This Week in Science

Science  25 Oct 2013:
Vol. 342, Issue 6157, pp. 401
  1. No Two Faces Are Alike


    Gene disruptions can cause severe dysmorphologies like cleft palate, but what causes the subtle shifts in facial morphology that make each face unique? Studying mice, Attanasio et al. (1241006) identified over 4000 candidate genetic enhancers around genes driving craniofacial development. To avoid the challenge of recognizing individual mouse faces, optical projection tomography was used to link changes in facial morphology with alterations in the function of specific enhancers.

  2. Bite Me!

    As unpleasant as it is, pain serves a purpose, to alert the body to potential damage. This protective function may explain why few predators have evolved resistance to the painful venoms used as a defense by their prey. The grasshopper mouse, however, is insensitive to one of the most painful stings in the animal kingdom—that of the bark scorpion. Rowe et al. (441; see the Perspective by Lewin) now show that grasshopper mice use the toxins present in the scorpion venom to block voltage-gated pain transmission, temporarily reducing their sensitivity to nonvenom-induced pain. Thus, grasshopper mice use the scorpion's painful defense to their advantage and have evolved a mechanism that allows for reduction of pain sensitivity only when it is needed.

  3. On the Brink of Melting

    The considerable pressures and temperatures of Earth's iron-rich inner core make it a challenge to compare measurements made in experimental systems with observed seismic data. Computational simulations of core materials may reconcile any apparent differences. Martorell et al. (p. 466, published online 10 October) used ab initio simulations to predict the elastic properties of iron at core pressures. As temperatures approached the melting point of pure iron, the material was predicted to weaken to the point that seismic waves would be slowed considerably. An inner core with a small percentage of light elements like oxygen and silicon near its melting temperature would correspond well with measured seismic velocities.

  4. Not Very Many

    In physics, the behavior of a system sometimes becomes easier to grasp when the number of particles is large and statistics begin to matter, but knowing how large the system needs to be for that to happen is a challenging computational problem. Wenz et al. (p. 457) used a one-dimensional trapped gas of 6Li atoms to study this crossover from few to many. To simplify the problem, they worked with one “impurity” atom that was in a state unlike the other—“majority”—atoms. For weak and intermediate interactions, the system approached the many-body limit with as few as four majority atoms.

  5. Explaining Anomalous Early Isotopes

    Meteorites contain some of the oldest materials formed in the solar system, including silicate minerals similar in composition to those on Earth. These meteorites, however, often differ in their oxygen isotopic composition, implying that their formation involved a different chemical mechanism than younger solar-system materials. Chakraborty et al. (p. 463) performed a series of experiments and found that mixing of SiO gas and OH leads to a mass-independent isotope fractionation in SiO2 solid products similar to those observed in the oldest meteorites. This oxidation reaction may thus have served as the first source for silicates with anomalous oxygen isotopic compositions in the solar nebula.

  6. Making an Impact

    How big a role do unconventional combinations of existing knowledge play in the impact of a scientific paper? To examine this question, Uzzi et al. (p. 468) studied 17.9 million research articles across five decades of the Web of Science, the largest repository of scientific research. Scientific work typically appeared to draw on highly conventional, familiar mixtures of knowledge. The highest-impact papers were not the ones that had the greatest novelty, but had a combination of novelty and otherwise conventional combinations of prior work.

  7. Dynamic Surfactants


    Surfactants are used to form a stable interface between two nonmiscible liquids, like oil and water, so that droplets of one fluid can be entrained in the other. Cui et al. (p. 460) designed a surfactant based on the association of a hydrophilic nanoparticle with a functionalized oleophilic molecule that self-assembles at a water-oil interface to produce a composite surfactant. Once adsorbed, the nanoparticles tend to remain in place causing them to accumulate and “jam” at the interface. When a drop was deformed, more surfactant could assemble at the surface, allowing droplets of various shapes to be produced.

  8. Conform to the Norm

    Human societies have always enforced compliance with norms of acceptable behavior among their members by threatening punishment. It has been proposed that the human brain may have developed neural processes that support norm enforcement behavior and generate appropriate behavioral responses to social punishment threats. However, evidence for the neural circuitry underlying sanction-induced norm compliance in humans is limited. Using noninvasive brain stimulation, Ruff et al. (p. 482, published online 10 October) observed that alteration of the activity and excitability of the right lateral prefrontal cortex affected norm compliance, without affecting awareness of the content of the respective norms, or the expected sanctions. These alterations were much larger in a social, as compared to a nonsocial, context.

  9. Guardian of the Gut

    The intestine is able to tolerate continual exposure to large amounts of commensal bacteria and foreign food antigens without triggering an inappropriate inflammatory immune response. In the large intestine, this immunological tolerance is thought to occur via a physical separation between environment and host imposed by a continuous mucous layer built up from the secreted mucin protein, MUC2. However, in the small intestine, this mucous layer is porous, necessitating an additional layer of immune control. Shan et al. (447, published online 26 September; see the Perspective by Belkaid and Grainger) now report that in the small intestine, MUC2 plays an active role in immunological tolerance by activating a transcription factor in resident dendritic cells, thereby selectively blocking their ability to launch an inflammatory response. This work identifies MUC2 as a central mediator of immune tolerance to maintain homeostasis in the gut and possibly at other mucosal surfaces in the body.

  10. Topological Replicas

    When a periodic perturbation couples strongly to electrons in a solid, replicas of the original electronic levels are predicted to develop at certain energies—the so-called Floquet-Bloch states. Such conditions can be achieved by shining light on a solid, but the effect is challenging to observe. Wang et al. (p. 453) used time- and angle-resolved photoemission spectroscopy to photoexcite Bi2Se3 and observe its dispersion at various delay times. The replicas were seen at expected energy shifts, along with the gaps predicted to occur at the new energy-level crossings caused by the appearance of the replicas. Because Bi2Se3 is a topological insulator, the breaking of the time-reversal symmetry caused by circularly polarized light resulted in the appearance of an energy gap at the Dirac point, indicating an interesting route toward manipulating electronic states in such materials.

  11. Piecing Together Hydrogenase

    Microbial hydrogenase enzymes generally use iron to catalyze the reversible formation of hydrogen from protons and electrons. Key to their efficiency is a set of iron-coordinating ligands, including CO and cyanide. Kuchenreuther et al. (p. 472) examined how the HydG maturase enzyme breaks down the amino acid tyrosine to derive these diatomic ligands for assembly of the diiron class of hydrogenases. The first step involves abstraction of an H atom from the phenolic OH substituent of the side chain. Electron paramagnetic resonance spectroscopy revealed a radical intermediate that subsequently results from heterolysis of the bond tethering the side chain to the α-carbon. With the side chain thus jettisoned, the residual dehydroglycine could be transformed into CO and CN.

  12. Exploiting Redundancy

    The genetic code is redundant—multiple codons can code for the same amino acid. So-called synonymous codon changes within genes can nonetheless have substantial affects on protein expression, which have been attributed to changes in the structure of 5′ messenger RNAs, among other factors. Goodman et al. (p. 475, published online 26 September) built and measured the expression of a synthetic library of 14,000 variant N-terminal sequences of 137 Escherichia coli genes to show that, unexpectedly, rare codons had a bigger effect on increasing protein expression than more common codons. Increased RNA structure downstream of translation initiation appeared to represent the major determinant of expression differences owing to codon usage.

  13. Farming or Fishing


    Evidence has been mounting that most modern European populations originated from the immigration of farmers who displaced the hunter-gatherers of the Mesolithic. Bollongino et al. (p. 479, published online 10 October) present analyses of palaeogenetic and isotopic data from Neolithic human skeletons from the Blätterhöhle burial site in Germany. The analyses identify a Neolithic freshwater fish–eating hunter-gatherer group, living contemporaneously and in close proximity to a Neolithic farming group. While there is some evidence that hunter-gatherer women may have admixed into the farming population, it appears likely that marriage or cultural boundaries between the groups persisted for over two millennia. Thus, the transition from the Mesolithic involved a more complex pattern of coexistence among humans of different genetic origins and cultures in the Neolithic, rather than a more abrupt transition.