Editors' Choice

Science  28 Aug 2009:
Vol. 325, Issue 5944, pp. 1048
  1. Geology

    Not of a Certain Age

    1. Brooks Hanson

    At present, there are 174 characterized impact structures on Earth, ranging from the largest—the 2-billion-year-old Vredefort that stretches 300 km across South Africa—to about 20 or so structures smaller than the 1-km-diameter Barringer crater in Arizona; most of these are less than 1 million years old. Although these structures represent only the preserved fraction of the overall series of asteroid impacts on Earth, their age-size relationships might aid in assessing the impact flux or the relation of impacts to events such as extinctions (for example, that marking the Cretaceous-Paleogene boundary). As pointed out by Jourdan et al., however, many of the craters are in fact poorly dated. About half have only approximate ages based on stratigraphy. Moreover, although 25 of the craters have ages reported with uncertainties of less than 2%, many of these reports, on investigation, contain generous or misleading estimates; in the authors' assessment, only 11 of the 25 craters have been assigned accurate ages. Thus, it is difficult to infer much from the current crater data about periodicities or changes in impacts through time.

    Earth Planet. Sci. Lett. 10.1016/j.epsl.2009.07.009 (2009).

  2. Geochemistry

    Heavy Down Below

    1. Nicholas S. Wigginton

    Many chemical and biological processes skew, or fractionate, atomic isotope ratios in products relative to their initial relative abundances. For example, most of the iron in Earth's crust exists as 56Fe, but iron that undergoes reduction/oxidation (redox) reactions is on average slightly lighter (that is, enriched in 54Fe—even by just a few parts per thousand).

    To determine the extent of contemporary Fe redox cycling in different marine settings, Homoky et al. compared Fe isotope signatures from pore water collected on a shallow continental shelf to those from pore water associated with sediments as deep as 4222 m below sea level, at the bottom of the Southern Ocean. The significant fractionation of 56Fe isotopes in shallow sediments indicated extensive Fe reduction by active communities of bacteria; in contrast, the Fe isotopes in the deep sediments showed almost no fractionation, thereby suggesting that deep marine sediments undergo very little redox cycling. Although limiting organic matter is expected to restrict the activity of iron-reducing bacteria at depth, the degree of fractionation is also dependent upon the reactivity of Fe-oxide minerals. Defining the controls on stable isotope fractionation in sediments will not only aid in understanding modern biogeochemical processes, but will also allow for a more accurate interpretation of Earth's history based on isotopic signatures found in the rock record.

    Geology 37, 751 (2009).

  3. Cell Biology

    Size Matters

    1. Stella M. Hurtley
    CREDIT: DINARINA ET AL., CELL 138, 502 (2009)

    The mitotic spindle, which pulls apart the duplicated chromosomes during cell division in animal and plant cells, is composed of microtubules that have been found to self-assemble in the presence of chromosomes. Dinarina et al. have developed a cellfree system in order to monitor the effects of chromosome size and shape on spindle morphology. Microscopic chromatin-coated beads were arrayed on a glass slide to which cytoplasmic extracts from Xenopus laevis were applied, which made it feasible to look at many individual spindles assembling simultaneously. When chromatin (blue) was deposited as circular spots 15 µm in diameter, typical bipolar spindles (red) were observed, and larger spots tended to produce spindles with additional poles. In contrast, for chromatin patterned in a chromosome-like rectangle with dimensions of 6 x 18 µm, a typical bipolar spindle orthogonal to the long axis of the chromosome was seen. Longer chromatin arrangements generated multipolar, disorganized spindles, which also generally crossed the long axis of the chromosome, and thicker arrangements generated multiple half-spindles along one side of the patch of chromatin. Thus, the size and shape of chromosomes can define microtubule spindle shape and orientation.

    Cell 138, 502 (2009).

  4. Plant Science

    Adding Oregano to Corn

    1. Laura M. Zahn

    Many plants emit signals to augment their defenses against attack by insects; some of these signals are known to work by attracting predators of the attackers. Maize roots under threat from larvae of the western corn rootworm normally emit the terpene caryophyllene, which serves to mobilize nematodes that then kill these larvae. However, most cultivated maize has lost the capacity to produce this terpene.

    Degenhardt et al. have engineered transgenic maize plants that carry a caryophyllene synthase gene from oregano. This manipulation restored the production of this compound, with the consequence that nematodes reduced the number of rootworms by more than half, resulting in much less root damage. This finding confirms that nematodes can be recruited effectively by caryophyllene and provides the basis for a pest biocontrol strategy to improve cultivated plants.

    Proc. Natl. Acad. Sci. U.S.A. 106, 13213 (2009).

  5. Biomedicine

    Blood Relations

    1. Helen Pickersgill
    CREDIT: AMUNDADOTTIR ET AL., NAT. GENET. 41, 10.1038/NG.429 (2009)

    The mortality rate for pancreatic cancer approaches the rate of incidence, in part because early stages of the disease are asymptomatic and diagnosis at later stages remains difficult. As with many cancers, a family history of the disease is a known risk factor; identification of genetic variants associated with pancreatic cancer might improve early detection. Amundadottir et al. have performed a genomewide association study to identify common genetic variants found specifically in patients suffering from pancreatic cancer. In a Manhattan plot, they found within the first intron of the ABO blood group gene variants that were associated with susceptibility to the disease. The ABO gene encodes a glycosyltransferase that modifies a cell surface antigen. The three alleles (A, B, and O) of this gene determine blood type, which was first associated with gastric and pancreatic cancer more than half a century ago. What pathways might mediate this association? Variants within the same intron of the ABO gene have also been linked with circulating levels of the cytokine tumor necrosis factor α, which is involved in the inflammatory response. Other possible factors contributing to tumorigenesis include alterations in cell adhesion and immune surveillance.

    Nat. Genet. 41, 10.1038/ng.429 (2009).

  6. Biomaterials

    Delivering to the Senses

    1. Marc S. Lavine

    Though nerve cells can be stimulated externally by electrical impulses, controlled delivery of molecular neurotransmitters would enhance the selectivity of the activation process across cell types. Simon et al. used conducting organic polymers to create an encapsulated device for delivering neurotransmitters such as glutamate in either continuous or pulsed fashion under electronic control. The device was initially constructed in a planar geometry for in vitro testing, which achieved glutamate-induced spikes in intracellular Ca2+ concentration in astrocytes. For in vivo testing, the device was reconfigured into a syringe-like geometry in order to deliver glutamate to the cochlea of anaesthetized guinea pigs, thereby triggering a variation in auditory brainstem response. Based on the current configurations, the devices could be operated in either continuous delivery mode for up to 1 hour or in pulsed delivery mode for much longer periods of time.

    Nat. Mater. 8, 10.1038/nmat2494 (2009).

  7. Immunology

    Paracrine Pass-Through

    1. Kristen L. Mueller

    Upon encounter with infectious agents such as viruses, host organisms must rapidly mount a strong system-wide immune response, lest the infection spread dangerously wide. How do they accomplish this task at the early stage when so few cells are typically infected? Patel et al. demonstrate that one mechanism may rely on intercellular communication through gap junctions. They showed that when cells were exposed to double-stranded DNA (dsDNA), which can be found during infection by some types of viruses, not only the cell that directly sensed the dsDNA, but also the surrounding cells, were able to produce pro-inflammatory and antiviral cytokines that are required to initiate the full antiviral immune response. If the gap junctions were blocked through chemical inhibition or by genetic interference, the surrounding cells no longer produced cytokines when their neighbors were exposed to dsDNA. Thus, gap junctions allow the rapid mobilization and amplification of an antiviral immune response by transmitting the activating signal directly from infected cells to uninfected cells.

    Proc. Natl. Acad. Sci. U.S.A. 106, 12867 (2009).