This Week in Science

Science  16 May 2014:
Vol. 344, Issue 6185, pp. 671
  1. Out of the Air

    New-particle formation from gaseous precursors in the atmosphere is a complex and poorly understood process with importance in atmospheric chemistry and climate. Laboratory studies have had trouble reproducing the particle formation rates that must occur in the natural world. Riccobono et al. (p. 717) used the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN to recreate a realistic atmospheric environment. Sulfuric acid and oxidized organic vapors in typical natural concentrations caused particle nucleation at similar rates to those observed in the lower atmosphere.

  2. Controlling Chiral Light Emission

    Circularly polarized light plays important roles in a number of applications such as displays, communication, and sensing. Thus, the ability to produce compact and readily controllable polarized light sources is important, and dichalcogenide materials such as tungsten diselenide may provide a route to such sources. Zhang et al. (p. 725, published online 17 April; see the Perspective by Zaumseil) formed an electric-double-layer transistor structure with WSe2 and used a gated ionic liquid to control the carrier density. Electrical control of the output light was achieved with the polarization being switched by reversing the polarity of the applied field and injected charge.

  3. Recyclable Thermoset Polymers

    The high mechanical strength and durability of thermoset polymers are exploited in applications such as composite materials, where they form the matrix surrounding carbon fibers. The thermally driven polymerization reaction is usually irreversible, so it is difficult to recycle the constituent monomers and to remove and repair portions of a composite part. García et al. (p. 732; see the Perspective by Long) now describe a family of polymers formed by condensation of paraformaldehyde with bisanilines that could form hard thermoset polymers or, when more oxygenated, produce self-healing gels. Strong acid digestion allowed recovery of the bisaniline monomers.

  4. Stick to the Bush

    Can the underlying genetic changes driving the divergence of populations into new species be predicted or repeated? Soria-Carrasco et al. (p. 738) investigated the genetic changes observed after one generation when stick insect (Timema cristinae) populations were transplanted from their preferred host plants to alternative hosts. Diverged genetic regions were relatively small, with most loci showing divergence in a single population pair. However, the number of loci showing parallel divergence was greater than expected by chance. Thus, selection can drive parallel phenotypic evolution via parallel genetic changes.

  5. Fundamentals of Bone Formation

    In vitro models can help guide research for tissue engineering or drug delivery, but the extent to which results from in vitro experiments may mimic in vivo ones will depend on the robustness of the model. For complex tissues like the extracellular matrix or bone, this means matching the chemical organization of the tissue at both the atomic scale and the structural level. Chow et al. (p. 742) used nuclear magnetic resonance (NMR) spectroscopy to analyze a sample on both these length scales. First an isotope-enriched mouse was produced to enhance the NMR signal. Samples from these mice were then used to study the extracellular matrix of developing bone and the calcification front during fetal bone growth.

  6. American Beauty

    Modern Native American ancestry traces back to an East Asian migration across Beringia. However, some Native American skeletons from the late Pleistocene show phenotypic characteristics more similar to other, more geographically distant, human populations. Chatters et al. (p. 750) describe a skeleton with a Paleoamerican phenotype from the eastern Yucatan, dating to approximately 12 to 13 thousand years ago, with a relatively common extant Native American mitochondrial DNA haplotype. The Paleoamerican phenotype may thus have evolved independently among Native American populations.

  7. Antarctic Collapse


    The West Antarctic Ice Sheet (WAIS) is particularly vulnerable to ocean warming-induced collapse. The Thwaites Glacier of West Antarctica is one of the largest WAIS regional contributors to sea level rise, and has been considered to be potentially unstable for many years. Joughin et al. (p. 735) used a combination of a numerical model and observations of its recent geometry and movement to investigate the stability of the Thwaites Glacier. The glacier has already entered the early stages of collapse, and rapid and irreversible collapse is likely in the next 200 to 1000 years.

  8. Virus-Enhanced Sulfur Oxidation

    How do microbial viruses affect subsurface microbial communities? Anantharaman et al. (p. 757, published online 1 May) investigated the interactions between ubiquitous marine lithotrophs found at hydrothermal vents and their viruses. The genes for sulfur oxidation in viruses that infect abundant marine chemosynthetic sulfur-oxidizing bacteria enhanced sulfur oxidation, thereby influencing the biogeochemical sulfur cycle.

  9. Scents and Sperm


    Once sperm enter the female reproductive tract, they have an arduous task to find an egg at a distant, often concealed, location. McKnight et al. (p. 754) show that Caenorhabditis elegans make this task more or less difficult, depending on pheromones in the external environment. Pheromones perceived by female sensory neurons modulate the synthesis of ovarian prostaglandins, which provide sperm positional information. Thus, environmental cues can indirectly impact sperm function even when the sperm themselves are not directly exposed.

  10. The Lignin Landscape

    Lignin is a chemically complex polymer that lends woody plants and trees their rigidity. Humans have traditionally either left it intact to lend rigidity to their own wooden constructs, or burned it to generate heat and sometimes power. With the advent of major biorefining operations to convert cellulosic biomass into ethanol and other liquid fuels, researchers are now exploring how to transform the associated leftover lignin into more diverse and valuable products. Ragauskas et al. (10.1126/science.1246843) review recent developments in this area, ranging from genetic engineering approaches that tune lignin properties at the source, to chemical processing techniques directed toward extracting lignin in the biorefinery and transforming it into high-performance plastics and a variety of bulk and fine chemicals.

  11. The Hormone's Message


    The receptor for growth hormone is a well-studied representative of a family of cytokine receptors through which binding of hormone molecules at the cell surface is converted into a biochemical signal within the cell. Brooks et al. (10.1126/science.1249783; see the Perspective by Wells and Kossiakoff) used a combination of crystal structures, biophysical measurements, cell biology experiments with modified receptors, and molecular dynamics and modeling to decipher how the receptor actually transmits the information that a hormone molecule is bound. The results suggest that the receptors exist in inactive dimeric complexes in which two associated JAK2 protein kinase molecules interact in an inhibitory manner. Binding of growth hormone causes a structural change in the receptor that results in movement of the receptor intracellular domains apart from one another. This relieves the inhibition of the JAK2 molecules and allows them to activate one another, thus initiating the cellular response to the hormone.

  12. Degrees of Separation

    Proteins embedded in membranes represent an interesting point of communication between the cell and its environment, but their localization to membranes can make them difficult to study. Jones et al. (p. 711) found an approach to catalog thousands of interactions involving membrane proteins and membrane-associated signaling machinery—including many previously uncharacterized proteins. With a focus on the model plant Arabidopsis, several of the identified interactions fill gaps in important signal transduction chains, while others point to functions for enigmatic unknown proteins.

  13. Untwisting the Spin Spiral

    Ultracold Fermi gases in the so-called unitary regime—where the diverging interactions between atoms make their thermodynamics universal—are an excellent test bed for an array of strongly interacting matter systems. The transport characteristics in this regime are particularly intriguing, and a discrepancy between two- and three-dimensional transport coefficients has been observed. Bardon et al. (p. 722) studied the demagnetization dynamics of a three-dimensional Fermi gas. The gas was initially polarized along a single direction and was noninteracting. An applied magnetic field gradient then caused a spin spiral to form; as the gas relaxed from this state, the authors extracted the diffusion coefficient and observed the buildup of interactions between the atoms.

  14. Crystal Growth

    Two main routes for the growth of crystalline species are either via molecule-by-molecule attachment to existing nuclei or via the addition of preformed metastable precursors, but do these mechanisms need to be mutually exclusive? Lupulescu and Rimer (p. 729; see the Perspective by Dandekar and Doherty) developed an in situ atomic force microscopy (AFM) technique to study the crystallization of materials under extreme conditions of temperature (25° to 300°C) and alkalinity (up to a pH of 13). The growth of the zeolite silicalite-1 involved both the attachment of metastable precursors and of individual molecules.

  15. Hunters and Farmers

    The Neolithic period in Europe saw the transition from a hunter-gatherer lifestyle to farming. Previous genetic analyses have suggested that hunter-gatherers were replaced by immigrant farmers. Skoglund et al. (p. 747, published online 24 April) sequenced one Mesolithic and nine Neolithic Swedish individuals to examine the transition from hunter-gatherers to farmers. Substantial genetic differentiation was observed between hunter-gatherers and farmers: There was lower genetic diversity within the hunter-gatherers and gene flow from the hunter-gatherers into the farmers but not vice versa.

  16. Signaling Dynamics

    The signaling pathways that activate the transcription factor NF-κB are key regulatory pathways in cells of the immune system, and their dynamic properties are still being elucidated. In B cells, analysis of single-cell responses has shown that the stimulation of the B cell receptor causes a “digital” all-or-none response of cells to a stimulus. Shinohara et al. (p. 760) used a combination of mathematical modeling and experiments to show that this property of the system results from the presence of a positive feedback loop among the signaling components activated in response to the receptor. Studies in cells expressing mutated signaling components resolved key phosphorylation events that provide the threshold responses observed and identified potential molecular modifications that might modify the threshold or other aspects of the dynamic response.