by tectonic thrust faulting. Previously, lunar lobate scarps could only be identified clearly in high-resolution Apollo Panoramic Camera images confined to the lunar equatorial zone. Now, an analysis by Watters et al. (p. 936) of images returned by the Lunar Reconnaissance Orbiter Camera reveals 14 previously unknown lobate scarps and shows that lunar lobate scarps may be globally distributed. Their appearance suggests that lunar scarps are relatively young landforms (less than 1 Ga), possibly formed during a recent episode of global lunar radial contraction.

  4. Directed Emission

    The atomlike features of quantum dots—the discrete energy levels and subsequent emission of light at discrete wavelengths—make them key building blocks in optoelectronic circuits and optical communication. However, the emitted light tends to be omnidirectional, which limits applications that require accurate transmission between sender and receiver. Curto et al. (p. 930; see the Perspective by Giessen and Lippitz) have designed an optical antenna, a shrunk-down version of the Yagi-Uda design used in microwave and radio communication, and show that coupling the quantum dot to the antenna provides control over the direction of the emitted light.

  5. Keeping Nanoparticles Small

    Heterogeneous catalysts that consist of small metal nanoparticles absorbed on oxide supports can deactivate over time through a process called sintering. Elevated temperatures increase the rate of diffusion of metal atoms over the support, and larger, less reactive particles grow at the expense of smaller ones. Some supports that contain reducible metals, such as cerium oxide, tend to resist sintering better than oxides such as alumina. Farmer and Campbell (p. 933) present an analysis of previous calorimetry data for silver nanoparticles on magnesium oxides and cerium oxide surfaces and show that nanoparticles smaller than 1000 atoms are bound much more strongly to reduced cerium oxide. The energetic driving force for creating larger particles is quite low on these surfaces and increases the lifetime of smaller particles.

  6. Reversing the Trend

    Terrestrial net primary productivity (NPP, the amount of atmospheric carbon fixed by plants and accumulated as biomass) increased from 1982 through 1999, which has been attributed to factors such as nitrogen deposition, CO2 fertilization, forest regrowth, and climatic changes. Zhao and Running (p. 940) used satellite data to estimate global terrestrial NPP over the past decade and found that the earlier trend has been reversed and that NPP has been decreasing. Combining this result with climate change data suggests that large-scale droughts are responsible for the decline. Future widespread droughts caused by global warming may thus further weaken the terrestrial carbon sink.

  7. The Pitfalls of Re-Replication

    The cell has several layers of regulation to ensure that the genome is replicated once and only once during cell division, presumably to avoid the formation of duplicated sequences, which have the potential to recombine and compromise genome stability. In agreement with this idea, Green et al. (p. 943; see the Perspective by Kaochar et al.) now show that re-replication of a marked yeast replication origin causes a significant increase in copy number variation in the region encompassing the origin. The observed duplications range in size from 135 to 470 kilobases and are almost all tandemly arrayed in a head-to-tail orientation. The duplicated regions are bounded by Ty repetitive elements and arise from nonallelic homologous recombination between the re-replicated repetitive elements.

  8. Sauce for the Goose?

    CREDIT: SARAH R. PRYKE

    Extra-pair copulations benefit males by increasing their potential number of offspring. However, whether they have adaptive significance for females has long been debated. Pryke et al. (p. 964), provided female finches four different scenarios to test when extra-pair copulations occurred and measured the paternity of the resulting offspring. By varying the potential fathers from a supposedly adaptive, compatible male through a neutral context, to a supposedly maladaptive scenario, females biased paternity to the extra-pair male when his contribution was adaptive and away from the extra-pair male when his contribution was maladaptive. Thus, despite bias in copulation frequency in favor of the social pair, cryptic female choice may show a fertilization bias toward compatible genes.

  9. Chloroplast Division Machinery

    The machinery for photosynthesis, which captures the Sun's energy to generate carbohydrates, generally resides in subcellular chloroplasts of plant cells. Chloroplasts must divide as the plant cell divides, but to do so requires their own plastid dividing machinery. Yoshida et al. (p. 949: see the cover) have now analyzed the plastid dividing machinery of the single-celled alga Cyanidioschyzon merolae, whose cells each contain a single chloroplast. The plastid dividing machinery is made up of polysaccharide chains and the proteins that make them, which together generate a ring that constricts to physically divide the chloroplast.

  10. Fungal Defenses

    One of the major driving forces of evolution is the constant arms race between plants and animals and the microbial pathogens that infect them. The fungus Cladosporium fulvum causes leaf mold on tomato plants. One of the ways tomato plants sense infections by C. fulvum is by detecting chitin, a component of fungal cell walls. In response, the fungus has evolved strategies to evade detection. De Jonge et al. (p. 953) have now identified one such mechanism in C. fulvum, mediated by the effector protein Ecp6. Secreted Ecp6 is able to bind to chitin oligosaccharides that are released upon degradation of the fungal cell wall and sequester them so that they are not detected by tomato chitin receptors. Proteins with domain structure similar to Ecp6 are conserved throughout the fungal kingdom, which suggests that chitin sequestration may represent a general mechanism used by fungi to evade immune detection.

  11. Line Up for Movement

    CREDIT: G.W. GANT LUXTON, ZSOLT LAZAR, GREGG G. GUNDERSEN

    The nuclei of animal cells can move to specific locations and help to polarize migrating and differentiating cells. Luxton et al. (p. 956; see the Perspective by Starr) found that linear arrays of nuclear membrane proteins assembled on, and moved with, actin cables toward the rear of the cell during nuclear movement in polarizing fibroblasts. Interfering with the components of these linear arrays prevented nuclear movement and centrosome reorientation. Thus, nuclear membrane proteins assemble into actin-dependent arrays during force transduction.

  12. Antidepressant Action of Ketamine

    In contrast to the weeks or months of treatment required for standard antidepressant medication, ketamine administration produces an antidepressant response within 4 to 6 hours in depressed patients. What lies behind the rapid actions of ketamine? Li et al. (p. 959; see the Perspective by Cryan and O'Leary) found that ketamine administration resulted in fast activation of mammalian target of rapamycin (mTOR) signaling and increased levels of synaptic proteins in the rat prefrontal cortex. Ketamine rapidly increased the density and function of the dendritic spines of layer V pyramidal neurons in the prefrontal cortex. Thus, the behavioral actions of ketamine in models of depression and antidepressant response are dependent on mTOR signaling.

  13. Zebrafish Development in 3D

    Vertebrate development has classically been characterized qualitatively, but—by combining expertise in physics, mathematics, and biology—Olivier et al. (p. 967) used label-free conformal nonlinear time-lapse microscopy and image analysis to calculate the spatiotemporal cell lineage of zebrafish embryos throughout their first 10 division cycles. The work reconstructs complete lineage trees, annotated with cell-shape measurements, and allows for visualization with interactive tools.

  14. PTIP in Immunoglobulin Switching

    One of the hallmarks of humoral immunity is the ability of immunoglobulins (Ig) to undergo class switch recombination (CSR). Through genetic recombining of the Ig heavy chain, Igs maintain their antigen specificity but gain the ability to interact with different cell surface receptors required for successful pathogen clearance. CSR requires transcription at the Ig heavy chain locus to initiate genetic rearrangement. Changes in chromatin accessibility are thought to promote CSR-associated transcription. Daniel et al. (p. 917, published online 29 July; see the Perspective by Singh and Demarco) now show that trimethylation of histone 3 at lysine 4 (H3K4me3) controls the accessibility of the Ig heavy chain locus to CSR and that PTIP (Pax interaction with transcription-activation domain protein-1), a component of the histone methylase complex, is required for this modification. Mouse PTIP-deficient B cells exhibited impaired CSR. PTIP was required both for the recruitment RNA Polymerase II and for subsequent chromatin remodeling, including histone acetylation, which occurs during CSR. Largely independent of its function in transcription initiation in CSR, PTIP also associated with double-stranded DNA breaks during CSR and promoted genome stability. These dual functions of PTIP may be important for the precise coordination of chromatin accessibility and recombination required during CSR.

  15. Legionella Hijacks Rab

    Legionella pneumophila can infect eukaryotic cells and takes up residence within intracellular vacuoles, where it multiplies. In order to produce and maintain this intracellular niche, the pathogen must manipulate membrane trafficking within the host cell. Now, Müller et al. (p. 946, published online 22 July) describe the ability of Legionella pneumophila to manipulate vesicular trafficking by the covalent modification of the small guanosine triphosphatase (GTPase) Rab1, which normally regulates the transport of endoplasmic reticulum–derived vesicles in eukaryotic cells. The Legionella protein DrrA is released into the cytosol of infected cells, where it specifically AMPylates a tyrosine residue of one of the regulating regions of Rab1. The modification renders the Rab protein inaccessible to GTPase-activating proteins and thus locks it in its active guanosine triphosphate–bound state.