Editors' Choice

Science  03 Jul 2009:
Vol. 325, Issue 5936, pp. 12
  1. Evolution

    Unable to Diversify

    1. Sherman J. Suter
    CREDIT: U.S. BUREAU OF RECLAMATION

    The decline in fitness with increasing distance from an adaptive peak can constrain morphological divergence between sister lineages. Collar et al. examine whether the functional demands of feeding on fish have limited morphological diversification in the Centrarchidae, an endemic clade of North American freshwater teleosts that includes sunfish and bass. To reconstruct ancestral feeding strategies, they map diet data synthesized from the literature onto a well-resolved, time-calibrated, multilocus phylogeny of 29 extant species. After measuring functional aspects of the skull, they carried out principal-components analysis of seven size-corrected morphological variables and total length. For each principal component (PC), the authors compare the fit of several evolutionary models, allowing parameters to vary across lineages with different degrees of piscivory. Two adaptive peaks (piscivory and nonpiscivory) appear to have influenced the evolution of PC1, with strongest loadings on the pharyngeal jaw adductor muscle and lower jaw out-lever. Brownian motion models reveal slow evolution of PC2 (muscle closing oral jaws) and PC4 (gape width) in fish-eating lineages. These findings indicate that piscivory has curtailed the diversification of feeding morphology in the centrarchids and that the effects have been strongest in highly piscivorous lineages.

    Evolution 63, 1557 (2009).

  2. Geochemistry

    Guiding Mercury's Meandering

    1. Nicholas S. Wigginton
    CREDIT: ISTOCK.COM

    Mercury's toxicity impels wide-ranging study of the binding and redox behavior of the element under varying environmental conditions. Sulfate-reducing bacteria in soils and sediments are known to enhance mercury accumulation in higher species such as fish and humans by methylating oxidized Hg(II) ions. To what extent do geochemical reactions at mineral surfaces play a role in controlling the fate of Hg? Lee et al. uncover sharp variations in Hg(II) adsorption behavior on muscovite as they shift the local concentration of naturally abundant organic acids; complexation by the organics in free solution appeared to enhance adsorption more effectively than did a preformed film on the surface. In a related study, Wiatrowski et al. show that when Hg(II) adsorbs onto redox-active mineral surfaces such as magnetite, soluble Hg(II) is quickly reduced to highly mobile gaseous Hg(0). It remains unclear how organic layers on redox-active mineral surfaces specifically influence these and similar electron transfer reactions. Taken together, the two studies suggest that mineral-associated processes such as adsorption and phase transformation may provide stiff competition for the bacteria that catalyze Hg methylation reactions in mediating Hg fate and mobility.

    Environ. Sci. Technol. 43, 10.1021/es900214e;10.1021/es9003608 (2009).

  3. Molecular Biology

    Liverish Days and Nights

    1. Beverly A. Purnell

    Biological processes with circadian rhythms show periodicity that correlates with the day/night cycle, and many metabolic pathways display such a regulatory mechanism. Prior work has suggested that between 2 and 10% of the liver transcriptome is under circadian control. Gatfield et al. show that miR-122, an abundant hepatocyte microRNA, is under circadian control. The abundance of the miR-122 precursor varies 4- to 10-fold during the day. However, accumulation is constant in the liver of mice lacking the orphan nuclear receptor REV-ERBα. When miR-122 is knocked down, hundreds of mRNAs are expressed. Peroxisome proliferation–activated receptor (PPAR) β/δ and the coactivator SMARCD1/BAF60a, two known metabolism regulators, are specific miR-122 targets, linking circadian rhythms with the PPAR family of nuclear receptors and liver metabolism.

    Genes Dev. 23, 1313 (2009).

  4. Immunology

    Maintaining Diversity

    1. Kristen L. Mueller

    Exposure to infectious agents has exerted selective pressure on the evolution of one group of immune system genes. The hygiene hypothesis, proposed two decades ago, posits that the reduced exposure of inhabitants of modern industrialized societies to microbes and macropathogens (such as parasitic worms) has increased susceptibility to inflammatory conditions such as allergies and autoimmune disease. Taking a population genetics approach, Fumagalli et al. investigated how infectious agents have affected the genetic variability of interleukins, which are critical signaling molecules of the immune system, and their associated receptors. They found that pathogen richness, a measure of pathogen diversity in a specific geographic area, has driven the selection of several interleukin family genes; in particular, balanced selection, a type of selection that maintains genetic variation within populations and is unusual in humans, has contributed to the evolution of five interleukin genes. Finally, the authors demonstrated that six of the nine known risk alleles associated with inflammatory conditions such as celiac disease and Crohn's disease correlated with pathogen richness, but in contrast to the hygiene hypothesis, more so with bacteria, viruses, and fungi rather than worms. Hence, pathogen-influenced evolution is a double-edged sword: The same bugs that helped us to develop an immune response to a wide array of pathogens also may have contributed to the appearance of debilitating inflammatory diseases.

    J. Exp. Med. 206, 1395 (2009).

  5. Applied Physics

    Half Metal, Full Spin

    1. Ian S. Osborne

    For optimal efficiency in spintronic applications (which replace ordinary charge flow with spin current), host materials should ideally be completely spin-polarized. Unfortunately, not many materials have this property. Heusler alloys constitute an emerging prospect. These are metals, or rather half-metals, in which the conduction band is split into two polarizations—one half occupied, the other empty—thus providing 100% spin polarized current transport. However, the materials studied so far have shown strongly temperature-dependent behavior due to the band structure, making them unsuitable for practical device applications. Shan et al. now show that Fermi-level tuning by the introduction of appropriate dopant atoms, a technique similar to electron or hole enrichment in semiconductors, can be applied to the Heusler alloy Co2FeAl0.5Si0.5. With a spin-polarization exceeding 90% and a weak temperature dependence up to room temperature, the resulting material highlights the potential of Fermi-level tuning for attaining high performance in spintronic applications.

    Phys. Rev. Lett. 102, 246601 (2009).

  6. Geochemistry

    Well-Aged Granite

    1. Maria Cruz

    Granite is not unique to our planet. Granite-like rocks have been found, albeit comparatively rarely, on Mars, Venus, and the Moon and within meteorites. Terada and Bischoff report the age of a granite-like fragment found in a stony meteorite (Adzhi-Bogdo) that fell in Mongolia in 1949. Their analysis of the fragment using a high-resolution ion microprobe implies that its parental granite-like magma crystallized 4.53 billion years ago. Granites on Earth and the Moon crystallized at a later stage, making this fragment the earliest example of granite-like material in the solar system. Its age reveals that such material must have formed very early in the solar system's history. On Earth, granite formation is thought to require the presence of water and plate tectonics. This fragment, however, must have formed under dry conditions and without plate tectonics, calling for a different formation mechanism.

    Astrophys. J. 699, L68 (2009).

  7. Biophysics

    Tissue-Engineering Principals

    1. Gilbert Chin
    CREDIT: KILARSKI ET AL., NAT. MED. 15, 657 (2009)

    If Robert Moses, the driving force behind the construction of the congested artery known as the Brooklyn-Queens Expressway, were alive today, he would marvel at the cellular engineers who relocate still-functioning blood vessels from nearby healthy tissue into healing wounds. After an injury, force-generating cells have previously been shown to reinforce the initial fibrin-based matrix and to latch onto and contract subsequently deposited extracellular matrix (ECM) components. Kilarski et al. show that myofibroblasts, in response to tensile stress and soluble growth factors, pull intact blood vessels with them as highly perfused granulation tissue (the reddish patch in a healing wound) migrates into the injured area [represented by a fibrin-collagen gel in an in vitro system or by a sutured cornea in an in vivo preparation (above)]. The mediating mechanisms are not yet precisely defined, but Legant et al. describe their microfabricated tissue gauge (or µTUG) and use it to quantitate the interplay between matrix mechanics and fibroblasts during remodeling of collagen mesh. Increasing matrix stiffness, both in vitro and in a computational model, elicited higher cellular contractility (which is roughly 20 nN per cell), whereas increasing mechanical stress evoked higher levels of expression of cytoskeletal and ECM proteins.

    Nat. Med. 15, 657 (2009); Proc. Natl. Acad. Sci. U.S.A. 106, 10097 (2009).

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