Editors' Choice

Science  11 Jul 2008:
Vol. 321, Issue 5886, pp. 176

    The Ups and Downs of Island Life

    1. Andrew M. Sugden

    The equilibrium theory of island biogeography, developed in the 1960s by MacArthur and Wilson, has been the principal reference point for ecologists investigating how the dynamic processes of colonization, speciation, and extinction affect biodiversity in insular habitats. However, the theory has been less successful when applied to longer-term evolution on oceanic islands, where geological dynamics such as erosion come into play. Whittaker et al. have developed a general dynamic model (GDM) that takes account of the humped trend in an oceanic island's carrying capacity over its entire life span. The model generates predictions about the biotic properties (species diversity, rates of speciation and extinction) of oceanic islands that fit snugly with data from oceanic archipelagoes, including the Hawaiian islands and the Galápagos (shown above) among others. By unifying evolutionary and ecological time scales and processes in a single theoretical framework, the GDM adds to the understanding of island biology. — AMS

    J. Biogeogr. 35, 977 (2008).


    Weighing Sverdrup Sediment

    1. Brooks Hanson

    The huge extinction at the end of the Permian period (∼252 million years ago) is marked globally by a large drop in the carbon isotope ratio (13C/12C) of organic matter preserved in sedimentary rocks. Low values persist for millions of years after the extinction, in part marking the delayed recovery of Earth's biota. In addition to providing a marker of the event, the magnitude and abruptness of the drop are important constraints on the likely cause(s) and pace of the extinction. Many sections show a relatively abrupt decrease, though some of these may be partly compressed by erosion; the Permian was a time when most of the land masses were assembled together in one supercontinent (Pangea), and sea level fluctuated markedly. Yet some sections seem to show a more complicated or gradual decrease. Grasby and Beauchamp describe several isotope records preserved across the Sverdrup Basin, Arctic Canada, which is now known to contain thick sections of Permian and Triassic rocks. Sections at the margin of the basin, where some erosion is documented to have occurred, show an abrupt carbon isotope drop at the boundary. Sections in the center, which seem to record continuous deposition and a thicker boundary layer than most other sections globally, show a more gradual decline over about 3 m, after a period of relative stability that is not well resolved elsewhere. Although the sections thus provide important detail on the extinction record and perhaps a period just before it, detailed dates are not yet available to calibrate absolute rates of change.— BH

    Chem. Geol. 10.1016/j.chemgeo.2008.05.005 (2008).


    The Roots of Hair Growth

    1. Pamela J. Hines

    Adult hair follicles sustain repeated cycles of hair loss and regrowth. Stem cells reside within a small niche, called the bulge, located in the upper part of the hair follicle. These stem cells are responsible for driving this cycle of growth and can repopulate follicles and surrounding epidermis damaged by wounding. However, the first hair follicles in mice arise from the developing epidermis, not from preexisting bulge stem cells, and these first-time hair follicles do not exhibit the conspicuous bulge that only becomes apparent some weeks after birth.

    Nowak et al. have analyzed the origins of the hair follicle stem cell niche in developing mice. The stem cells of the bulge are in fact established much earlier than previously suspected and begin to form in the embryo. These cells, which are characterized by expression of the transcription factor Sox9, not only contribute to the formation of the initial hair follicle, but also give rise to the adult bulge stem cells that are responsible for the maintenance of the hair follicle itself. Ablation of Sox9 left the embryos without bulge cells, and the mice never grew any hair and did not have any sebaceous glands. Moreover, the skin did not repair epidermal wounds well when Sox9 was missing. These results implicate Sox9 in establishment of the hair follicle stem cell population and show that early stem cells can contribute to skin morphogenesis before assuming their role as adult stem cells. — PJH

    Cell Stem Cell 3, 33 (2008).


    Every Little Second Counts

    1. Jake Yeston

    Part of the appeal of nuclear magnetic resonance phenomena is that the two-level system being manipulated (nuclear spin up, or nuclear spin down) is comparatively easy to model in a quantum mechanical framework. In this context, application of a very short and strong (“hard”) electromagnetic pulse to a spin ensemble can be approximated as having an instantaneous effect—a hard pi pulse, for instance, immediately rotating the aggregate spin vector 180°. Of course, such pulses are not precisely instantaneous, and Dong et al. show that it is possible to exploit their small but still finite durations to manipulate coherence in experiments that apply many of them, one after the other, in a train. The technique substantially reduces linewidth in inhomogeneously broadened samples, most strikingly by nearly five orders of magnitude for 29Si resonances in antimony-doped silicon powder. The authors also apply the technique to 13C probing in C60 samples. — JSY

    Phys. Rev. Lett. 100, 247601 (2008).


    Resistance to Infection

    1. Caroline Ash

    In 1987, W. D. Hamilton wrote that the pressure of parasites is one of the factors favoring genetic diversity. The ant Formica selysi is found in colonies with single or multiple queens. Colonies with one queen have low genetic diversity, but workers tend to live longer than in polygynous colonies with high genetic diversity. In the wild, the multiqueen colonies are large, and individual workers are small and have short lives, mainly it seems because they do not provision so efficiently. So why does polygyny persist? The advantages of diversity became apparent when Reber et al. brought ants into experimentally controlled conditions to reduce the impact of compensating factors such as environmental variables. Artificial ant colonies revealed a strong advantage conferred by diversity when challenged by a fungal parasite; colonies of lower genetic diversity were decimated by infection. — CA

    Ecol. Lett. 11, 682 (2008).


    Radical Stabilization

    1. Phil Szuromi

    The properties and reactions of single hydrogen atoms are of interest because of their inherent quantum mechanical behavior; experimentally, they can be generated and stabilized at very low temperatures (4 K) by high-energy irradiation of solid molecular hydrogen. Yeon et al. show that icy organic hydrates, which contain small cages that can trap guest molecules, can be used to create and trap H atoms at higher temperatures. They trapped H2 in deuterated tetrahydrofuran hydrates (D2O and THF-d8) at 123 K, using γ-ray irradiation to form stabilized H atoms that were detected by electron spin resonance (ESR) and magic-angle spinning proton nuclear magnetic resonance (MAS NMR) spectroscopy. Irradiating THF afforded ESR assignments for free D atoms and THF radicals that were also created. The temperature evolution of the MAS NMR signals from 173 to 183 K indicated that the formation of radical products H2+ and H2 likely occurred directly as opposed to being mediated by reaction with the ice framework.— PDS

    J. Am. Chem. Soc. 130, 10.1021/ja802952p (2008).


    A Ligand, Not a Carrier

    1. Elizabeth M. Adler

    Volatile odorants are generally thought to bind to G protein-coupled receptors, and this event then activates downstream signaling pathways. In Drosophila, various odorant-binding proteins are secreted into the lymph around subsets of olfactory neurons; the function of these pheromone- and odorant-binding proteins, however, has been unclear. Building on earlier work that showed that the odorant-binding protein LUSH is required for sensitivity to the pheromone 11-cis vaccenyl acetate (cVA), Laughlin et al. have compared the x-ray crystal structure of LUSH bound to cVA with the previously determined structure of uncomplexed LUSH and found that binding of cVA (which was almost completely enclosed by LUSH) induced a conformational change in LUSH. A LUSH mutant bearing an amino acid substitution predicted to minimize this conformational change was less effective than the wild-type protein at conferring cVA sensitivity to T1 neurons, which mediate the response to this pheromone; in contrast, a mutation predicted to enhance the conformational change produced a more potent ligand. Moreover, a mutation yielding an uncomplexed LUSH conformation that resembled that of the cVA-bound form stimulated T1 neurons even in the absence of cVA. Thus, the authors conclude that LUSH is an inactive ligand that is converted into an active form through cVA binding. — EMA

    Cell 133, 1255 (2008).

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