Editors' Choice

Science  04 Feb 2011:
Vol. 331, Issue 6017, pp. 512
  1. Evolution

    Mountain Goat Movements

    1. Sherman J. Suter

    Patterns of genetic differentiation within species are influenced by both current dispersal and historical processes such as geographic isolation and subsequent recolonization. To gauge the relative importance of these factors, Shafer et al. examined spatial variation in genetic diversity across the natural range of the North American mountain goat (Oreamnos americanus). Fossil evidence supports the existence of a southern geographic refuge during the Last Glacial Maximum. Had mountain goats simply spread back from this region, one would expect a northward decrease in genetic heterozygosity and a nested phylogeny. Instead, analyses of microsatellite DNA reveal the presence of a second core area of elevated genetic diversity in northern British Columbia and southeastern Alaska. Mitochondrial DNA offers strong support for the separation of northern and southern clades, which molecular dating suggests split before the onset of the last glaciation. High gene flow links contiguous mountain ranges, and there are signs of contemporary long-distance dispersal. Similar considerations of population differentiation will be important for efforts to conserve alpine biota and ecosystems in the face of climate change.

    Evolution 65, 125 (2011).

  2. Paleontology

    Extinction's Cause and Effect

    1. Nicholas S. Wigginton
    CREDIT: GRASBY ET AL., NAT. GEOSCI. 4, 10.1038/NGEO1069 (2011)

    At the Permian-Triassic boundary ∼250 million years ago, nearly 90% of marine animals went extinct. Unlike the dinosaurs' demise attributed to a giant asteroid impact ∼185 million years later, this extinction event was at least partially related to a long series of volcanic eruptions. A series of feedbacks in the ocean-atmosphere system as a response to these eruptions would have turned the oceans anoxic and suffocating to animals, but how marine life recovered is not well constrained. To track primary productivity after the mass extinction, Meyer et al. examined the carbon isotope signature from limestone deposits in south China. Large variations in the isotopic record suggest that autotrophic productivity was so high after the mass extinction that it maintained anoxia and choked out larger marine life, though the isotopes say little about how such a situation would have originated. Toward that end, a study by Grasby et al. suggests that volcanism also combusted nearby coal deposits in a series of events that are roughly coincident with downturns in productivity leading up to the mass extinction. The charred remains (shown left in comparison with modern combustion residue), which were found in sedimentary deposits in the Canadian arctic, may have added toxins, such as chromium, to ocean basins that could also have contributed to the declining state of marine biodiversity.

    Earth Planet Sci. Lett. 10.1016/j.epsl.2010.12.033 (2011); Nat. Geosci. 4, 10.1038/ngeo1069 (2011).

  3. Neuroscience

    Brain Stimulation In Depth

    1. Peter Stern

    Transcranial magnetic stimulation (TMS) is a noninvasive clinical method of stimulating the human brain, which has been tested for the treatment of depression and other neurological disorders. Depending on the stimulation protocol used, TMS can increase or decrease cortical excitability; however, how it does so is poorly understood. Benali et al. thus used electrophysiological recording techniques and immuno-histochemistry to analyze the changes that occur in the rat brain after different repetitive TMS protocols. They found that two theta-burst stimulation (TBS) protocols that in the past have been shown to modify human cortical excitability in opposite ways also affected rat cortical activity and protein expression. Although both intermittent and continuous TBS affected cortical inhibitory neuronal systems, the cell types affected, and thus the overall effects of the stimulation, differed. These findings sound a note of caution because such impairment of the cortical inhibitory system may also take place in the human cortex. They also raise the critical question of whether these novel and increasingly popular high-frequency TMS protocols are really safe for use in human cognitive or clinical neuroscience.

    J. Neurosci. 31, 1193 (2011).

  4. Chemistry

    Baby Bubbles

    1. Marc S. Lavine

    Emulsions consist of droplets of one fluid immersed in a second, immiscible fluid and stabilized through the addition of surfactants or emulsifiers. Examples include oil and vinegar salad dressing and some cosmetics or perfumes. Over time, droplets may adhere together and form a cream or sediment, but this is a reversible process. Droplets can also irreversibly coalesce together or undergo Ostwald ripening whereby the largest droplets grow even larger at the expense of the smaller ones. In studying droplets smaller than 150 nm, formed by sonication, Delmas et al. find that droplet size evolution is an exponential function of the sonication time for a given sonication power but is not influenced much by surfactant concentration or type, or the viscosity of the samples. They are able to obtain a master curve correlating the average diameter to the sonication energy. For these small droplets, destabilization only occurs by Ostwald ripening, but this can be prevented by kinetic trapping of molecules in the droplet interiors or at the membrane between the two phases.

    Langmuir 27, 10.1021/la104221q (2011).

  5. Cell Biology

    Daughters Divided But Equal

    1. Stella M. Hurtley

    Bacterial cells grow and divide rapidly and must ensure that upon division, both daughter cells receive all of the components necessary for future development. In Escherichia coli, Min proteins help the cell to select the appropriate site for cell division so that each daughter cell contains its own complement of DNA. During this process, however, the Min proteins themselves are distributed unevenly throughout the cell, oscillating from pole to pole. Di Ventura and Sourjik explored whether self-organizational principles may underlie the partitioning of Min components during cell division. Microscopic analysis of the Min proteins in dividing cells revealed that when the cells are close to division, each daughter cell displays its own oscillating Min system. As the septum constricts, the amounts of Min proteins in the two daughter cells equilibrate. The data could be successfully modeled simply by including the known membrane interaction properties of the Min proteins and the level of separation between the new cells, depending on the size of the newly forming septum (above). Thus, the self-organizing behavior of the Min proteins together with the geometrical constraints on protein movement as cytokinesis proceeds leads to the generation of two daughter cells with a full complement of functional Min proteins.

    Mol. Syst. Biol. 7, 457 (2011).

  6. Applied Physics

    TRUE Vision

    1. Phil Szuromi

    Optical imaging within biological tissue has to overcome the problem of multiple scattering events. Xu et al. show how to encode optical laser light with ultrasonic frequencies so that it effectively focuses to a point despite scattering. They split the light into three components: two reference beams and a sample beam. The latter is sent though two acousto-optic modulators to add ultrasonic-frequency side bands, allowing a focal point to be created in the scattering medium with ultrasound, which scatters about a thousandth as strongly as optical light. The light from the sample is collected in a photorefractive crystal as a hologram with the use of the reference beams, and then read out to create a time-reversed image; only the light that was encoded with ultrasound is unaffected by multiple scattering. These time-reversed ultrasonically encoded, or TRUE images, have a resolution determined by the ultrasonic frequency, in this case, 0.63 mm for light with a wavelength of 532 nm.

    Nat. Photon. 5, 10.1038/NPHOTON.2010.306 (2011).

  7. Biochemistry

    Alternate Antioxidant Pathway

    1. Valda Vinson

    The low-molecular-weight thiol glutathione (GSH) is essential for growth in eukaryotes and plays a central role in stress responses and the maintenance of redox homeostasis in certain bacteria. Surprisingly, most aerobic bacteria lack the GSH biosynthesis enzyme glutamate cysteine ligase (GshA), although many have genes encoding proteins expected to require GSH for activity. To understand how these bacteria thrive, Veeravalli et al. isolated suppressor mutations in GshA-deficient Escherichia coli cells that expressed GSH. These cells carried mutations in the first two genes in the proline biosynthesis pathway, proB and proA. The mutations facilitated the accumulation of γ-glutamyl phosphate. This reacts with cysteine to give γ-glutamyl cysteine, which is converted to GSH by the enzyme glutathione synthase. Bioinformatic analysis revealed homologs of the suppressor ProB in bacterial strains that make GSH-dependent proteins but do not encode GshA. These results suggest that the proline biosynthetic pathway may have a role in GSH synthesis in prokaryotes.

    Nat. Chem. Biol. 7, 101 (2011).

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