This Week in Science

Science  21 Nov 2008:
Vol. 322, Issue 5905, pp. 1159
  1. Martian Reservoirs


    Most of the visible water on Mars is locked up in its polar caps. There have been hints and evidence of some water flow at lower latitudes, but evidence for large reservoirs has been lacking. A series of large lobate landforms have been identified in several mid-latitude areas, and one hypothesis is that these may be covered glaciers or ice-filled rock piles. Holt et al. (p. 1235) have used the radar on the Mars Reconnaissance Orbiter to probe two of these deposits. The regions are indeed made predominantly of water ice covered with recent debris that probably formed during a previous climatic episode when Mars's orbit was more inclined. If these two deposits are representative, these landforms contain the largest volume of Martian water outside the poles.

  2. Visions from Afar

    Events distant in time and objects distant in space are processed in psychologically similar ways. Liberman and Trope (p. 1201) review the evidence describing commonalities among faraway objects, far-in-the-future events, and imagined (versus real) happenings. How do the various kinds of distance induce these events or objects to become more strongly associated with each other, as in the phrase “long ago, in a faraway place”? More distal objects are represented in terms of more abstract characteristics, which leads to grouping distant objects into fewer and more general categories than objects closer in time or space. In everyday life, forming high-level or abstract constructions that incorporate perceptions and beliefs may increase the value of future events and facilitate self-control and the delay of gratification.

  3. Massive Stars and MAGIC Crab Nebula

    Energetic pulsars may produce gamma rays. Gamma rays have been observed emanating from several locations, but many specific sources remain unidentified (see the Perspective by Bignami). Abdo et al. (p. 1218, published online 16 October) report that the recently launched Fermi Gamma-Ray Space Telescope has detected a gamma-ray pulsar and tied the emissions to a neutron star. Many other unidentified gamma-ray sources may have a similar origin, implying a greater number of massive stars than thought. The Crab pulsar is a neutron star remaining following a supernova in 1054. Gamma rays from it have been observed from space to energies of about 5 GeV. Ground-based observations have failed to detect pulsed emissions at energies above their usual minimum threshold of about 60 GeV. The threshold of the MAGIC Cherenkov telescope has now been lowered to 25 GeV, and The MAGIC Collaboration (p. 1221, published online 16 October) has been able to detect pulsed emissions from the Crab pulsar. The emissions were observed at a flux lower than expected from extrapolation of the lower energy data, which challenges models for the location and source of the emission in the outer parts of the neutron star.

  4. GPCR Structural Diversity

    G protein-coupled receptors (GPCRs) respond to diverse extracellular signals to activate different signaling pathways. They are important pharmacological targets, but, so far, structural information that might aid drug design is limited to the rhodopsin and adrenergic receptors, and determinants of ligand binding specificity among different GPCRs are unclear. Now Jaakola et al. (p. 1211, published online 2 October) report the structure of the human A2A adenosine receptor, a receptor that is blocked by caffeine, in complex with an antagonist. The binding pocket is distinct from that in the rhodopsin and adrenergic receptors, suggesting that, rather than a conserved binding pocket with specific amino acid side chains determining specificity, the GPCR binding pocket itself can vary in position and orientation.

  5. Light from Below


    A strong oscillating laser field can pull electrons from the outer layer of an atom or molecule and then send them careening back, releasing high-energy photons upon collision. This process underlies recent techniques for generating the shortest light pulses, with durations below 10−15 second. Two studies now show that more tightly bound electrons can also undergo this ricochet process in strong fields (see the Perspective by Doumy and DiMauro). Li et al. (p. 1207; published online 30 October) observed distinct emission intensities at different stages in the vibrational period of a vibrating N2O4 molecule. Similarly, McFarland et al. (p. 1232, published online 30 October) found distinct spectral signatures depending on the spatial orientation of N2 molecules relative to the polarization of the applied laser field. In both cases, the results are attributed to involvement of electrons in both the highest and second-highest energy-occupied orbitals. The findings open the door to wide-ranging probes of electronic coupling to molecular vibrations and rotations on time scales previously too fast to monitor.

  6. Promoting Polyandry

    Females of almost all animals mate with more than one male, but the evolutionary drivers of this behavior, known as polyandry, remain enigmatic. Price et al. (p. 1241) show that selfish genetic elements, which affect male fertility in sperm relative to those lacking such elements, are associated with increased rates of remating. These data document the potential impact of selfish genetic elements within populations and provide a general explanation for the evolution of polyandry.

  7. Modeling Subcellular Morphogenesis

    Within cells during mitosis, the subcellular architecture, particularly the arrangement of cellular microtubules, is substantially remodeled to promote the faithful segregation of chromosomes to daughter cells. Athale et al. (p. 1243, published online 23 October) report how enzymatic networks self-organize around chromosomes to affect the symmetry of the stochastic dynamics of microtubules nucleated by centrosomes in an open cell cytoplasm (frog egg extracts). A theoretical model shows how a true reaction diffusion mechanism can generate complex steady-state patterns of microtubules from enzymatic reactions. The coupling of a stochastic system (microtubule dynamics) and a deterministic regulatory gradient may thus provide a generic mechanism for subcellular morphogenesis.

  8. Dinospore Control


    Harmful algal blooms, or red tides, are of increasing concern in warm coastal waters polluted by nitrogen runoff from agricultural land. The blooms are commonly produced by dinoflagellate protozoans, some species of which can produce dangerous toxins and are a major problem for the shellfish industry. During the 1980s, red tides swept the estuaries of northern France, but in the past decade these have largely disappeared. Chambouvet et al. (p. 1254) have been monitoring the algal succession in one of these estuaries and observed that during each summer, waves of different dinoflagellate species are successively knocked out by waves of distinct and highly specific parasitic species. Phylogenetic analysis indicated that these host and parasite associations have been established only in the past decade in these estuaries, but now persist in the environment throughout the year and help to suppress the formation of the toxic blooms.

  9. Wnts and the Brain Vasculature

    Whereas all tissues in our bodies share a vasculature, different organs display distinct modes of vascularization during development, and the vessels that form are not alike. For example, blood vessels in the brain form a tight seal, the blood-brain barrier, enabling normal brain function but also restricting access of potentially therapeutic drugs. Now Stenman et al. (p. 1247; see the Perspective by Lammert) provide in vivo evidence that neural Wnt-signals, Wnt7a and Wnt7b, promote both the formation and differentiation of central nervous system vasculature by direct signaling to endothelial cells through a canonical Wnt pathway.

  10. A Drive Along the Metabolic Information Highway

    Metabolic regulation in mammals requires communication between multiple organs and tissues. Studying mouse models, Imai et al. (p. 1250) found that obesity stimulates pancreatic β cell proliferation through a neural-mediated relay of metabolic signals from the liver to the pancreas. Obesity-induced activation of extracellular regulated kinase (ERK) signaling in the liver was identified as a key component of this metabolic relay. In mouse models of insulin-deficient (type 1) diabetes, liver-specific activation of ERK signaling resulted in an increase in β cell mass and normalization of serum glucose levels. Thus, interorgan metabolic communication systems may serve as valuable therapeutic targets for diabetes.

  11. No Escape for Insect Pathogens

    Insects respond to infection by an immediate nonspecific recruitment of phagocytic blood cells, followed by the generation of small peptides that act as slightly more specific antibiotics. Even though the production of antimicrobial peptides can persist for weeks, insects rarely, if ever, suffer the problems of antibiotic resistance that we see in clinical settings. Haine et al. (p. 1257; see the Perspective by Schneider and Chambers) propose that the long-term peptide response in insects mops up any bacteria that have resisted the original cellular response and thereby prevents the emergence of resistant organisms. Thus, ecological analysis of the role of antibiotics in natural settings could prompt a reappraisal of therapeutic antibiotic use.

  12. Long Live the Standard Model

    The Standard Model of particle physics was developed to explain the observed families of particles and their interactions that make up our visible universe—the protons and neutrons making up the atoms, the constituent particles of quarks and other baryonic matter. Lattice Quantum Chromodynamics is the numerical approach used to determine the masses, but it is so computationally intensive that shortcuts and approximations need to be taken, which introduce inevitable errors in the calculations. Dürr et al. (p. 1224; see the Perspective by Kronfeld) present one of the largest computational efforts to date to calculate particle masses and find good agreement with experimental results, confirming that the Standard Model is the correct description of particle mass.

  13. Accelerated Imaging

    Electron microscopy is widely used to obtain molecular-scale images of materials and biological structures. However, space-charge considerations in the electron pulses impinging on the sample have limited the simultaneously attainable temporal resolution. Barwick et al. (p. 1227) have surmounted this problem by constructing an electron microscope that images substrates using one electron at a time. By generating the free electrons with an ultrafast optical pulse, they can measure real space femtosecond structural dynamics in gold and graphite films with Angstrom spatial resolution. The study uncovers distinct responses to thermal stress (induced by a preceding laser pulse) in different regions of the gold substrate, whereas the graphite undergoes a more coherent response from which a characteristic Young's modulus at the atomic scale can be extracted.

  14. Mussel-ing in on Evolution

    The identification of fossil evolutionary trajectories can help assess macroevolutionary processes over geologic time. However, such analyses require the reconciliation of patterns and rates across widely varying temporal scales and among disparate taxa. Appropriate methods are needed to analyze trends and samples that are well resolved both stratigraphically and phylogenetically and that can capture and characterize appropriate morphological variables. Grey et al. (p. 1238, published online 23 October) address these concerns and the possibility that the evolutionary mode exhibited by a taxon might vary in different parts of its geographic range. Exploring the modes of evolutionary change in the fossil record of ancient relatives of the mussel revealed how patterns of evolution vary with geography.

  15. University Challenge

    In an effort to study how scientific research is changing, Jones et al. (p. 1259, published online 9 October) analyzed the collaboration arrangements that produced over four million papers over a 30-year period. Within the U.S., team science has become increasingly composed of coauthors located at different universities, and these “teams” produce higher-impact work than comparable colocated teams or solo scientists. Teams located at different universities were the fastest-growing type of authorship structure by far. Such teams are increasingly composed exclusively of top-tier universities, rather than universities of different status rankings. It seems that through multiuniversity collaborations, elite universities are more exclusively interconnected and are increasingly becoming the source of highly cited scientific ideas.