This Week in Science

Science  29 Jun 2007:
Vol. 316, Issue 5833, pp. 1809
  1. Early Reef Builders


    The diversity of ways in which living cells secrete mineralized structures is being revealed through the integration of molecular, chemical, and physical analyses. Using the coralline demosponge Astrosclera willeyana, a reef-building organism that has survived since the Mesozoic, Jackson et al. (p. 1893, published online 31 May; see the Perspective by Taylor et al.) studied the evolution of biocalcification mechanisms. They isolated an (α-carbonic anhydrase (α-CA) that is involved in biocalcification and identify a subclass of this protein that is the sister group to other known (α-CAs. The last common metazoan ancestor may have possessed a single copy of this gene, which was subsequently duplicated in sponges and other animals to provide the genetic foundation of the diversity of physiological processes in which it is involved today.

  2. Core Structure

    The high pressures and temperatures of Earth's core changes the structure of the iron-nickel material and affects the physical properties that can be probed by seismic observations. Dubrovinsky et al. (p. 1880) used x-ray diffraction to observe a transition in a 90% iron-10% nickel alloy in an internally heated diamond anvil cell which suggests that it adopts a bodycentered cubic structure, rather than a closepacked structure, at pressures above 225 giga-pascals and temperatures above 3400 kelvin. Such a change affects the density and rheology of the core as well as the partitioning of light elements between differently structured regions.

  3. When Galaxies Collide


    Large galaxies grow through collisions of many smaller ones (see the Perspective by Coppi). When two galaxies collide, the giant supermassive black holes that sit in their centers eventually meet and spin around one another as a binary system. In the absence of any braking forces, the black holes would continue to orbit one another for at least billions of years. However, large galaxy cores host single black holes, so other astrophysical processes must help the black hole pairs coalesce more rapidly. Mayer et al. (p. 1874, published online 7 June) performed hydrodynamical simulations which show that gas within merging galaxies slows their black holes enough so that they can bind together within just 1 million years. In simulating the decay of a binary black hole system within a gas-rich galaxy that has recently formed from the merger of two smaller spirals, the authors tracked the black holes before their coalescence to within a few light years from each other. Max et al. (p. 1877, published online 17 May) have obtained very-high-resolution infrared images of a nearby pair of spiral galaxies called NGC 6240 that have already collided—their stars and gas wrapping are around one another. Using adaptive optics techniques on the Keck telescope in Hawaii, they pinpoint the positions of two black holes that once dotted the centers of the original galaxies. Around the black holes, cones of gas and new stars are seen that may have formed in the wake of the black holes as they spiraled in toward one another. This separation indicates the effects of dynamical friction stirring the gas as it mixes together.

  4. Selective Dehydrogenation at Surfaces

    Voltage pulses from the tip of a scanning tunneling microscope have been used to induce chemical reactions of adsorbed species on conducting surfaces. Katano et al. (p. 1883) now report the reversible cycling of selective dehydrogenation and rehydrogenation reactions of methyl-aminocarbyne (CNHCH3) adsorbed on the Pt(111) surface at 4.7 kelvin. Pulses of ~3 volts removed a hydrogen atom to form methyl isocyanide but did not affect the C-H bonds of the adjacent methyl group. Exposure to hydrogen at room temperature regenerated CNHCH3. Higher voltage pulses caused irreversible bond cleavages.

  5. Ancient Farm Transitions

    The early development of agriculture in the New World must have involved early farming in settlements at high elevations in the Andes, but the records have been sparse. Dillehay et al. (p. 1890; see the news story by Balter) now document the transition to intensive farming of several crops beginning about 10,000 years ago in this region based on a large number of agricultural sites in central Peru. New radiocarbon dates show that cultivation of squash began around 10,000 years ago, followed by peanuts about 8500 years ago, and cotton by 6000 years ago.

  6. Domestication Past and Present

    The original wild ancestors of wheat would have been tough to farm and tough to eat. However, domestication of wheat as a crop some 10,000 years ago captured advantageous changes in grain size, threshability, and retention of grains on the plant spike. Dubcovsky and Dvorak (p. 1862) review recent insights from molecular genetics and genomics to understand how gene mutations and genome ploidy paved the way for successful domestication of our modern cultivated wheat varieties. Kareiva et al. (p. 1866) review human influences on the global ecosystem and suggest humans are in the process of domesticating the world. On balance, human modifications of the environment have historically provided net benefits, but the point may have been reached such that harmful impacts outweigh the benefits.

  7. Timeless Changes

    Diapause, a developmental suspension in insects often occurring in the winter, is induced by temperature and light conditions, which for the fruit fly Drosophila vary significantly over its European range (see the Perspective by Bradshaw and Holzapfel). In Drosophila, the circadian rhythm gene timeless affects diapause, and Tauber et al. (p. 1895) identify an allelic variant of timeless that can generate only one of the two known alternative forms of this protein. The coding variant, which affects the time when insects enter diapause, is found at higher frequencies at its putative origin and decreases in frequency in all directions along a latitudinal cline throughout Europe, which suggests the influence of environmental selection. Sandrelli et al. (p. 1898) show that this variant results in more stable protein-protein interactions between TIMELESS protein and its partners, which may explain the selective difference in the timing of diapause among individuals of different genotypes.

  8. Pooling Assets

    Collective endeavors among individuals are often accompanied by risk. Defectors (those who do not invest but who share in the return) fare better than cooperators (who do invest), but a third type of participant, the punisher, who acts against the defectors, can stabilize a cooperative group of individuals. Hauert et al. (p. 1905; see the Perspective by Boyd and Mathew) now provide a theoretical basis for the emergence of such punishers, who incur costs that mere cooperators do not and would thus be expected to suffer in evolutionary terms. Allowing for a fourth type of individual—the abstainer—leads to population dynamics where punishers flourish. In essence, it appears that voluntary submission to social norms is a prosocial act.

  9. ESCRTed from Cytokinesis to Viral Budding


    Midbody abscission physically separates daughter cells during cell division. Retroviral budding requires a membrane fission event that is topologically identical and differs from the fusion events involved in processes like endocytosis or exocytosis. Carlton and Martin-Serrano (p. 1908, published online 7 June) establish a functional analogy between abscission and retroviral budding that is key to interpret the defects in cytokinesis observed upon disruption of two proteins of the so-called ESCRT machinery (endosomal sorting complex required for transport) known to be involved in viral budding. Thus, the ESCRT machinery is recruited to the midbody where it may promote membrane fission events required for the completion of cell division.

  10. Precision Excision

    After infecting a cell, HIV integrates as a provirus into the DNA of the host. Some therapeutic approaches have been aimed at preventing this step, but most have focused on blocking cell entry by the virus. Sarkar et al. (p. 1912; see the Perspective by Engelman) describe the lab-based evolution of a specific recombinase protein that can recognize retroviral target sequences and efficiently excise integrated HIV provirus from the genome of infected cells. Although such an approach is still a long way from practical application in treating HIV, this study offers a proof of principle that excision of integrated virus is possible on a genome-wide scale, and may also be useful in other applications.

  11. Probing Exotic Electronic Order

    In most metallic systems, electrons can usually be treated as interacting only weakly with one another at low energies and are described as Fermi liquids. However, a number of materials with exotic electronic ordering, such as low-dimensionality materials, display non-Fermi liquid (NFL) behavior, so that properties such as conductivity have a different temperature dependence compared with Fermi liquids. In three-dimensional (3D) NFL systems, it has been assumed that their NFL behavior arises near quantum critical points, which are second-order phase transitions in the limit of zero temperature and some tuning parameter such as pressure. Pfleiderer et al. (p. 1871) examine one such 3D NFL material, MnSi, with neutron Larmor diffraction to explore the nature of the quantum critical point (QCP) at elevated pressure. Contrary to previous expectations, the NFL behavior for pressures greater than 14.6 kilobars does not arise because of proximity to a QCP but is instead characteristic of a new metallic phase that forms through a first-order transition.

  12. Weak Feedback, Strong Response

    Complex patterns and signals can arise when simple rhythmic oscillators interact through nonlinear coupling; feedback between the elements can change the degree to which they are in or out of phase with one another. Ideally, in order to control such systems (such as pacemakers for heartbeat or antipacemakers used in brain stimulation to disrupt pathological signals), applied feedback signals should be able to tune the desired response but be weak enough so not to perturb the main system dynamics. Kiss et al. (p. 1886, published online 24 May; see the Perspective by Kath and Ottino) have used phase modeling to create complex dynamics from simple electrochemical oscillators (the dissolution of nickel electrodes in sulfuric acid, which can vary periodically in potential every few seconds) by using delayed nonlinear feedback to couple their responses. For a group of four oscillators, they can produce a predetermined pattern in which the system slowly switches between unstable states, and with all 64 oscillators, they can desynchronize the oscillations and create high-order clustering.

  13. Decision-Making in the Fly

    Even the humble fruit fly has choices to make, and uses a rudimentary brainlike organ known as the mushroom body in the process. Zhang et al. (p. 1901) studied choice in Drosophila while the animals were confronted with competing visual cues. Decision-making was based on two processes, one linear and the other nonlinear. The switch from linear to nonlinear decision-making involved dopamine function in the mushroom body. Without the integrity of dopamine-mushroom body circuits, flies could make simple perceptual decisions based on the subtraction of the saliency of the conflict cues (color versus position), but lost the ability to amplify the discrepancy at some crucial points.

  14. Yeast In Sync

    In the laboratory, yeast is usually grown in the presence of high glucose, and lab yeast is adapted for those conditions. If strains more typical of wild yeast are maintained under conditions with few nutrients, they will show a synchronous cycle of alternating respiratory and glycolytic metabolism. Cell division is confined to the glycolytic (or reductive) phase. Chen et al. (p. 1916) hypothesized that this occurs to prevent replication from taking place in an oxidizing environment, which could cause mutations. When cells were forced to divide during the oxidizing phase of the metabolic cycle by introducing one of several cell cycle mutations or treating with H2O2, an increased level of DNA mutation was indeed observed. In addition, a mutation in a DNA checkpoint kinase that links the cell and circadian cycles in other fungi disrupts cell synchrony, suggesting parallels between the metabolic and circadian cycles.