This Week in Science

Science  23 Sep 2011:
Vol. 333, Issue 6050, pp. 1676
  1. Promiscuous Sisters

    CREDITS: LUKASZ MICHALCZYK, IAN BARR, ANNA MILLARD AND MATTHEW GAGE

    In populations founded by relatively few individuals, a genetic bottleneck may result in lowered genetic diversity and inbreeding. One way females may avoid the deleterious effects of inbreeding is by mating with more males to increase the genetic diversity among their offspring. Michalczyk et al. (p. 1739) investigated the fitness of singly mated and multiply mated red flour beetles after a bottleneck and found that females who mated with more males showed higher fitness than females that were singly mated. Furthermore, inbred females exposed to 10 males mated with more males, and spent more time copulating than non-inbred females. Thus, inbreeding may explain the evolution of female promiscuity in most species.

  2. It Usually Takes Two to Tango

    In 1992, a system of two planets was detected around a millisecond pulsar, a neutron star that spins around hundreds of times per second. Since then, hundreds of planets have been detected around normal stars, but not around millisecond pulsars. Bailes et al. (p. 1717, published online 25 August; see the Perspective by Rasio) report the detection of a planet with a mass similar to that of Jupiter's around the millisecond pulsar PSR J1719-1438. This planet is much denser than Jupiter and is likely to be made of carbon and oxygen. This detection may help explain why some millisecond pulsars are solitary, while most are in binary systems, with other stars.

  3. Spatial Bounds on Coherent States

    Imaging techniques often make a tradeoff between time and spatial resolution. For example, methods that use ultrafast laser pulses are often limited by diffraction limits. Aeschlimann et al. (p. 1723, published online 11 August) overcame diffraction limits for imaging coherent electronic states in metal samples by using photoelectrons to create images. Four-wave coherent mixing enabled observation of coherent states with different spatial features in a nanostructured silver film.

  4. Popping Open

    CREDIT: FORTERRE AND DUMAIS

    Plant seed pods can change shape in response to environmental factors like humidity. The absorption of water, for example, can cause nonuniform swelling of the pod tissue. This swelling causes the seed pod to twist as stresses accumulate and can cause it to burst open. Armon et al. (p. 1726; see the Perspective by Forterre and Dumais) asked whether the twisting motion is dependent on the material properties of the pod, or the way that it is assembled. When latex strips were stretched uniaxially and then glued together, depending on the difference in orientation of the two layers, a rich behavior was observed in the twisted structures driven by the tendency of each of the layers to bend in different ways.

  5. From Ductile to Brittle

    When some ductile (that is, stretchable) metals come into contact with certain liquid metals, they can become brittle. Luo et al. (p. 1730) used aberration-corrected scanning transmission electron microscopy to investigate the liquid phase encroachment of nickel by bismuth atoms. The bismuth atoms penetrated along nickel grain boundaries and formed single-layer complexes on either side of the boundary. The bismuth atoms then bonded to the nickel much more strongly than to each other, causing the nickel to become brittle.

  6. Hydrogen Liberation

    Sometimes the Sun doesn't shine, and the wind doesn't blow. Solar and wind-powered electricity generation thus require a secondary means of storing energy for use in those dark, still periods. One commonly promoted scalable proposal is to embed the energy in electrochemically synthesized molecular hydrogen; however, the hydrogen itself then needs to be stored. Boddien et al. (p. 1733; see the Perspective by Ott) have raised the prospect of using formic acid for this purpose. An iron catalyst was able to accelerate release of hydrogen from this molecule under environmentally benign conditions.

  7. Big Footprints

    Megacities are huge sources of many types of air pollution, of which nitrogen oxides (NOx) are some of the most important in terms of their effects on air quality, public health, tropospheric chemistry, and climate. However, it can be difficult to determine emission inventories in many of these large urban areas, particularly in developing countries where ground-based measurements are limited. Beirle et al. (p. 1737) present a method to measure both NOx emissions and lifetimes using instruments aboard satellites, by analyzing downwind patterns of NO2 concentration. This should allow the types of results obtained for well-studied cities to be extended to those less well characterized by traditional methods.

  8. Orchid Bees and Their Orchids

    Many plant-insect interactions show a high degree of specialization. Ramírez et al. (p. 1742) investigated the relationships between neotropical orchids and their pollinators—euglossine bees—using molecular phylogenies, fossil dating, and analyses of which bees are pollinating which flowers by collecting orchid pollen attached to the bees. Whereas diversification of orchids tracked that of the bees, bee evolution showed much less dependency on the orchids.

  9. Hold and Release

    The hepatitis C virus encodes an RNA helicase, NS3, which is essential for viral RNA replication and is a model for the large family of non-hexameric helicases. Cheng et al. (p. 1746) used optical tweezers to follow the unwinding of an RNA hairpin by NS3 at single-nucleotide resolution. Consistent with previous models, binding of one adenosine triphosphate catalyzed unzipping of one base pair. However, the helicase could sequester nucleotides from either nascent single RNA strand with their release decoupled from base-pair opening. The resulting model is consistent with disparate previous observations and could provide a mechanism for the enzyme to coordinate the speed of movement along nucleic acids.

  10. Not So Competitive After All

    There is a long-standing belief that wild herbivores and cattle cannot coexist because of competition for food, which has often meant less space for wild species as more space for cattle is needed. Now Odadi et al. (p. 1753; see the Perspective by du Toit) challenge this assumption. While competition reduced cattle performance in the dry season, native herbivores actually increased cattle weight gain in the wet season. Furthermore, the facilitation that occurred during the wet season nearly compensated for any loss due to competition during the dry season. Native herbivores may eat plants that are less palatable to cattle and thus provide easier access to more “cattle-friendly” plants. Thus, coexistence between livestock and wildlife may perhaps even be beneficial.

  11. Arranging Nucleosomes

    Nucleosomes, the core constituents of chromatin, show a stereotypical alignment with respect to transcribed genes. Specific DNA sequences have been proposed to contribute to the establishment of this pattern, yet purified histones assembled onto DNA in vitro only partially recapitulate in vivo patterns. Gkikopoulos et al. (p. 1758) show that loss of any one of a group of three chromatin remodeling enzymes had a relatively minor effect. However, removal of all three enzymes caused a dramatic loss of nucleosome organization with respect to the transcription start site and the coding regions of genes, together with a global reduction in the regular spacing of nucleosome arrays and a loss of chromatin compaction.

  12. Sperm-Egg Binding

    CREDIT: PANG ET AL.

    Human fertilization is initiated when sperm bind to the outer covering of the egg, known as the zona pellucida. Sialyl-Lewisx is a universal carbohydrate ligand for selectins, a family of cell adhesion proteins that mediate the binding of immune and inflammatory cells to stimulated endothelium. Pang et al. (p. 1761, published online 18 August; see the Perspective by Wassarman) isolated unfertilized human oocytes and used ultra-sensitive mass spectrometry to identify chemical compounds attached to the human zona pellucida. Sialyl-Lewisx was abundantly expressed on the zona pellucida and mediated sperm binding to the egg.

  13. Mobile Resistance Fronts

    Although we understand quite a lot about the evolution of antibiotic resistance, we know relatively little about its dynamics. Zhang et al. (p. 1764; see the Perspective by Frisch and Rosenberg) designed a microfluidic chip in which to test the influence of gradients of antibiotic concentration that microorganisms might encounter in the heterogeneous range of niches in soil or within an animal's body. In a concentration gradient of ciprofloxacin, within hours of exposure, motile Escherichia coli were selected that were resistant to the antibiotic. No such resistance was seen when the bacteria were grown in stagnant culture conditions. Genome sequencing of resistant bacteria repeatedly revealed four single-nucleotide polymorphisms in bacterial gyrase, a ribose transporter component, and a drug efflux pump master regulator.

  14. Open Minds

    A disposition toward believing that another person's beliefs are malleable can be conducive to achieving a negotiated settlement in situations of interpersonal conflict. Does a similar disposition toward groups exist, and if so, might it influence one's willingness to negotiate? Halperin et al. (p. 1767, published online 25 August) explored this possibility in the context of conflict in the Middle East by surveying 500 Israeli Jews and found a correlation between a belief in malleability and willingness to compromise. In experimental studies with both Israeli Jews and Palestinians, the belief in malleable versus fixed natures was manipulated, and greater support for a negotiated settlement was measured in subjects whose beliefs had been tilted toward the malleable end of the spectrum.

  15. Nonlinear Plasmons

    Surface plasmons are light-induced electronic excitations tightly confined to the surface of a metal. Much work on surface plasmons has focused on the linear regime, with the demonstration of passive plasmonic devices such as waveguides. Cai et al. (p. 1720) now demonstrate the ability to produce and tune second-harmonic generation of light from a plasmonic nanocavity filled with a nonlinear medium. Combining the subwavelength confinement and dual electrical and optical functionality with nonlinear effects should prove useful for applications in active optoelectronic circuitry and networks.

  16. Getting Over the Hump

    Primary productivity has been considered to represent a dominant control of plant species richness, generating a hump-shaped unimodal relationship. But meta-analyses of the literature suggest that data are somewhat inconsistent. Adler et al. (p. 1750; see the Perspective by Willig) use data from a globally distributed research network to throw further light on this debate. The Nutrient Network, an international consortium of ecologists, moved beyond meta-analysis, relying instead on standardized sampling conducted at 47 grassland sites on five continents. No consistent relationship was observed between productivity and species richness either within sites, across regions, or across the globe. This empirical result should spur ecologists to use more mechanistic approaches to understand diversity.

  17. Assessing Variation

    One of the most striking patterns in the natural world is that species richness in local ecological communities declines with increasing latitude and elevation. Ecologists have long focused on β diversity (a measure of compositional differences among communities) as a means of understanding this observation. However, β diversity can be influenced both by variation in local ecological processes and by differences in the size of species pools (γ diversity). Kraft et al. (p. 1755; see the cover) introduce a measure that more accurately reflects variation in the strength of local ecological processes and apply it to a forest inventory data set spanning 197 locations and more than 100° of latitude, as well as a smaller data set spanning a 2250-meter gradient in Ecuador. The data sets, at first pass, show the widely reported pattern for many organisms where β-diversity measures increased toward the tropics and toward low elevations. However, these trends are erased once corrected for differences in γ diversity, such that there is no systematic trend with latitude or elevation. Thus, variation in biogeographical and regional processes appear to set the size of the species pool, and the combined influence of local processes acts consistently across these gradients to produce the patterns of species turnover that are observed across the planet.

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