This Week in Science

Science  20 Apr 2012:
Vol. 336, Issue 6079, pp. 274
  1. Tic-Toc Segmentation Clock


    Molecular oscillators are an essential component of vertebrate segmentation, but whether they exist in segmented animals in general has been controversial for almost a decade. Sarrazin et al. (p. 338, published online 8 March; see the Perspective by Roth and Panfilio) demonstrate the existence of a segmentation clock in the growth zone of insects. Microsurgical manipulation and embryo culture revealed cyclic expression of the segmentation gene Tc-odd in the beetle Tribolium castaneum, which suggests that segmentation clocks are a widely shared mechanism that mediates animal segmentation.

  2. Going More Slowly

    Himalayan glaciers sometimes are called the “Third Pole” because of the amount of snow and ice they contain. Despite their importance as a global water reservoir and their essential role in Asian hydrology, how their mass is changing in response to global warming is not well known. Bolch et al. (p. 310) review the contemporary evolution of glaciers in the Himalayan region, including those of the less well sampled region of the Karakoram to the Northwest, in order to provide a current, comprehensive picture of how they are changing. Most Himalayan glaciers are retreating at rates comparable to glaciers elsewhere in the world. In the Karakorum, on the other hand, advancing glaciers are more common.

  3. Unnatural Bases

    The genetic basis of all life on the planet is comprised of deoxyribonucleic acid (DNA) with four nitrogenous nucleotide bases, abbreviated to A, G, C, and T. But there are variations on this theme, and Pinheiro et al. (p. 341; see the Perspective by Joyce) describe the directed evolution of unnatural nucleic acid–like genetic polymers. Variant enzymes were developed that efficiently transcribed DNA to anhydrohexitol (HNA), cyclohexenyl (CeNA), locked (LNA), and threofuranosyl (TNA) nuceic acid analogs. Further variant enzymes were developed to reverse-transcribe these analogs back to DNA. Thus, man-made nucleic acid analogs can be designed and selected that have the potential to operate in a way analogous to the natural process of heredity and evolution.

  4. Old Plates and the Sea


    Estimates for the area and age of the ocean floor are at odds with assumptions for mantle convection, which imply that an older sea floor—rather than a new one—would be preferentially subducted over time. Previous efforts to explain these relationships have been based on geologic evidence and simple models. Coltice et al. (p. 335) created numerical three-dimensional convection models representing more realistic physical boundaries, including a spherical Earth, the existence of continents and supercontinents over time, and realistic rheologies. A combination of continents and plate-like behavior of the ocean floor sufficed to produce the observed relationship between plate area and plate age, which explains why some old oceanic crust still remains.

  5. A Specific Oxidative Catastrophe

    Three different classes of antibiotics induce bacterial cell death by the production of hydroxyl radicals. Hydroxyl radicals are powerful oxidizing agents in living cells and will oxidize the nucleic acid base, guanine, to form 8-oxoguanine, which is potentially mutagenic because it can pair with both cytosine and adenine and form lethal double-strand DNA breaks. Foti et al. (p. 315) discovered that overproduction of the nucleotide sanitizer MutT, which hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, gives striking protection against cell death.

  6. Bonded at the Source

    Asymmetric catalysis is a relatively mature field in the laboratory, with a diverse array of techniques available for the selective transformation of organic compounds. However, scaling up these techniques for industrial application remains challenging, in part because many catalysts act best on reagents that have been expensively modified, although this process often generates copious waste. Zbieg et al. (p. 324, published online 22 March) combat this challenge with a ruthenium-based catalyst that couples an unmodified bulk commodity feedstock (butadiene) with alcohols, forming carbon-carbon bonds to generate complex products with high selectivity.

  7. Hydrated in a Hurry

    Water has a major influence on the conformation of proteins and related biomolecules. However, so many water molecules participate in the hydrogen bonding networks that it can be difficult to pinpoint which specific interactions play the biggest role. Nagornova et al. (p. 320) sought to answer this question for the case of a 10–amino acid ring—the antibiotic compound Gramicidin S—by probing the conformational impact of successive additions of one to 50 water molecules to the naked gas-phase structure. The primary changes in the overall ring geometry came from the addition of just the first two waters.

  8. A Sturdy Electrode Coating

    To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices.

  9. Ancient Bears

    Polar bears are well known for adapting to their cold Arctic climate. Some recent studies, based on mitochondrial DNA, concluded that they are a relatively young species and that these adaptations occurred quite quickly. Although mitochondrial DNA is regularly used to estimate evolutionary history, it has some well-known drawbacks, including sex-biased dispersal and hybridization. Thus, Hailer et al. (p. 344) looked at neutral genetic data that are distributed more widely across the genome of a relatively large sample of polar, brown, and black bears. Consistent with fossil-based studies, the analysis reveals polar bears as a sister lineage to all brown bears, with an estimated divergence time of 300,000 to 900,000 years ago. Thus, polar bears are indeed of a more ancient lineage, and more recent estimates based on mitochondrial DNA are likely to have been affected by past hybridization with brown bear.

  10. Protein Tipping Point


    Amyloid fibrils are insoluble protein aggregates that play a role in various degenerative diseases. Recent experiments have provided insight into fibrillar structures; however, the mechanisms of aggregation remain unclear. Neudecker et al. (p. 362; see the Perspective by Eliezer) report the structure of a transient folding intermediate in a protein SH3 domain known to undergo aggregation. The intermediate is stabilized by non-native interactions and exposes an aggregation-prone β strand. Thus, for this protein, folding from the intermediate state will compete with aggregation.

  11. Bad News for Bees

    Neonicotinoid insecticides were introduced in the early 1990s and have become one of the most widely used crop pesticides in the world. These compounds act on the insect central nervous system, and they have been shown to be persistent in the environment and in plant tissues. Recently, there have been controversial connections made between neonicotinoids and pollinator deaths, but the mechanisms underlying these potential deaths have remained unknown. Whitehorn et al. (p. 351, published online 29 March) exposed developing colonies of bumble bees to low levels of the neonicotinoid imidacloprid and then released them to forage under natural conditions. Treated colonies displayed reduced colony growth and less reproductive success, and they produced significantly fewer queens to found subsequent generations. Henry et al. (p. 348, published online 29 March) documented the effects of low-dose, nonlethal intoxication of another widely used neonicotinoid, thiamethoxam, on wild foraging honey bees. Radio-frequency identification tags were used to determine navigation success of treated foragers, which suggested that their homing success was much reduced relative to untreated foragers.

  12. Thoroughly MODern Yeast

    It is not clear if prion induction in yeast is truly linked to physiological roles. Suzuki et al. (p. 355) show that the yeast prion protein Mod5 (a transfer RNA isopentenyltransferase) responds to an environmental stressor by converting to an aggregated amyloid form, which leads to phenotypic changes in cell metabolism and drug resistance. Introduction of Mod5 amyloid into yeast resulted in the formation of a dominantly heritable prion state [MOD+], in which Mod5 is aggregated. [MOD+] yeast showed high ergosterol levels and acquired resistance to several antifungal agents. Selective pressure by antifungal drugs on nonprion [mod] yeast induced the [MOD+] prion state, formation of amyloid, and increased cell survival.

  13. Joint Decisions

    In many instances, decisions made by relatively homogeneous groups (two or more people) coalesce around the choice that people are most confident in, and this in turn stems from the sampling of representations that individuals perform when making their choices. Koriat (p. 360; see the Perspective by Hertwig) found that if most of the group members are able to form accurate judgments, then confidence and accuracy coincide and the consensus choice is the correct one. By contrast, if few people know the right answer, then heterogeneity in people's representations appeared to offer a surer path to accuracy.

  14. Upper Mantle Dislocations

    The driving forces behind plate tectonics act on a relatively weak upper mantle, such that the stress accumulated from colliding plates in the crust dissipates with depth. The physical properties of common mantle minerals, such as olivine, may be important in controlling mantle rheology, but they are difficult to measure directly. Farla et al. (p. 332) monitored the deformation of randomly oriented olivine crystals at the pressures and temperatures of the upper mantle. Linear defects, known as dislocations, dissipated energy in samples that should normally be stable in several regions of the mantle, including below oceanic crust and around actively subducting slabs. Contrary to previous models, dislocations may dampen low-frequency seismic waves traveling through Earth's interior.

  15. Climb Every Mountain

    Mountaintop floras across Europe appear to be responding to climatic change in terms of upslope species range shifts. Pauli et al. (p. 353) systematically analyzed data gathered from standardized permanent plots on 66 high-mountain environments across Europe. On average, mountaintop species numbers have increased significantly during the last decade. However, this increase is a net effect of gains and losses, with losses particularly affecting mountains of Mediterranean regions and their endemic species. This turnover is largely consistent with model predictions and indicates that high-altitude species, and in particular the rich endemic alpine flora of many Mediterranean mountain ranges, will come under increasing pressure in the predicted warmer and drier climates in this region.