This Week in Science

Science  21 Mar 2014:
Vol. 343, Issue 6177, pp. 1288
  1. There Goes the Neighborhood

    A mass-accreting black hole in steady-state cannot produce more radiative energy than its gravity can counterbalance, achieving what is known as the Eddington limit. However, mass accretion can also be converted into kinetic energy via mechanical outflow. Using x-ray observations, Soria et al. (p. 1330, published online 27 February; see the Perspective by King and the cover) identified a compact shock-ionized radio/optical nebula in spiral galaxy M83, powered by a black hole, inferred that the black hole emits a spherical wind that exceeds the Eddington limit tenfold and succeeded in estimating it's mass in the range of 5 to 15 solar masses. It is possible that rapidly accreting black holes have greater influence on their host galaxy than once appreciated.

  2. Sourcing Martian Meteorites

    There are nearly 150 recognized martian meteorites, but where exactly they came from on Mars is not known. Werner et al. (p. 1343, published online 6 March) present evidence that the <5 million-year-old Mojave impact crater on Mars is the single ejection site of one type of martian meteorites: the shergottites. The Mojave crater formed on an ancient terrain on Mars, and so the shergottites represent old martian crustal material.

  3. Productive Dustiness


    The idea that biological productivity in the surface ocean is limited by a lack of available iron has been widely accepted, but it has been difficult to show that this effect might have operated in the geological past. Martínez-García et al. (p. 1347) investigated the isotopic composition of foraminifera-bound nitrogen in samples from an Ocean Drilling Project sediment core and found millennial-scale changes in nitrate consumption correlated with fluxes in the iron burial and productivity proxies over the past 160,000 years. Hence, in the Southern Ocean the biological pump was strengthened when dust fluxes were high, which explains a significant part of the difference in atmospheric CO2 concentrations observed to occur across glacial cycles.

  4. Cytologically Informative Fossils


    Fossilization processes tend to destroy fine-cell structure but, exceptionally, Bomfleur et al. (p. 1376) have found examples of fossil ferns from the Jurassic in which subcellular structures, including organelles such as nuclei and chromosomes, are well-preserved. Comparative and quantative analyses show that these cells closely resemble the fossil nuclei of extant cinnamon ferns, Osmundastrum cinnamomea, which indicates that this group of ferns has remained virtually unchanged for 180 million years.

  5. All the Smells of the World

    How many odorant stimuli can a normal human being discriminate? During psychophysical tests of odor mixture discrimination, Bushdid et al. (p. 1370) were surprised to find that humans can discriminate among more than a trillion different smells. Because the authors reduced the complexity by investigating only mixtures of 10, 20, or 30 components drawn from a collection of 128 odorous molecules, this astonishingly large number is probably the lower limit of the potential number of olfactory stimuli that humans can distinguish.

  6. Predation Favors Parasitism

    Parasitism in birds often results in ejection or starvation of the host's nestlings. Consequently, many host bird species have evolved protective behavior such as mobbing and parasite egg rejection. Curiously, some host species show no parasite avoidance behaviors; for example, the crow Corvus corone corone tolerates cuckoo chicks among its own brood. In a long-term study, Canestrari et al. (p. 1350) found that crow nests containing a cuckoo chick had lower rates of predation because the parasite's chicks secrete a noxious repellent substance. Overall, in years of high predation pressure, the presence of cuckoos improves the crow's breeding success, but when there are fewer predators around, parasitism reduces crow fitness.

  7. Giving Electrocatalysts an Edge

    Platinum (Pt) is an excellent catalyst for the oxygen-reduction reaction (ORR) in fuel cells and electrolyzers, but it is too expensive and scarce for widespread deployment, even when dispersed as Pt nanoparticles on carbon electrode supports (Pt/C). Alternatively, Chen et al. (p. 1339, published online 27 February; see the Perspective by Greer) made highly active ORR catalysts by dissolving away the interior of rhombic dodecahedral PtNi3 nanocrystals to leave Pt-rich Pt3Ni edges. These nanoframe catalysts are durable—remaining active after 10,000 rounds of voltage cycling—and are far more active than Pt/C.

  8. Coordinated Hair Growth

    Wnt/β-catenin signaling is a key pathway that plays a conserved role in regulating stem cell function during adult tissue regeneration. Using time-lapse imaging of live mice, Deschene et al. (p. 1353) show that genetic activation of β-catenin within hair follicle stem cells generates axes of hair growth by coordinated cell divisions and cell movements, even when the normal niches—the dermal papillae—are laser-ablated. Activated β-catenin enhances Wnt ligand secretion, and these ligands can then activate Wnt signaling in adjacent cells that do not have activated β-catenin, indicating how activated stem cells could influence neighboring cells during normal growth and in cancer.

  9. HIV Virulence

    A major focus of research on HIV is on host responses to infection—understandably, because the virus targets the immune system and because of the interest in vaccine development. In reviewing what little research has been done on viral virulence determinants, Fraser et al. (10.1126/science.1243727) present evolutionary explanations for some of the poorly understood phenomena that mark HIV infection, including long-term survivorship, latency, rapid within-host evolution, and inheritability of between-host virulence.

  10. Oscillate to Synchronize

    In the social amoeba Dictyostelium discoideum, periodic waves of the small-molecule cyclic adenosine monophosphate (cAMP) guide chemotactic migration and cell differentiation. Cai et al. (10.1126/science.1249531; see the Perspective by Wollman) found that a GATA family transcription factor shuttles between the nucleus and the cytoplasm of the amoeba in response to oscillatory cAMP signals acting through G protein–coupled receptors. Each oscillation generates a transient burst of gene activation, such that gene expression is linked to the number rather than the level of stimulus, which then brings about transcriptional synchrony in populations of cells.

  11. Ultrafast Manipulation

    Multiferroic materials commonly show both magnetism and ferroelectricity, such that the electric field can be used to manipulate the magnetic order, and vice versa. Kubacka et al. (p. 1333, published online 6 March) used a strong terahertz electromagnetic pulse in resonance with an electromagnon—an excitation based on both electric and magnetic ordering—to control the spin dynamics of the multiferroic TbMnO3 on a sub-picosecond time scale and induce the rotation of the spin-cycloid plane of the material.

  12. The Cuprate Pseudogap

    The properties of copper-oxide superconductors are changed by chemical doping, but, if doping is suboptimal, the transition temperature Tc drops. Conversely, the so-called pseudogap, a depression in the density of states around the Fermi level that may or may not be related to superconductivity, gains strength. The cuprate YBa2Cu3O6+x shows a charge density order that grows as Tc is approached from both low and high temperatures. Hayward et al. (p. 1336) have developed a model in which classical fluctuations of a six-component order parameter, encompassing both superconducting and charge orders, reproduce the characteristic concave temperature dependence of the x-ray scattering intensity and thus provide a framework for the understanding of the pseudogap regime.

  13. Life and Cell Death

    Trying to protect animals from one form of cell death may lead to death by another. Two protein kinases, known as RIPK1 and RIPK3 promote signaling that leads to cell death by necroptosis. However, Newton et al. (p. 1357, published online 20 February; see the Perspective by Zhang and Chan) found that inhibition of RIPK3 was not always beneficial. Instead, mice expressing a form of RIPK3 with no catalytic activity died from increased apoptotic cell death, but animals lacking the RIPK3 protein entirely, did not die perhaps because RIPK3 restrains apoptosis mediated by caspase-8 by an independent mechanism.

  14. Transcripts Visualized in Situ

    Despite advances, current methods for single-cell sequencing are unable to resolve transcript location within the cell, so Lee et al. (p. 1360, published online 27 February) developed a method of fluorescent in situ RNA sequencing (FISSEQ) that works in vivo to show messenger RNA localization within cells. The method amplifies complementary DNA targets by rolling circle amplification, and then in situ cross-linking locks amplicons to produce ample, highly localized templates for three-dimensional sequencing. The technique was tested in fibroblasts to reveal the differences between individual cells during wound repair.

  15. Serial Mutation

    Mutations that affect gene function and, ultimately, the phenotype of an organism are grist to the mill of evolution. While examining the genetic basis for a stable polymorphism observed in bacteria during a long-term mutation experiment, Plucain et al. (p. 1366, published online 6 March) identified three specific, successive mutational events exhibiting synergistic epistatic and frequency-dependent interactions that enabled one lineage to invade the other and to be maintained. Thus, a series of specific mutations conferred the invasion phenotype and allowed the use of novel resources only when all mutations were present.

  16. Translocation in Injury


    The translocator protein TSPO is essential for the import of cholesterol and porphyrins into mitochondria. TSPO expression increases in areas of brain injury and during neuroinflammation and, thus, has diagnostic and therapeutic implications. Jaremko et al. (p. 1363) used nuclear magnetic resonance spectroscopy to determine the high-resolution structure of the 18- kilodalton mammalian TSPO with the ligand PK11195, which stabilized the structure and resolved the conformation as a tight bundle of five helices.

  17. Real Network Control

    Understanding how complex networks are controlled has implications for a variety of real-world networks, from traffic safety to transcriptional control. Ruths and Ruths (p. 1373; see the Perspective by Onnela) have developed a theoretical framework for analyzing individual controls within networks based on numbers of sources and sinks for information flow. By this method, the number of controls required by a network can be predicted and direct comparisons for the basis for control across networks of differing size, structure, and function can be made. Although three broad classes of real networks were observed, current, established random models of networks were insufficient to model their control structures.