Editors' Choice

Science  07 Jun 2013:
Vol. 340, Issue 6137, pp. 1142
  1. Evolution

    Hop to Evolution

    1. Laura M. Zahn

    The mechanisms by which speciation occurs have been a mystery because they are generally best uncovered after populations have diverged and are reproductively isolated. In order to identify the genetic changes that occur after species are geographically isolated, Wessel et al. examined Hawaiian leafhoppers (the monophyletic radiation encompassing the Oliarus polyphemus species complex). Leafhoppers have colonized lava tubes, thus providing replicates within similar environmental complexes. These insects exhibit cryptic speciation on the basis of their mating calls, and the authors identified differentiation in acoustic and morphometric characters that is random with respect to cave age and distribution. Analysis of mitochondrial DNA suggested a correlation between genetic distance and geographic distance but not with age of the cave, whereas phenotypic variability decreased with cave age. These results suggest that stochastic genetic effects are likely responsible for the rapid evolution observed in this group and that this system warrants further investigation to understand the consequence of the founder effect on speciation.

    Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1301657110 (2013).

  2. Chemistry

    Bridging Color

    1. Marc S. Lavine

    The ability to solution-process small light-emitting molecules makes it possible to fabricate organic light-emitting devices using large-scale methods and onto a wide range of substrates. In order to get reasonable values for the photoluminescence efficiency, larger oligomeric molecules are required; however, the increased size rapidly decreases the solubility. Solubility can be increased through the addition of alkyl side groups while also decreasing intermolecular quenching, but this often also diminishes the intermolecular charge transfer, which is the key step in converting electrical charge into light. Christensen et al. reacted a series of chromaphore molecules to form Ar-S-Ar symmetrically bridged sulfides, where the Ar ligand was either bithiophene, terthiophene, naphthalene, or pyrene, and where the sulfur bridge could be subsequently oxidized to form a sulfoxide or sulfone. The addition of the sulfur bridge enhanced the solubility of the molecules; however, it was the oxidiation of the sulfur that was the key to the enhanced photoluminescence seen in all cases. The authors speculate that the electron deficiency of the sulfoxide or sulfone groups enhances the formation of charge-transfer states. Interestingly, the photoluminescence yields of the bithiophene and terthiophene sulfone molecules were greater than the values seen for nonbridged thiophenes with an equivalent number of aromatic units.

    J. Am. Chem. Soc. 10.1021/ja401383q (2013).

  3. Geochemistry

    The Nd of the Innocence

    1. Nicholas S. Wigginton

    There are few remaining geologic clues about the structure and geodynamics operating on Earth from its initial differentiation around 4.5 billion years ago until modern plate tectonics began. Debaille et al. analyzed the geochemistry of an ancient layered lava flow from Ontario, Canada, that serves as a window into a partially melted mantle plume from around 2.7 billion years ago. Excess 142Nd contents, which are daughter products of now extinct radiogenic 146Sm, imply that the mantle contained heterogeneities generated by relatively sluggish mixing—especially when considering that the temperatures and convection rates at that time would have been much higher than they are in the modern mantle. Using numerical models, the authors show that the preservation of this signature could occur in a stagnant regime of plate tectonics independently of high convection rates, meaning that early subduction events occurred only in brief and sporadic episodes. Combined with other geochemical data, the ancient lava flow data are consistent with the onset of modern (i.e. continuous) plate tectonics around 2.7 to 3.0 billion years ago.

    Earth Planet Sci. Lett. 10.1016/j.epsl.2013.04.016 (2013).

  4. Biomedicine

    Misplacing RAS (Again)

    1. Paula A. Kiberstis

    The discovery that the mutational activation of RAS proteins drives the growth of human cancer cells catalyzed a dogged—but ultimately unsuccessful—search for drugs that inhibit RAS activity. Interest in pharmacologically targeting RAS has been revived by cancer genome studies, which revealed KRAS to be the most frequently mutated gene in the cancer types that are most common in the population and/or most refractory to therapy, such as pancreatic, lung, and colorectal cancer.

    Because KRAS signaling activity is dependent on the protein's localization at the cell membrane, Zimmermann et al. investigated whether compounds that interfere with KRAS localization have anticancer activity. In a high-throughput screen, they identified small molecules that prevent KRAS from binding to PDE-δ, a protein that facilitates KRAS trafficking to the membrane. An optimized compound, deltarasin, was found to inhibit KRAS signaling and growth of KRAS-mutant human pancreatic cancer cells in vitro and in mice. Although these results are promising, the bar for deltarasin and its derivatives will be high, because previous drugs designed to disrupt KRAS membrane localization in a different way proved to be ineffective in clinical trials.

    Nature 10.1038/nature12205 (2013).

  5. Cell Biology

    Seeing the Light

    1. Valda Vinson

    The ability to visualize live cells using genetically encoded bioluminescent and fluorescent proteins has provided insight into many biological processes. Sellmyer et al. report a strategy to visualize cell-cell interactions in live animals using bioluminescence, which occurs when lucerifase enzymes are activated by a luciferin substrate. One cell population (the reporter cells) expressed luciferase, whereas a different cell population (the activator cells) expressed an uncaging enzyme and luciferin. The uncaging of activator cells allowed the release of luciferin, which could then diffuse into nearby reporter cells, react with luciferase, and cause the emission of light that was dependent on the distance between the activator and reporter cells. The method was shown to assay cellular proximity in cultured cells and was able to identify metastatic sites in a mouse breast cancer model. Further improvements in the technique will likely allow the visualization of many other cellular interactions, which could lead to new insight into processes such as adaptive immunity or stem cell niche development.

    Proc. Natl. Acad. Sci. U.S.A. 110, 8567 (2013).

  6. Evolution

    Cooperative Yeast Break Free

    1. Guy Riddihough

    Cooperation is pervasive in nature, but how it remains a successful evolutionary strategy in the face of defectors and free riders is not yet fully understood. This is especially true in models of the classical Prisoner's Dilemma game, in which, theoretically, cooperators should always lose to defectors.

    Van Dyken et al. engineer baker's yeast, Saccharomyces cerevisiae, into cooperators and defectors and pit them against each other in a Prisoner's Dilemma game. They are mutated so they cannot take up sucrose from the medium. Cooperators secrete invertase, which breaks sucrose down into monosaccharides. Defectors are mutant for invertase. Neither cooperators nor defectors can take up sucrose, but both can take up monosaccharides. When a growth cost is imposed for cooperation, and defectors are able to achieve limited monosaccharide-independent growth, defectors initially outcompete cooperators. But as the colonies grow, the cooperator populations expand at the expense of the defectors. The cooperators form genetically demixed sectors, analogous to "genetic surfing" seen in frontier populations. Simulations support the idea that an expanding colony frontier favors (cooperative) genotypes that maximize group productivity and that this could apply to range expansions seen in many species, including humans.

    Curr. Biol. 23, 919 (2013).

  7. Physics

    Seeing Nodes

    1. Jake Yeston

    A hundred years ago, Niels Bohr introduced the concept of discrete, or quantized, electronic energy levels in hydrogen atoms to account for the discrete lines in their optical spectra. Within a couple of decades, the model had been elaborated to establish the framework for quantum mechanics, with successive levels corresponding to spatial probability distributions of electronic wave functions manifesting a rising number of nodes. Since then, a vast and diverse array of physical and chemical phenomena has bolstered the validity of this framework. Yet visualization retains a special power to elucidate. Stodolna et al. have now directly projected the nodal pattern of a hydrogen atom's electronic wave function onto a detector. Theory had predicted that the preparation of Stark states, in a static electric field, would foster direct imaging upon ionization. The authors note that the addition of a magnetic field to the experimental setup could probe further intricacies of atomic electron dynamics.

    Phys. Rev. Lett. 110, 213001 (2013).

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