This Week in Science

Science  26 Apr 2013:
Vol. 340, Issue 6131, pp. 405
  1. Saturn's Meteoroid Crash

    During Saturn's equinox in 2009, when the Sun illuminated its rings edge-on, images taken by the Cassini spacecraft showed dust clouds appearing as bright streaks above the rings. Similar streaks were detected in 2005 and 2012 when Cassini observed the C ring at close range. Tiscareno et al. (p. 460) suggest that the cause of each observed feature is likely to be the impact of a stream of recently disrupted material originating from a meteoroid impact onto the ring and derive the influx rate of meteoroids at Saturn.

  2. Hot Enough to Melt Iron

    Earth's core is divided into a fluid outer core and a solid inner core, both composed predominately of iron at extremely high pressures and temperatures. The boundary between these two regions is largely controlled by the melting point of iron at ∼330 GPa, which in turn influences heat transfer and geodynamo generation. Anzellini et al. (p. 464, see the Perspective by Fei) compressed iron in a laser-heated diamond anvil cell, tracking its structure and texture by using time-resolved x-ray diffraction as the pressure increased to 200 GPa. The melting curve suggests the possibility of high heat flux and partial melting at the core-mantle boundary.

  3. Double Vision


    Vaterite is the least stable form of anhydrous crystalline calcium carbonate. While rarely found in geological contexts, it is an important biological precursor and occurs as a minor component in the shells of some organisms. The crystal structure of vaterite has long been debated with no model able to explain all the experimentally observed diffraction spots. Kabalah-Amitai et al. (p. 454) show that vaterite contains two coexisting crystallographic structures that form a pseudo-single crystal.

  4. Early Mayan Architects

    Lowland Mayan civilizations began to construct dramatic pyramids beginning about 900 BCE. The cultural origins of these civilizations have been uncertain, particularly the connection to earlier Olmec cultures along the Gulf of Mexico. Inomata et al. (p. 467; see the cover; see the News story by Pringle) describe excavations and extensive radiocarbon dating at the early Mayan city of Ceibal in Guatamala. The site shows incipient plaza and pyramid construction beginning before those seen in the lowlands or Olmec areas, suggesting a broader cultural exchange through to the Pacific Coast as Mayan cultures evolved.

  5. Where Parkin Parks

    Damaged mitochondria are removed from cells in a process known as mitophagy. Failure of this quality-control mechanism contributes to Parkinson's disease. When damaged mitochondria lose membrane depolarization, the protein kinase, PINK1, accumulates on the mitochondrial surface, recruits Parkin, and promotes mitophagy. Chen and Dorn (p. 471) describe another component of this process, mitofusin 2, which appears to function as the receptor for Parkin on the surface of damaged mitochondria.

  6. Viruses and Congenital Disorders

    Mutations in genes involved in α-dystroglycan O-linked glycosylation result in posttranslation modifications associated with the congenital disease Walker-Warburg syndrome (WWS). This cellular modification is also required for efficient Lassa virus infection of cells. Jae et al. (p. 479, published online 21 March) screened for genes involved in O-glycosylation that affected Lassa virus infection and identified candidates involved in glycosylation. Individuals from different pedigrees exhibiting WWS had unique mutations among genes identified in the genetic screen. Thus, comprehensive forward genetic screens can be used to define the genetic architecture of a complex disease.

  7. Animal Culture

    Cultural transmission of information occurs when individuals learn from others with more experience or when individuals come to accept particular modes of behavior as the local norm. Such information transfer can be expected in highly social or long-lived species where contact and time for learning are maximized and are seen in humans (see the Perspective by de Waal). Using a network-based diffusion analysis on a long-term data set that includes tens of thousands of observations of individual humpback whales, Allen et al. (p. 485) show that an innovative feeding behavior has spread through social transmission since it first emerged in a single individual in 1980. The “lobtail” feeding has passed among associating individuals for more than three decades. Van de Waal et al. (p. 483), on the other hand, used a controlled experimental approach in vervet monkeys to show that individuals learn what to eat from more experienced individuals within their social group. Not only did young animals learn from observing older animals, but immigrating males switched their food preference to that of their new group.

  8. Daylight Determines Dopamine


    Expression of the appropriate neurotransmitters is essential for the function of neural circuits. Can neurons change their transmitter phenotype to deal with alterations in the environment? Dulcis et al. (p. 449; see the Perspective by Birren and Marder) exposed adult rats to different photoperiods mimicking summer and winter daylengths. Neurotransmitter expression switched between dopamine and somatostatin in hypothalamic neurons that regulate release of corticotropin-releasing factor. Transmitter switching occurred at the transcriptional level and was accompanied by changes in postsynaptic receptors.

  9. Pulsar Tests Gravity


    Because of their extremely high densities, massive neutron stars can be used to test gravity. Based on spectroscopy of its white dwarf companion, Antoniadis et al. (p. 448) identified a millisecond pulsar as a neutron star twice as heavy as the Sun. The observed binary's orbital decay is consistent with that predicted by general relativity, ruling out previously untested strong-field phenomena predicted by alternative theories. The binary system has a peculiar combination of properties and poses a challenge to our understanding of stellar evolution.

  10. How Copper Clicks

    The copper-catalyzed coupling of azides and alkynes has been termed a “click” reaction on account of its efficiency and versatility, but despite its widespread use, the mechanism remains somewhat unclear. Through a series of kinetic and isotopic labeling studies, Worrell et al. (p. 457, published online 4 April) show that, in the case of terminal alkynes (capped at one end by an H atom), two equivalents of copper participate in activating each molecule's reactivity toward azide. Surprisingly, the reaction also appears to proceed through an intermediate in which the two copper centers become equivalent and functionally exchangeable, despite initially coordinating to distinct sites on the alkyne.

  11. Tracking Secreted Proteins

    The proteins secreted by cells provide a flow of information within tissues and are thus of particular interest. However, systematic detection of secreted proteins is tricky because they tend to be present in small amounts within complex mixtures of proteins where other components are very abundant. Meissner et al. (p. 475) developed a method for screening the secreted proteins from cultured mouse macrophages in response to cues that cause inflammation. The amount of contaminating proteins was reduced by culturing the cells without added serum and then sensitive mass spectrometry techniques were used to detect and quantify secretion of nearly 800 different proteins. Secretion was compared from cells lacking the signaling adaptor proteins MyD88 or TRIF, or both. Secretion of some proteins were regulated redundantly and were secreted without one of the adaptors, but others required both signals for release. Some anti-inflammatory proteins were released at later times in response to synergistic signals from both adaptor proteins, perhaps as a fail-safe mechanism to prevent excessive inflammation.

  12. Balancing Act

    Life cycles are strongly influenced by seasonal and interannual environmental and climate events. Such phenological timings are likely to shift as our climate changes, but species exist in communities, and not all species can be expected to shift in concert. Reed et al. (p. 488) used long-term data on European great tits to reveal how the negative consequences of phenological mismatch can be buffered: Lower fitness in individuals subject to mismatch-driven reductions of food availability was balanced by reductions in competition. Thus, overall, the population is resilient and has not declined.

  13. One Protein, Two Probes

    A central challenge in the use of x-ray diffraction to characterize macromolecular structure is the propensity of the high-energy radiation to damage the sample during data collection. Recently, a powerful accelerator-based, ultrafast x-ray laser source has been used to determine the geometric structures of small protein crystals too fragile for conventional diffraction techniques. Kern et al. (p. 491, published online 14 February) now pair this method with concurrent x-ray emission spectroscopy to probe electronic structure, as well as geometry, and were able to characterize the metal oxidation states in the oxygen-evolving complex within photosystem II crystals, while simultaneously verifying the surrounding protein structure.

  14. A Wing by Another Name?

    Wings and their derivatives on the second and third thoracic segments represent the only known dorsal appendages in modern insects; however, the existence of wing homologs in non-winged segments has been suggested from fossil insects. Ohde et al. (p. 495, published online 14 March) identify part of the body wall on the first thoracic segment and pupal dorsolateral outgrowths as modified pairs of wings in the mealworm beetle. These organs could be transformed into wings and thus share the central wing developmental mechanism.

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