This Week in Science

Science  19 Jul 2013:
Vol. 341, Issue 6143, pp. 214
  1. More Magnetoresistance


    When data is read off your computer's hard drive, chances are that the read head is using the phenomenon of magnetoresistance (MR)—the dependence of electrical resistance on applied magnetic field—to interpret the magnetic signature of the data on the disk. Devices that have the large MR necessary for such tasks are usually made of layers of magnetic materials. Mahato et al. (p. 257, published online 4 July) observed a large MR effect in a nonmagnetic material—organic molecules squeezed into a zeolite crystal. Importantly for potential future applications, the effect was observed at room temperature and at low magnetic fields.

  2. Accretion Analog

    Mass flow from a circumstellar disk onto a young star's surface plays an important role in the final stages of star formation but the details of this complex process are not well understood. Reale et al. (p. 251, published online 20 June) analyzed a solar flare that led to bright impacts of plasma onto the solar surface. Numerical simulations suggest that these events can be seen as analogs to accretion of matter onto stars and can thus be used to understand stellar accretion.

  3. Mars' Atmosphere from Curiosity

    The Sample Analysis at Mars (SAM) instrument on the Curiosity rover that landed on Mars in August last year is designed to study the chemical and isotopic composition of the martian atmosphere. Mahaffy et al. (p. 263) present volume-mixing ratios of Mars' five major atmospheric constituents (CO2, Ar, N2, O2, and CO) and isotope measurements of 40Ar/36Ar and C and O in CO2, based on data from one of SAM's instruments, obtained between 31 August and 21 November 2012. Webster et al. (p. 260) used data from another of SAM's instruments obtained around the same period to determine isotope ratios of H, C, and O in atmospheric CO2 and H2O. Agreement between the isotopic ratios measured by SAM with those of martian meteorites, measured in laboratories on Earth, confirms the origin of these meteorites and implies that the current atmospheric reservoirs of CO2 and H2O were largely established after the period of early atmospheric loss some 4 billion years ago.

  4. Major Meltdown

    The ice shelves and floating ice tongues that surround Antarctica cover more than 1.5 million square kilometers—approximately the size of the entire Greenland Ice Sheet. Conventional wisdom has held that ice shelves around Antarctica lose mass mostly by iceberg calving, but recently it has become increasingly clear that melting by a warming ocean may also be important. Rignot et al. (p. 266, published 13 June) present detailed glaciological estimates of ice-shelf melting around the entire continent of Antarctica, which show that basal melting accounts for as much mass loss as does calving.

  5. Ancient Warriors or Murderers?

    Some have suggested that the human predilection for war is ancient, perhaps dating back to the emergence of our species, while others maintain that evidence for such early warring is scant. Past studies that looked at nomadic foraging bands as models of early humans and their potential for conflict concluded that war is in our blood. Fry and Söderberg (p. 270), however, reexamined the standard cross-cultural sample, the main repository for behavioral data on forage bands, and found little evidence for large-scale conflicts or wars. Instead, the majority of incidences of lethal aggression in these societies were homicides driven by a variety of factors relevant at the individual or family scale.

  6. Accessory to Obesity?

    Melanocortin receptors are a family of cell membrane receptors that control diverse physiological functions. Mutations in the gene encoding melanocortin 4 receptor (MC4R) are a cause of familial early-onset obesity. Asai et al. (p. 275) studied the function of an accessory protein for MC4R signaling, MRAP2, and found that mice genetically deficient in MRAP2 develop severe obesity. Sequencing of MRAP2 in unrelated, severely obese humans revealed one individual with a clearly disruptive genetic variant, suggesting that MRAP2 mutations might also be a rare cause of human obesity. In a zebrafish model, Sebag et al. (p. 278) studied two paralogs of the MRAP2 accessory protein, one of which enhanced MC4R responsiveness to α–melanocyte-stimulating hormone, which regulates feeding and growth.

  7. Romancing the Frog


    In túngara frogs, auditory and visual components of mate calling do not naturally occur together. Taylor and Ryan (p. 273, published online 6 June) now show that two signals that are unattractive to female frogs when presented alone become highly attractive when presented together. In a kind of “perceptual rescue,” the unique combination of two signals increased the receiver's interest in the previously uninteresting signals.

  8. Stem Cells in Wound Healing

    Although excessive numbers of stem cells (SCs) may increase the risk of cancer, elevated SC numbers may be desirable, at least transiently, for the promotion of tissue repair and regeneration. Fuchs et al. (p. 286, published online 20 June) found that mice deficient for the proapoptotic Sept4/ARTS gene have elevated numbers of apoptosis-resistant hair follicle SCs and display dramatic improvement in wound healing and regeneration. Inactivation of the caspase inhibitor XIAP, a direct target for the proapoptotic activity of ARTS, abrogated these phenotypes and impaired wound healing.

  9. Setting Hydrogen Free

    Oxidation of organic compounds has traditionally been considered to involve the transfer of hydrogen atoms in the molecular framework to an oxidant such as O2, peroxide, or a metal oxide complex. Gunanathan and Milstein (1229712) review the ongoing development of an alternative process, in which a catalyst coaxes the H atoms to depart on their own in the form of H2. These acceptorless dehydrogenations are appealing because they generate so little waste. In one class of reactions, the liberated H2 gas is actively expelled from the reaction mixture and collected for potential use elsewhere. In another class, the H atoms return to the source molecule after it has undergone an intermediate transformation in their absence.

  10. Magnetic Self-Assembly


    During self-assembly, objects spontaneously assemble into larger ordered patterns as observed, for example, in the phase segregation of block copolymers or the assembly of micrometer-sized objects and components in electronics. In dynamic self-assembly, the ordered patterns require an external energy source, but still form because of intrinsic interactions within the system. Timonen et al. (p. 253; see the Perspective by Hermans et al.) studied the organization of magnetic droplets, in the form of a ferrofluid, placed on a low-friction surface. A time-varying magnetic field transformed the statically arranged droplets into a dynamic pattern.

  11. Sensing Reduced Translation

    The interplay between metabolic pathways and the cellular survival programs that enable tumors to grow are poorly understood. Heat shock factor 1 (HSF1) coordinates an unexpectedly diverse transcriptional network involved in oncogenesis. Santagata et al. (1238303; see the Perspective by Gandin and Topisirovic) found that reduced translation may be used to sense a cell's metabolic status and regulate transcription, in particular by inactivating HSF1 with consequent affects on its targets. Small-molecule drugs that affected this link were able to inhibit the growth of transformed cells in culture and of an animal tumor model.

  12. Zeus' Revenge

    Sediment-dwelling amoebae appear to have an unhappy affinity for huge viruses. Giant icosahedral Mimiviruses with genomes of the order of 1 megabase (Mb) were first identified in Acanthamoeba. Digging into antipodean sediments has once again been fruitful where Philippe et al. (p. 281; see the cover) discovered some enormous viruses in Acanthamoeba, visible by light microscopy and having genomes up to 2.5 Mb. The Pandoraviruses are phagocytosed by target cells and, after fusing with the phagosome membrane, their contents are released into the cytoplasm where they wreak terrible havoc on its nucleus. These viruses are encased into a tegument-like envelope and lack genes for capsid proteins, and there are no genes for protein translation, adenosine triphosphate generation, or binary fission—confirming their classification as viruses.

  13. Background Extinction

    Diversity results through both the processes of species origination and extinction. However, studies of extinction have tended to focus on mass extinctions, despite the fact that the background extinction represents a greater loss in terms of the absolute number of extinct taxa. In order to identify what factors affect this rate of background extinction, Quental and Marshall (p. 290, published online 20 June) explored the dynamics of 19 mammalian clades and compared the rates of expansions and declines among taxa to expected models assuming random processes. Most clades decline to extinction in a “driven” manner—that is, faster than expected by chance alone.

  14. Playing the Tape of Life

    Should the tape of life be replayed, would it produce the same music? Many influential evolutionary biologists, notably Stephen J. Gould, have argued that the answer is “no.” However, patterns of convergence among different species filling similar niches all over the world have argued that the answer is neither so simple nor perhaps so negative. Classic cases of convergence, such as marsupials on the Australian continent or cichlids across the African rift lakes, have demonstrated that similar ecological pressures can result in species with similar ecological traits. Such classic examples, however, do not allow for the influence of niche filling based purely on chance. Mahler et al. (p. 292) take advantage of the well-studied species clades of Caribbean anoles to examine patterns of adaptation and niche filling across species and islands. Across-islands convergence on a few distinct adaptive peaks (or niches) has driven diversification of species. Anomalies from these ecotypes are only found on the largest, most diverse islands. Thus, ecological niches powerfully shape species and convergence on particular forms is an inherent component of adaptation. Thus, it seems that the tape of life might play the same music, despite being produced by different instruments.

  15. Digoxin Dangers

    A proportion of patients treated with digoxin, a cardiac glycoside used to treat heart function abnormalities, generate the inactive metabolite, dihydrodigoxin, resulting in poor efficacy. Haiser et al. (p. 295) examined a potential culprit responsible for this transformation—the actinobacterium, Eggerthella lenta—to probe the microbiota-digoxin interaction. Microbe growth was promoted by arginine, and differential expression analysis revealed a two-gene cardiac glycoside reductase (cgr) operon that was induced by digoxin in low arginine conditions. Not all strains of E. lenta could reduce digoxin and, when fecal samples from healthy people were tested, a spectrum of digoxin inactivation was detected. When the digoxin-reducing strain of E. lenta was given to germ-free mice that were fed a high-protein (that is, high-arginine) diet, digoxin levels stayed high in serum, and drug inactivation was suppressed.