Editors' Choice

Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 10
  1. Evolution

    Cicada Cycles

    1. Andrew M. Sugden

    The bizarre life cycle of the cicadas (genus Magicada) of eastern North America entails many years spent underground as larvae, before emerging synchronously as adults for a few brief weeks of sexual activity. Southern species have 13-year life cycles, whereas northern species have 17-year cycles. At any location, there are generally three species groups, all of which are synchronized with each other. The species groups are thought to have diverged allopatrically (i.e. geographically separated) before becoming sympatric as they are now. Several selective factors leading to the evolution of these life history patterns have been hypothesized, but in the absence of comprehensive phylogenetic data, they have proved difficult to test. Sota et al. have now addressed this gap, by conducting a thorough phylogenetic analysis of all known Magicicada species using nuclear and mitochondrial DNA markers. The resulting combination of phylogenetic and geographical patterns suggests a complex history of evolution—some of it quite recent—accompanying the habitat shifts in North America during the Pleistocene glacial cycles. The results are consistent with the allopatric divergence model and show that the splits into 13-year and 17-year species occurred multiple times within each species group. It appears that selection favored the evolution of synchronized population cycles between invading and resident species, simultaneously affording invaders protection from predation and avoidance of the adverse reproductive consequences of low population density.

    Proc. Natl. Acad. Sci. U.S.A., 110, 10.1073/pnas.1220060110 (2013).

  2. Immunology

    Feeding Hungry T Cells

    1. Kristen L. Mueller

    In order to mount productive responses against pathogens, T cells need to be fed. That is, the metabolic demands of T cell increase upon activation and as a result, T cells need to take up more amino acids. How this process is regulated, however, is not well understood. Sinclair et al. investigate and find that the System L amino-acid transporter, Slc7a5, which mediates the uptake of large neutral amino acids (LNAAs) such as leucine, was required for T cell activation in mice. In response to activation, T cells increased their expression of Slc7a5 and their uptake of LNAAs in a Slc7a5-dependent manner. T cells from mice with a T cell–specific deletion of Slc7a5 did not proliferate well or acquire effector cell functions in response to activating stimuli. This was correlated with a failure to activate the serine-threonine kinase complex mTORC1 and to induce protein expression of the transcription factor c-Myc, both important components required to mediate the energetic demands of activated T cells.

    Nat. Immunol. 14, 10.1038/ni.2556 (2013).

  3. Planetary Science

    Vortex Complexity

    1. Maria Cruz

    The European Space Agency Venus Express spacecraft has been orbiting the second planet from the Sun since April 2006. Understanding its atmosphere has been one of the main goals of the mission. Using data from the Visible and Infrared Thermal Imaging Spectrometer onboard the spacecraft, Garate-Lopez et al. examined the planet's south polar vortex by measuring cloud motions at two different altitudes. The data show that the vortex is long-lived, but its dynamical behavior and thermal structure are more complex than previously thought.

    Nat. Geosci. 10.1038/ngeo1764 (2013).

  4. Cell Biology

    Polo's Game in Meiosis

    1. L. Bryan Ray

    The protein kinase Polo functions to promote cell division and is overactive in some cancer cells. Bonner et al. demonstrate a distinct role of the enzyme in regulation of meiosis: In this case, cells that fail to inhibit activity of Polo show improper chromosome segregation in meiosis or complete failure of meiosis. The protein Matrimony (Mtrm) directly interacts with Polo and appears to quench its activity. It appears that this may be the primary role of Mtrm in the cell, because the genetic effects of loss of Mtrm can be restored in cells in which Polo is also depleted. Phosphorylation-dependent binding of Polo to its targets is well characterized, but the interaction with Mtm occurs in a different manner that may be more characteristic of Polo's interaction with its regulatory partners. Two domains of Mtrm appear to trap Polo and restrain its activity. Both proteins accumulate during meiosis I, and if Mtrm is lacking, proper meiosis is not achieved.

    Proc. Natl. Acad. Sci. U.S.A. 110, E1222 (2013).

  5. Physics

    Purely Magnetic Impurities

    1. Jelena Stajic

    Magnetically doped topological insulators (TIs) have been predicted to exhibit a host of exotic properties stemming from the breaking of time-reversal symmetry caused by magnetic impurities. However, dopants such as iron or manganese also have a charge effect, which distorts the original electronic structure and makes it difficult to isolate the effects of magnetism. Schlenk et al. devised a procedure for magnetic doping that aims to circumvent this problem and which should help to provide insight into the effects of time-reversal symmetry-breaking in TIs. They deposited Fe atoms on the surface of cold samples of the topological insulator Bi2Se3 and then annealed them at much higher temperatures. Scanning tunneling microscopy indicated that the annealing caused the dopants, initially adatoms, to descend into the material; angle-resolved photoemission found the electronic structure, initially distorted by depositing the adatoms, to be restored after the annealing step. First-principles calculations further indicated that the Fe atoms were probably substituting Bi atoms, and that whereas adatoms acted as electron donors, the buried Fe atoms were either neutral or acceptors; the magnetic interaction with the host material was also found to be enhanced with respect to the case of adatoms.

    Phys. Rev. Lett. 110, 126804 (2013).

  6. Ecology

    Marsh Basin Dynamics

    1. Guy Riddihough

    Coastal salt marshes—found at temperate and high latitudes—form on tidal flats. Such marshes are both ecologically important and highly biologically productive. Although often protected, their fragile nature and location make them particularly vulnerable to human activities and climate change. Mariotti and Fagherazzi analyze marsh basins—rounded tidal flats surrounded by salt marshes—at three sites on the U.S. Atlantic coast, as enlargement of marsh basins can drive marsh loss. They develop a simple dynamic model for the morphological evolution of marsh basins and show that the model has a single unstable equilibrium point for basin size. Below this size, basins shrink, and above it, they continuously enlarge. Aerial photos taken 50 years apart at the three locations reveal that for particular basin locations, the marsh boundary will grow and the basin will shrink, whereas at other locations, all basins are shrinking. The model suggests that sediment supply is the primary controlling factor for marsh boundary growth or recession, and historical data from the three locations, where river dredging has in some cases significantly influenced sediment supply, support this idea.

    Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1219600110 (2013).

  7. Geochemistry

    Unlocking Sulfur's Secrets

    1. Nicholas S. Wigginton

    The sulfur isotope signatures of sediments and aerosol particles provide a rich record of biological and chemical processes in modern environments and in the geologic record. For example, the ratio of two or more of sulfur's several stable isotopes can indicate whether it was enzymatically reduced by microorganisms or if it underwent photolysis in the atmosphere after exposure to ultraviolet (UV) radiation. There are myriad proposed mechanisms to explain the underlying reactions that induce isotopic fractionation; disentangling them from one another is critical for interpreting their preservation. In a series of photochemical experiments, Ono et al. demonstrate that the partial pressure of SO2 strongly influences the sulfur mass-independent fractionation, at least in part due to self-shielding from UV photolysis by other SO2 molecules with a different isotopic composition. The experimental isotopic patterns are consistent with sulfate aerosols generated in the stratosphere, where UV photolysis is mostly confined; however, around 3 billion years ago, Earth's atmosphere may have been much more transparent to UV radiation, creating a more extensive record of mass-independent fractionation. Farquhar et al. present an explanation for why the sulfur isotope signal generated by sulfate-reducing microorganisms—which were thought to have been present at that time—does not overprint mass-independent fractionation signals generated in Earth's early atmosphere. Spot analysis of sedimentary pyrites from Archean rocks suggests that there were two distinct sulfur pools contributing to most isotopic signatures: one of soluble oceanic sulfate and one of zero-valent sulfur derived from atmospheric deposition.

    J. Geophys. Res. 118, 10.1002/jgrd.50183; Proc. Nat. Acad. Sci. U.S.A. 110, 10.1073/pnas.1218851110 (2013).

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