This Week in Science

Science  21 Dec 2012:
Vol. 338, Issue 6114, pp. 1508
  1. The Meteor That Fell to Earth

    CREDIT: NASA, E. H. JAMES/P. JENNISKENS

    In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutter's Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed.

  2. Whence Species Variation?

    Vertebrates have widely varying phenotypes that are at odds with their much more limited proteincoding genotypes and conserved messenger RNA expression patterns. Genes with multiple exons and introns can undergo alternative splicing, potentially resulting in multiple protein isoforms (see the Perspective by Papasaikas and Valcárcel). Barbosa-Morais et al. (p. 1587) and Merkin et al. (p. 1593) analyzed alternative splicing across the genomes of a variety of vertebrates, including human, primates, rodents, opossum, platypus, chicken, lizard, and frog. The findings suggest that the evolution of alternative splicing has for the most part been very rapid and that alternative splicing patterns of most organs more strongly reflect the identity of the species rather than the organ type. Species-classifying alternative splicing can affect key regulators, often in disordered regions of proteins that may influence protein-protein interactions, or in regions involved in protein phosphorylation.

  3. Enhancing Heart Function

    The epicardium, a protective layer of tissue surrounding the mammalian heart, plays a critical role during embryogenesis because it supplies growth factors and multipotent progenitor cells essential for heart development. In adults, the epicardium is dormant but it becomes reactivated when the heart is injured, a response that leads to re-expression of developmental genes. Studying mouse models, Huang et al. (p. 1599, published online 15 November; see the Perspective by Rosenzweig) found that the C/EBP transcription factors activated the epicardium during development and injury. Blockade of C/EBP signaling in the epicardium of injured (ischemic) hearts reduced inflammation and improved heart function, a finding that could ultimately lead to new strategies for the repair of heart damage.

    CREDIT: BERG ET AL.
  4. Symmetry Semantics

    Topological insulators (TIs) are characterized by boundary states that are protected by time-reversal symmetry. A systematic study of this, and other symmetry-protected states, is possible in noninteracting systems, but complications arise when interactions are present. Chen et al. (p. 1604; see the Perspective by Qi) used group cohomology theory to predict symmetry-protected phases of interacting bosons. The analysis enabled the generalization of a known result in one dimension by using a path-integral formulation and suggests the existence of three counterparts of TIs in three dimensions, and one in two dimensions, as well as phases protected by other symmetries. The formalism is applicable to any symmetry group and dimension and is valid for interactions of arbitrary strength.

  5. Getting Rid of Negativity

    Problems in condensed matter physics involving strong interactions are notoriously hard to tackle analytically, and physicists often resort to numerical methods such as quantum Monte Carlo (QMC). However, in the most interesting cases, this method becomes computationally intractable because partition sums for fermionic systems generally involve integration over an oscillatory function with both negative and positive values. Berg et al. (p. 1606) find a way around this problem for a two-dimensional metal in the vicinity of an antiferromagnetic quantum phase transition, which is of relevance to electron-doped cuprates, iron-based superconductors, and heavy fermion compounds. Their lattice theory results in a positive integrand for the partition sums; thus, avoiding the sign problem; and yields the expected competition between antiferromagnetic order and unconventional superconductivity near the critical point.

  6. Inducing a Quiet Space

    The interaction between light and matter forms the foundation of many applications in communication and sensing, as well as provides insights into fundamental quantum-level processes. Optical coupling of a mechanical system can be used to study these processes. However, because the mechanical oscillator is unavoidably coupled to its environment, thermal noise can spoil the sensitivity of the optomechanical coupling. Dong et al. (p. 1609, published online 15 November) exploit the ability to form a mechanical “dark state” that can effectively isolate the mechanical system from thermal-induced noise. The formation of such a noise-free zone may provide a simpler route to probe quantum optomechanical systems that circumvents the need to cool the oscillator to its quantum limit where all thermal motion is frozen out.

  7. Earning a High Grade

    Most of the world's copper and molybdenum come from porphyry-type ore deposits in Earth's crust. The metals are deposited either as veins of concentrated metals in fractured rock or in a confined shell, associated with the edges of magma chamber plumes. But it remains unclear why a front of sharp temperature-pressure gradients, which allows the accumulation of high-grade metal deposits, remains stable. Weis et al. (p. 1613, published online 15 November; see the Perspective by Ingebritsen) constructed a hydrothermal model of a porphyry-type system given a supply of magmatic fluids with high metal concentrations. The thermodynamic properties of injected volatile fluids and dynamic variations in host rock permeability, driven by injection-induced fracturing, controlled the stability and evolution of the fronts and allowed for the formation of an extensive network of ore veins from within the magmatic chamber.

  8. A Grand Old Canyon

    In the southwestern United States, the Grand Canyon is a striking example of the power of erosion over time. Over millions of years, flowing river water carved out the canyon that today measures over 1.6 km deep and 29 km long. Most models posit that the majority of the canyon formed 5 to 6 million years ago. Using thermochronometry, Flowers and Farley (p. 1616, published online 29 November) examined the temperature-dependent diffusion of helium within mineral grains representative of the canyon basement, which cools as erosion brings crustal rocks near the surface. After validating the approach across the younger eastern canyon, the model suggests that the western canyon experienced an ancient cooling event induced by erosional processes, such that the canyon likely reached near modern depths by 70 million years ago—nearly 60 million years earlier than generally believed.

  9. Influenza Revealed

    CREDIT: ARNE MOELLER

    Influenza virus, a single-stranded RNA virus, is responsible for substantial morbidity and mortality worldwide. The influenza ribonucleoprotein (RNP) complex, which carries out viral replication and transcription, is central to the virus life-cycle and to viral host adaptation (see the Perspective by Tao and Zheng). Structural characterization of the viral RNP has been challenging, but Moeller et al. (p. 1631, published online 22 November) and Arranz et al. (p. 1634, published online 22 November) now report the structure and assembly of this complex, using cryo-electron microscopy and negative-stain electron microscopy. The structures reveal how the viral polymerase, RNA genome, and nucleoprotein interact in the RNP providing insight into mechanisms for influenza genome replication and transcription.

  10. Autism Genes, Again and Again

    Despite recent advances in sequencing technologies and their lowered costs—effective, highly sensitive, and specific sequencing of multiple genes of interest from large cohorts remains expensive. O'Roak et al. (p. 1619; published online 15 November) modified molecular inversion probe methods for target-specific capture and sequencing to resequence candidate genes in thousands of patients. The technique was applied to 44 candidate genes to identify de novo mutations in a large cohort of individuals with and without autism spectrum disorder. The analysis revealed several de novo mutations in genes that together contribute to 1% of sporadic autism spectrum disorders, supporting the notion that multiple genes underlie autism-spectrum disorders.

  11. Single-Cell Sequencing

    With the rapid progress in sequencing technologies, single-cell sequencing is now possible, promising insight into how cell-to-cell heterogeneity affects biological behavior. Achieving adequate genome coverage remains a challenge because single-cell sequencing relies on genome amplification that is prone to sequence bias. Zong et al. (p. 1622) report a new amplification method: multiple annealing and looping-based amplification cycles that allowed 93% genome coverage for a human cell. This coverage facilitated accurate detection of point mutations and copy number variations. Lu et al. (p. 1627) used the method to sequence 99 sperm cells from a single individual. Mapping the meiotic crossovers revealed a nonrandom distribution with a reduced recombination rate near transcription start sites.

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