This Week in Science

Science  02 Mar 2012:
Vol. 335, Issue 6072, pp. 1017
  1. The Constant Pollinator


    In ecology, reinforcement is the process by which species prevent hybridization and maintain species boundaries, but the underlying genetic mechanisms are unclear. Hopkins and Rausher (p. 1090, published online 2 February) examined reinforcement between two species of a wild flowering plant called Phlox that show incomplete hybrid sterility. Down-regulation of a flavonoid gene produces red flowers and operates in concert with a color intensity locus to adjust flower color and tone. A distinct geography of flower color has emerged in which it appears that dark coloration causes less hybridization between the species because the butterfly pollinators tend to favor light-blue flower color variants. If pollinators visit flowers with similar phenotypes more frequently than flowers with dissimilar phenotypes, this will decrease gene flow between the unlike flowers.

  2. Acid History

    As human activity continues to pump nearly 50-fold more CO2 into the atmosphere than any existing natural sources, the oceans absorb it. Over time, this vast quantity of excess oceanic CO2 is expected to decrease oceanic pH and have marked effects on calcifying marine species. Looking to the past for records of the consequences, other instances of ocean acidification in geologic history caused by large natural events, such as volcanism, may help predict the oceans' response to contemporary CO2 levels. Hönisch et al. (p. 1058) review the geological events that potentially altered oceanic pH, from the last deglaciation to the largest mass extinction in Earth's history. The current rate of anthropogenic CO2 input into the oceans is much faster than at any other instance in the past, but yet it is unclear whether or not future ocean pH will be significantly affected.

  3. Earth's Sluggish Past

    Based on their affinity for iron or silicon, some elements partition into different compartments of Earth's interior. In the early stages of the formation of the solar system, when Earth was rapidly accreting and differentiating to form a core and mantle, elements such as tungsten and hafnium went along for the ride. By studying the signatures of these elements and their isotopes in ancient volcanic rocks from the mantle, Touboul et al. (p. 1065, published online 16 February; see the Perspective by Bennett) reveal that there was a long-lived 182W reservoir that formed deep within Earth during the first 30 million years of the solar system. Because the reservoir persisted for over 1.7 billion years, it appears the mantle was poorly mixed and indeed it was so sluggish that it did not even homogenize during the massive collision that formed the Moon.

  4. Cold Dust

    Atmospheric dust affects air quality, air and ocean chemistry, ocean biology, and climate, so understanding its origins is important to many fields. Hot, dry, desert regions at low latitudes are well-understood sources, but the role of higher-latitude regions in dust production has not been considered. Prospero et al. (p. 1078) present a 6-year record of measurements made on an island south of Iceland, which revealed frequent episodes of dust-production associated with glacial outwash plains and outburst floods. Much of this dust is transported southward and deposited in the North Atlantic, making it a potentially important supply of iron to drive ocean production in that region.

  5. Tree Refugia

    Ideas of how and when boreal plants spread to the formerly glaciated parts of the world following the retreat of the glaciers 9000 years ago are long debated. Models of the postglacial spread of boreal plants argue for dispersal from southern refugia; however, Parducci et al. (p. 1083) have shown that both spruce and pine were present in small ice-free regions of Scandinavia much earlier than thought. DNA haplotyping confirmed that a remnant mitochondrial type of spruce, once unique to Scandinavia, now lives alongside the more common spruce originating from Eastern Europe. Evidence from lake cores collected from central and northern Norway indicated the survival of conifers as early as 22,000 years before the present, when apart from ice-free pockets, most of Scandinavia was covered by ice.

  6. Setting Atoms Free


    In a plasma, gaseous atoms and molecules accumulate so much energy that they fall apart into electrons and all manner of bigger charged fragments. Free electrons can also wreak a degree of havoc within a liquid environment as they generate local reactive fragments, which then couple with the surrounding solvent to form a slew of products. Alexander et al. (p. 1072) explored what happens if reactive electrons emerged at the surface of such a liquid. To generate these conditions, they bombarded a shallow film of deuterated glycerol with sodium atoms that ionized on impact and discovered, in contrast with bulk liquid, that solvent-derived deuterium atoms escaped from the surface into the gas phase before they could react any further.

  7. Random and Directed

    Natural selection drives populations to adapt to new environments; the raw material or “founder effects” provided by the first colonizing individuals can thus have a formative influence on the population's future. Kolbe et al. (p. 1086, published online 2 February; see the cover) tested the relative contributions of selection and founder effects in Bahamian lizards. Founders were taken from an island covered in forest: These lizards had long hindlimbs for sprinting across the broad expanses of tree trunks. Long-limbed lizards were introduced to seven smaller islands covered in scrub that, before hurricane Frances in 2004 swept them away, had been populated by lizards with short hindlimbs better suited for navigating a twiggy habitat. After several generations, all the new lizard populations had adapted to their new habitats by evolving shorter hindlimbs but they also retained other morphological and genetic signatures from their founding ancestors. Thus, evolution occurs by a combination of arbitrary events, as well as those shaped by selection.

  8. Shape-Shifting Signals


    Although orthogonal signaling systems seem to direct various developmental processes, few tissues remain in the same shape as they are at initiation to that of the final form. Arabidopsis leaves are free of the cell migrations that complicate animal development, and thus allowed Kuchen et al. (p. 1092) to track and model the trajectory of leaf growth under a variety of perturbations. Varying the values of parameters in their model produced outputs of different leaf shapes ranging from obcordate, ovate, and oval to elliptic, and offered predictions for genes that regulate the developmental process. The meristem at the growing tip of plants is home to stem cells and is the source of newly differentiating shoots and leaves. New leaves make their first appearance as bulges at the side of the dome-shaped meristem. Although these developmental events are under hormonal control, they also seem to be constrained by the physical properties of the meristem. Kierzkowski et al. (p. 1096) tested physical effects acting on the shoot apical meristem of growing tomato shoots that alter turgor pressure. Again, mathematical modeling combined with observations of plant tissue helped to define the different zones in the meristem that respond to diverse mechanical stimuli.

  9. Sulfa's Crystal View

    The sulfonamide antibiotics (sulfa drugs) have been used to treat infections for over 70 years; however, emerging resistance has eroded their clinical utility. Sulfa drugs target dihydropteroate synthase, a key enzyme in the bacterial folate pathway. By performing the reaction in the crystalline form of the enzyme, Yun et al. (p. 1110) have characterized the key structural intermediates. In combining structural data with theoretical and mutagenesis studies, they propose a detailed mechanism for dihydropteroate synthase catalysis. By resolving this structure with a sulfa drug bound to the enzyme, they showed how inhibition occurred and indicated how resistance could emerge.

  10. Acquire and Share

    Few would argue with the stance that human social cognition supports an unequaled capacity to acquire knowledge and to share it with others. Dean et al. (p. 1114; see the Perspective by Kurzban and Barrett) compared the extent to which these social and cognitive psychological processes can be elicited in children, capuchins, and chimpanzees through the use of a three-level puzzlebox task. Incentivized by improving rewards, 3- to 4-year-old children progressed from the first to the third level by imitating observed actions, taught other members of their social group how to solve the problem, and shared the rewards obtained. By contrast, neither the capuchins nor chimpanzees, very few of which ever reached the third level, exhibited these charactertistics.

  11. New Teacher Syndrome

    The employment retention of high-school science and math teachers in the United States has diminished over recent decades, such that now many students are being taught by novice teachers. Henry et al. (p. 1118) have analyzed the effectiveness of teachers by examining data collected from North Carolina public schools. Less-effective teachers seem to have a greater tendency to leave teaching and the greatest gains in teacher effectiveness are when the teacher is new—within the first 3 years on the job. Some subject areas are more likely to be affected by the preponderance of novice teachers than other subjects, including math and science.

  12. Critically Cold Atoms

    Unlike classical phase transitions, such as the freezing of water into ice, which is driven by lowering the temperature of the system, quantum phase transitions occur at absolute zero and are driven by other parameters, including magnetic field or pressure. In the vicinity of a quantum phase transition, a critical region forms where physical observables obey scaling laws as a consequence of the self-similarity of the system. Quantum phase transitions and quantum criticality are usually observed in solid state, but Zhang et al. (p. 1070, published online 16 February) used an optical lattice filled with a cold gas of atoms to simulate a quantum phase transition—from an insulator to a superflnuid in two dimensions. They observed the characteristic scaling of the equation of state, a finding that will facilitate the building of a platform in a tunable system for further investigations of quantum criticality.

  13. Exclusive Roaming

    How do polyatomic molecules fall apart? The basic model, supported by centuries of chemical theory and experiment, invokes a series of internal rearrangements that lead to a fleeting high-energy transition-state geometry from which lower-energy products emerge. Over the past decade, several molecules have been shown to manifest a competing dissociation mechanism such that alongside trajectories that pass through the transition state, there are energetically accessible pathways that roam around it. Grubb et al. (p. 1075; see the Perspective by Jordan and Kable) showed that the light-induced reaction of NO3 to form NO and O2 proceeded exclusively by roaming. Although there are distinct pathways to the products in two different electronic states, neither one passes through a conventional transition state.

  14. Outside In

    Acquisition and analysis of large data sets promises to move us toward a greater understanding of the mechanisms by which biological systems are dynamically regulated to respond to external cues. Now, two papers explore the responses of a bacterium to changing nutritional conditions (see the Perspective by Chalancon et al.). Nicolas et al. (p. 1103) measured transcriptional regulation for more than 100 different conditions. Greater amounts of antisense RNA were generated than expected and appeared to be produced by alternative RNA polymerase targeting subunits called sigma factors. One transition, from malate to glucose as the primary nutrient, was studied in more detail by Buescher et al. (p. 1099) who monitored RNA abundance, promoter activity in live cells, protein abundance, and absolute concentrations of intracellular and extracellular metabolites. In this case, the bacteria responded rapidly and largely without transcriptional changes to life on malate, but only slowly adapted to use glucose, a shift that required changes in nearly half the transcription network. These data offer an initial understanding of why certain regulatory strategies may be favored during evolution of dynamic control systems.

  15. Choosing a Path

    The β2-adrenergic receptor (β2AR) is a G protein–coupled receptor that recognizes diverse ligands to trigger signaling in the cell. Besides binding G proteins, activated β2AR can be phosphorylated and bind arrestin, which redirects signaling to other pathways. Some β2AR ligands are “biased” in that they differentially activate G protein or arrestin signaling. Liu et al. (p. 1106, published online 19 January; see the Perspective by Sprang and Chief Elk) used 19F-NMR spectroscopy to examine conformational changes associated with a range of ligands and discovered that biased ligands caused differential shifts in equilibrium between two conformational states—the G protein binding state and the arrestin binding state—and thus provide a basis for rational design of pharmacological ligands.