Editors' Choice

Science  12 Jun 2009:
Vol. 324, Issue 5933, pp. 1366
  1. Evolution

    Mapping Life's History

    1. Andrew M. Sugden

    The tropical region of the New World hosts the richest plant diversity in the world, a richness that owes much to the complex tectonic history of this part of the Earth. Focusing on two tribes of the family Rubiaceae (which includes the coffee plant among many others), Antonelli et al. have traced how the uplift of the Andes has influenced the spatial and temporal evolution of neotropical plant diversity—not only in the mountains but also in the lowlands. Molecular phylogeographic analysis shows how the initial uplift of the western and central Andes beginning in the Eocene inhibited dispersal and led to diversification in the lowlands, while creating new montane habitats and taxa. Further barriers to dispersal between Amazonia and the mountains were imposed by marine incursions and the formation during the Miocene of the huge, but now-vanished, wetland (referred to as Lake Pebas or the Pebas Sea) to the east of the Andes. The disappearance of Lake Pebas and the establishment of the modern Amazon drainage are reflected in the distribution and speciation of younger lowland taxa in western Amazonia. The Miocene uplift of the eastern Cordillera of the Andes (shown above) meanwhile promoted dispersal of montane taxa.

    Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0811421106 (2009).

  2. Biochemistry

    Fit to Function

    1. Valda Vinson
    CREDIT: DUNHAM ET AL., EMBO J. 28, 10.1038/EMBOJ.2009.120 (2009)

    In bacteria, low–copy number plasmids provide a model system for studying the segregation of DNA into daughter cells—partitioning requires only a centromere-like DNA site, a partition NTPase, and a centromere-binding protein. The type II systems use the actinlike partition protein ParM to drive plasmid separation. The Escherichia coli plasmid P1 uses the type I system of the partition ATPase ParA, the centromere binding protein ParB, and the DNA site parS. ParA with ATP bound interacts with centromere-bound ParB to mediate segregation; however, ADP-bound ParA acts as a transcription factor that represses parAB transcription.

    Using crystallography and biochemistry, Dunham et al. found that ParA forms a conformationally flexible dimer that relies on an interaction involving the N-terminal α helix. ADP binding locked the monomers (blue, yellow) into agreeable conformations for DNA binding and induced the folding of two positively charged regions (red) that docked onto the DNA (gray). In contrast, electron microscopy showed that ATP-bound ParA forms filaments that likely facilitate segregation.

    EMBO J. 28, 10.1038/emboj.2009.120 (2009).

  3. Chemistry

    Perils in Circular Dichroism

    1. Jake Yeston

    Circular dichroism (CD) spectra nominally measure the extent to which a particular chiral molecule preferentially absorbs one sense of circularly polarized light over the other sense. However, the spectra have traditionally been acquired using samples that comprise millions of trillions of molecules, and they have largely been treated as empirical data. In this context, a study that probed the interaction of circularly polarized light with one molecule at a time (Hassey et al., Reports, p. 1437, 1 December 2006) potentially offered deep mechanistic insight. The authors found that on an individual basis, single stereoisomers of a helicene molecule differed across a huge range in their CD response—some, in fact, appeared to favor light polarized opposite to the sense preferentially absorbed overall in an ensemble of like enantiomers. The method relied on fluorescence microscopy to achieve single-molecule resolution, and Tang et al. have now suggested that the data might be more easily explained by an artifact associated with the apparatus than by an inherent molecular property. Specifically, the second group of authors attempted to reproduce the findings, but observed that the circular polarization of incoming light was distorted into an ellipse upon reflection at a dichroic mirror, thus becoming more sensitive to molecular orientation. After correcting for this effect, they failed to detect distinct signals from the different single stereoisomers. They therefore propose that the previously observed wide-ranging CD response was actually a manifestation of linear dichroism (a function of orientation rather than chirality), and they argue for careful analysis of mirror-induced distortions in future related studies.

    J. Phys. Chem. A 113, 6213 (2009).

  4. Climate Science

    Life in the Long Run

    1. H. Jesse Smith

    The idea that we are already committed to a certain amount of surface air temperature increase and sea-level rise over the coming century, even if we could immediately halt all CO2 emissions, has become well known in scientific and science policy circles. The longer-term outlook is less well understood. Eby et al. use a complex, coupled climate-carbon cycle model to investigate how long anthropogenic climate change will persist as a function of how high the concentration of atmospheric CO2 rises. They calculate how long it will take for half of the total emissions to be removed from the atmosphere, what the maximum global average sea surface temperature increase will be, and how long it will take for 80% of that sea surface thermal anomaly to decay. The results suggest that atmospheric CO2 can persist at high concentrations for several thousand years, and that sea surface temperature increases can last many times longer than that. It looks, then, like we are in this for the long haul.

    J. Climate 22, 2501 (2009).

  5. Geology

    Holes in Rocks

    1. Nicholas S. Wigginton

    Most minerals in Earth's crust are built on a Si–O(2−)–Si framework. Commonly, these minerals contain minor O()–O() dimers as impurities distributed throughout the bulk of the crystal. When minerals within rocks experience mechanical stress, the weak bonds holding the dimers together are broken, creating a charge imbalance that gives rise to mobile positive holes. These holes travel through stressed mineral grains to other nearby minerals—like current passing through a p-doped semiconductor—until they eventually arrive at the rock's surface. Through a series of electrochemical experiments, Balk et al. demonstrate that such holes at the surface of rocks in simulated seawater rapidly oxidize water to form H2O2. Because crustal rocks are predominately in contact with water and are almost always experiencing some type of stress from processes such as plate tectonics, glacial movement, and lithostatic pressuring, the oxidation of water by stressed minerals might have helped to contribute oxygen to Earth's early atmosphere. Conceivably, H2O2 generated at rock-water interfaces could also have provided evolutionary pressure for local organisms to develop survival mechanisms in highly oxidizing microenvironments.

    Earth Planet. Sci. Lett. 283, 87 (2009).

  6. Human Genetics

    Imperfect Assortment

    1. Helen Pickersgill

    Cancer cells use multiple mechanisms to evade the normally stringent control on cell behaviors such as growth and differentiation. These mechanisms can involve chromosomal instability, a common occurrence in solid tumors whereby cells lose or gain whole chromosomes or parts thereof.

    Using microarray-based methods to analyze with high resolution the chromosomal changes in individual cells, Vanneste et al. have observed that chromosomal instability occurs at a remarkably high frequency in human cleavage-stage embryos. In 2 of 23 3- to 4-day-old embryos from young healthy couples, all of the cells isolated contained normal chromosomes. The remainder contained various numbers of cells with extra whole chromosomes or chromosomal fragments. Although this study analyzed embryos generated by in vitro fertilization, there may be in vivo implications, given that half of spontaneous abortions display chromosomal imbalances.

    Nature Med. 15, 577 (2009).

  7. Neuroscience

    I Hear What You're Saying

    1. Gilbert Chin

    The widespread adoption of multiple technologies for distinct channels of data communication—text, voice, and video—has made it abundantly clear to even the casual user that more bandwidth allows for higher rates of information transfer. But what happens on the receiving end? Presumably, recipients of phone calls are processing a lot more information, such as emotional overtones, than just the words that are spoken. Does this emotional content register in their brains? Ethofer et al. apply the method of multivariate pattern analysis and show that pseudowords spoken with five distinct emotional melodies (anger, sadness, relief, joy, or neutrality) do evoke recognizable neural responses within the auditory cortex. Each of these emotions could be discriminated against the others, and decoding algorithms trained on any nine of the speakers' voices were accurate in classifying the emotional identity of the tenth speaker's speech. Furthermore, the five distributed maps of neuronal activity segregated more closely to levels of arousal than valence, suggesting a possible affective organization within the auditory cortex.

    Curr. Biol. 19, 10.1016/j.cub.2009.04.054 (2009).