This Week in Science

Science  24 Apr 2009:
Vol. 324, Issue 5926, pp. 435
  1. Not Just Dinner on Legs


      Several thousand years ago, human beings realized the virtues of domesticating wild animals as easy meat. Soon other possibilities became apparent, and as revealed in a series of papers in this issue, early pastoralists became selective about breeding for wool, leather, milk, and muscle power. In two papers, Gibbs et al. report on the bovine genome sequence (p. 522; see the cover, the Perspective by Lewin, and the Policy Forum by Roberts) and trace the diversity and genetic history of cattle (p. 528), while Chessa et al. (p. 532) survey the occurrence of endogenous retroviruses in sheep and map their distribution to historical waves of human selection and dispersal across Europe. Finally, Ludwig et al. (p. 485) note the origins of variation in the coat-color of horses and suggest that it is most likely to have been selected for by humans in need of good-looking transport.

    1. Burn, Baby, Burn

        Wildfires can have dramatic and devastating effects on landscapes and human structures and are important agents in environmental transformation. Their impacts on nonanthropocentric aspects of the environment, such as ecosystems, biodiversity, carbon reserves, and climate, are often overlooked. Bowman et al. (p. 481) review what is known and what is needed to develop a holistic understanding of the role of fire in the Earth system, particularly in view of the pervasive impact of fires and the likelihood that they will become increasingly difficult to control as climate changes.

      1. Disappearing Stars

          One way to identify and study the stars that have exploded as supernovae is to look for stars that are spatially coincident with the supernovae in pre-explosion images. The only way to be sure of their identifications is to wait for the light from the supernovae to fade and check that the stars have vanished from the sky. Maund and Smartt (p. 486, published online 19 March), using the Hubble Space Telescope and the Gemini telescope, have now shown that the red supergiant stars that have been found in pre-explosion images of SN 2003gd and SN 1993J are no longer seen in recent images of the supernovae sites. Their absence confirms that they were the supernovae's true progenitors.

        1. Improving on Spider Silk?

            In a conventional atomic layer deposition, alternating pulses of a reactive metal-based precursor and a reactant, such as water, are deposited onto a solid surface to create a coating that conforms closely to the original surface. Lee et al. (p. 488) apply this process to fibers of spider silk, which are soft and porous. In addition to forming a coating on the fibers, some of the metal ions penetrate the fibers and react with the protein structure. These doped fibers show significant increase in toughness as measured by the work required to deform them.

          1. Boundary Issues of the Lithosphere

              The depth of Earth's tectonic plates is defined by the lithosphere-aesthenosphere boundary (LAB), but its seismic signature is more subtle compared with other deeper boundaries within Earth (see the Perspective by Romanowicz). Under oceanic plates, the LAB is often defined by where temperatures are hot enough to cause some melting. This boundary has been hard to detect in older oceanic plates, but it is important for understanding how these plates thicken with age or distance from ocean ridges, and for assessing heat flow through the oceanic crust. Kawakatsu et al. (p. 499) use a detailed seismic array to detect a seismic velocity reduction beneath the Philippine Sea and Pacific plates. The data imply that 5%, or less, melt forms horizontal layers, and that oceanic plate thicknesses do indeed deepen with age. Rychert and Shearer (p. 495) used 15 years of seismic data to explore the global distribution of an anomaly imaged by conversion of pressure waves to shear waves (waves associated with a sharp velocity drop). The data reveal a broad signal at depths of 70 kilometers (km) beneath ocean islands to 95 km beneath Precambrian shields. It is not clear whether this boundary is the lithosphere-aesthenosphere boundary or a layer with a distinct horizontal fabric.

            1. Methane from Wetlands


                At the end of the cold climate interval called the Younger Dryas, approximately 11,600 years ago, global temperatures began their final ascent to the warmth of the Holocene, and the concentration of methane in the atmosphere increased rapidly and substantially. There has been much speculation about the cause of that increase, with most recent evidence pointing to wetlands as the source. The most direct proof of that explanation requires the measurement of the radiocarbon content of that methane. Petrenko et al. (p. 506; see the Perspective by Nisbet and Chappellaz) analyzed 1000 kilogramsized samples of Greenland ice, which have sufficient methane to allow measurement of its 14C content. They show that wetland sources indeed must have been responsible for the majority of the rise in atmospheric methane levels at the end of the Younger Dryas.

              1. Hot Silent Quakes

                  Subduction zones tend to produce the largest and potentially most destructive earthquakes. Recent observations show that some deformation in several subduction zones seems to be occurring through small or “silent” quakes. The origin of these silent quakes, and their effect on the seismic hazard, is uncertain. Song et al. (p. 502) use a specific seismic signal to map out thin regions with low seismic velocities on the subduction zone beneath southern Mexico. The regions seem to occur at depths below the seismogenic zone where temperatures are higher. These high temperatures and the silent quakes may reflect the release and episodic trapping of fluids from metamorphic reactions.

                1. Resolving Isotopically Chiral Alcohols

                    Autocatalytic reactions, which are accelerated by their own product, can amplify small imbalances in the chiral distribution of starting materials. A particularly effective system is the alkylation of certain aldehydes by diisopropyl zinc, which becomes increasingly stereoselective as the chiral alkoxide product coordinates to unreacted zinc centers. Kawasaki et al. (p. 492, published online 26 March) show that the sense of enantioselection in this system can be influenced by a factor as subtle as chirality in an alcohol that arises only because two positions differ in having 12C and 13C atoms. Isotopically chiral ligands were carefully prepared by using methods that would avoid chiral contaminants, and each led to a distinct enantiomer with enantiomeric excesses exceeding 90%.

                  1. Early Birds and Night Owls

                      In humans, peaks and troughs in alertness and cognitive performance partly depend on the timing of daily activities. Using brain imaging from the extreme ends of the spectrum of individuals whose performance is better in the mornings to those who do best in the evenings, Schmidt et al. (p. 516) found that after more than 10 hours of wakefulness, morning types had less activity in brain areas linked to attention compared with evening types. Moreover, morning types felt sleepier and tended to perform slower on psychomotor vigilance tasks. It seems getting the balance of sleep at the right time is important for regulating the daily pattern of cognitive performance.

                    1. The Mystery of PolyP Polymerase

                        CREDIT: HOTHORN ET AL.

                        Inorganic polyphosphate (polyP) is found in all organisms. In bacteria it is involved in multiple cellular processes, but in eukaryotes its function is less clear and investigation is hampered because the identity of the polyP synthesizing enzyme has been elusive. Previous genetic screens suggested that a yeast vacuolar transporter chaperone may play a role in polyP metabolism. Hothorn et al. (p. 513) have used structural and biochemical studies to show that a domain in this chaperone complex generates polyP from ATP. Crystal structures from various stages of the reaction cycle supply clues for the mechanism and include a structure with a phosphate polymer bound in an enzyme tunnel. This polymerase has been found in a range of organisms where it appears to be important not only in deep-sea organisms contributing to global phosphate cycling, but also in symbiotic fungi exchanging phosphate with their hosts, through to phosphate storage in human protozoan parasites like Leishmania.

                      1. Model Progesterone Response

                          The hormone progesterone stimulates maturation of oocytes in the toad Xenopus laevis by binding to its receptor. Although receptor number and affinity for the hormone stay relatively constant, the dose of hormone required to stimulate meiotic maturation varies according to the environmental stimuli to which the animal is exposed, but little is known about how this is regulated. Justman et al. (p. 509; see the Perspective by Skotheim) combined experimental analysis and mathematical modeling to explore the role of glycogen synthase kinase—3β in desensitization, and a natural, sensitizing co-stimulus, the amino acid L-leucine. The results help to explain the layered complexity seen in signal transduction networks: If multiple stimuli act upon components of linked feedback loops, cells can tune their sensitivities dynamically to match their environment.

                        1. Playing More Games

                            The neural mechanisms underlying strategic decision-making and social preferences can be teased apart by experimental “games.” In one class of two-player games known as dominance solvable, a unique strategy emerges that is the optimal one to adopt because it cannot be overcome by what the other player does. In a second class of games, there is no unique solution, and the optimal strategy requires the players to coordinate, which, in the absence of explicit communication, arises from a “meeting of minds.” Kuo et al. (p. 519) have conducted a neuroimaging study of subjects playing these two kinds of games and found that brain regions previously associated with deliberate and effortful reasoning are activated during dominance-solvable games, whereas other brain regions linked to social processing take the lead during coordination game playing.

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