Editors' Choice

Science  30 Sep 2011:
Vol. 333, Issue 6051, pp. 1802
  1. Oceanography

    An Increase in Productivity

    1. H. Jesse Smith
    CREDIT: IMAGE ENHANCED (PHOTO SOURCE) © LEE THOMAS/ALAMY

    One of the most dramatic consequences of recent global warming has been the rapid and large reduction of summertime sea ice in the Arctic Ocean. The seasonal disappearance of so much ice exposed huge expanses of the ocean that were previously covered, and creates the possibility for greater marine productivity in the regions newly exposed to sunlight. Solar irradiation alone is not sufficient to cause increased biological productivity, of course: There also must be an adequate supply of nutrients to fuel it. Tremblay et al. used a combination of in situ and remote observations to show that new productivity in the Arctic coastal Beaufort Sea has increased because of climate warming there, and that the nutrients needed to fuel that new production are derived from deep waters that have upwelled in response to the stronger winds and reductions in summertime sea-ice coverage caused by climate change. Further increases in the amounts of new production can be expected as increasingly deep and frequent seaward retreat of the ice pack and stronger winds occur.

    Geophys. Res. Lett. 38, L18604 (2011).

  2. Development

    In with the New

    1. Valda Vinson
    CREDIT: CHAUDHARI ET AL., PNAS 108, 10.1073/PNAS.1112288108 © 2011 NAS, USA

    Growing insects must periodically shed their hard exoskeletons. At the beginning of the molting process, epidermal cells secrete a thin nonchitinous envelope and then secrete new cuticular chitin underneath this envelope. The envelope was believed to protect the new cuticle from molting-fluid chitinase enzymes that digest the inner layer of the old cuticle before shedding of the hard outer layers. Chaudhari et al. determined the localization of the molting-fluid chitinase in the red flour beetle, Tribolium castaneum, and found surprisingly that it colocalized with chitin in both the old and newly synthesized cuticle. With the goal of discovering the source of the new cuticle stability, they examined the gene knickkopf (Knk), which is required for the laminar organization of the cuticle during embryonic development in Drosophila melanogaster. Knockdown of the T. castaneum Knk ortholog, TcKnk, led to molting defects at all developmental stages, and TcKnk protein colocalized with chitin in the new cuticle. Knockdown of TcKnk RNAi led to loss of total chitin. Although chitin levels could be restored by knockdown of two chitinase genes, this did not restore proper cuticular organization, suggesting that TcKnk organizes chitin into laminae to protect it from degradation. This mechanism of cuticle protection may be conserved in all chitinous invertebrates.

    Proc. Natl. Acad. Sci. U.S.A. 108, 10.1073/pnas.1112288108 (2011).

  3. Evolution

    Sparrow Speciation

    1. Laura M. Zahn

    Homoploid hybrid speciation creates new species that arise from interbreeding between two species without a chromosomal doubling. Although long thought to be a rare mode of speciation in animals, a growing number of examples have been reported. Hermansen et al. have now identified a likely homoploid hybrid, the Italian sparrow, which is intermediate in phenotype between Spanish sparrows and house sparrows. Microsatellite loci and mitochondrial DNA sequences of sampled sparrows showed that Italian sparrows are genetically intermediate between the other two species. Italian and Spanish sparrows behave like distinct species in an area of geographical overlap; however, Italian and house sparrows do form hybrid zones. A genetic analysis of these species by Elgvin et al. found evidence that the Italian sparrow is genetically intermediate between its putative progenitors. Furthermore, they found that the sex (Z) chromosome has probably played an important role in this case of speciation.

    Mol. Ecol. 20, 3812; 3823 (2011).

  4. Molecular Biology

    Choosing Your 3' End

    1. Guy Riddihough

    Many eukaryotic genes contain noncoding sequences—introns—that must be removed before translation. The 5' slice site is determined by base-pairing interactions with the U1 snRNA of the spliceosome, a large ribonuclear-protein complex that catalyses the removal of introns. How the spliceosome finds the other (3') end of the intron—nominally marked by no more than the dinucleotide sequence AG—is less clear.

    While studying a budding yeast gene, Meyer et al. found that the potential destabilization of an RNA secondary structure downstream of the intron branch point (a sequence critical in the splicing reaction) had an adverse effect on splicing. Analyzing the RNA folding potential of sequences between the intron branch point and the 3' splice site for 282 yeast introns revealed that a substantial fraction had the potential to form secondary structures. In vivo, the RNA secondary structure functioned to bring distant 3' splice sites within a specific distance window, which is neither too close to nor too far from the intron branch point, thus allowing effective 3' splicing to occur and implying that the spliceosome has a limited “reach.” For another gene, choice of the 3' splice site is infl uenced by temperature, which is sensed through the thermal stability of the RNA secondary structure, implying that such structures can function in a regulatory capacity.

    Mol. Cell 43, 1033 (2011).

  5. Planetary Science

    Predicting Rain on Titan

    1. Maria Cruz
    CREDIT: NASA/JPL/SPACE SCIENCE INSTITUTE

    Saturn's largest moon, Titan, has an active weather cycle, with methane clouds and liquid methane rain. In September and October last year, a few distinctive clouds were detected by the Cassini spacecraft: arrow-shaped formations near the equator and sheared streaks at mid-southern latitudes. Mitchell et al. found similar features at similar latitudes in their three-dimensional model of the general circulation of Titan's atmosphere. The calculations reveal that the features are not isolated occurrences but are instead shaped by large-scale atmospheric waves that are similar to those that account for intraseasonal variability in Earth's tropics. The model predicts several centimeters of precipitation over regions on scales exceeding 1000 km associated with the clouds. Such amounts could cause Titan's low-latitude deserts to be shaped by seasonal fluvial erosion. Previous observations have shown sudden surface changes near Titan's equator after the arrow-shaped cloud outburst that was interpreted as signs of rainfall. Precipitation from the mid-southern latitude streaks has not been detected by observations, which suggests that the model overestimates mid-latitude precipitation during the current season.

    Nat. Geosci. 4, 10.1038/NGEO1219 (2011).

  6. Immunology

    Killer Cells for Killer Bacteria

    1. Kristen L. Mueller

    Natural killer T (NKT) cells are an unusual type of immune cell because they straddle the line between the innate and adaptive immune systems. They express an antigen receptor and require antigen presentation by a (nonclassical) major histocompatability complex molecule, but unlike most lymphocytes, they mount rapid responses to infection. Although shown to participate in immune responses to a wide variety of infections, a ligand from a highly pathogenic organism, which might explain the evolutionary conservation of NKT cells, and its invariant antigen receptor in particular, has not been identified. Kinjo et al. now report that NKT cells from both mice and humans are able to recognize diacylglycerol- containing glycolipids from Streptococcus pneumonia and group B Streptococcus, both of which can cause fatal infections in humans. The response to these glycolipids was dependent on the presence of vaccenic acid, which is expressed at low levels in mammalian cells and therefore might help NKT cells distinguish these microbes as foreign.

    Nat. Immunol. 12, 996 (2011).

  7. Materials Science

    Partners in Healing

    1. Marc S. Lavine

    The complexity of the interactions between growth factors, receptors, and the extracellular matrix makes it hard to replicate in vivo—like conditions when designing a clinical method for the delivery of growth factors. Some progress has been made by using multistaged controlled release methods or by covalently linking the growth factor to the matrix material, but, in these cases, the growth factors do not benefit from complex clustering effects that are needed to get the right cells to migrate, proliferate, and differentiate. Martino et al. devised a peptide that combined fibronectin fragments designed to cause integrin binding with fragments designed to cause growth factor binding. A third section contained coagulation factor XIIIa, which causes covalent immobilization of the fragments to the relevant part of a fibrin matrix. In comparison tests, with only one of the fibronectin fragments or with the two mixed together but not covalently linked, a synergistic healing effect was observed only when the two fragments were in close proximity on the same polypeptide chain. Enhanced regenerative healing was also observed in vivo when the engineered peptide was tested in a diabetic mouse model for chronic wounds and in a rat model for critical-size bone defects.

    Sci. Transl. Med. 3, 100ra89 (2011).

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