This Week in Science

Science  04 Jan 2008:
Vol. 319, Issue 5859, pp. 11
  1. The Ant and the Butterfly


    Complex channels of chemical communication affect both the ecology and evolution of interactions among species. In Denmark, the Alcon blue butterfly larvae are adopted by ants of the genus Myrmica, which raise them preferentially to their own young. Nash et al. (p. 88) show that similarity in host and parasite surface chemistry explains a significant proportion of the variation in virulence of the parasite. Patterns of host cuticular chemistry are consistent with an ongoing coevolutionary arms race with one of its host ants (Myrmica rubra), but not with an alternative host (Myrmica ruginodis).

  2. A Cooling Outburst

    The generally stable climate of the North Atlantic region during the Holocene was interrupted by sudden intense cooling about 8200 years ago. It often is assumed that this event was the result of weaker ocean overturning circulation caused by the injection of large amounts of freshwater melt into the North Atlantic Ocean. Kleiven et al. (p. 60, published online 6 December), using an ocean sediment core from a location in the Northwest Atlantic well-suited to record a flood of fresh water from the Laurentide Ice Sheet, show that the bottom water chemistry and flow speed of Lower North Atlantic Deep Water changed markedly at approximately the same time as the catastrophic freshwater outburst from Lake Agassiz.

  3. Motion in a Higher Plane

    In host-guest materials, the lattices of the host framework and captured guests can differ in an incommensurate fashion along one or more dimensions. In such cases, the full crystal structure must be described by using a higher-dimensional superspace framework. Toudic et al. (p. 69; see the Perspective by Coppens) probed such an incommensurate system—a urea lattice encapsulating nonadecane—using neutron diffraction and uncovered a phase transition that exclusively involves rearrangement in the fourth, superspace dimension.

  4. Supercontinents and Subduction

    Mapping of the ocean basin and geologic reconstructions provides direct evidence for subduction that occurred continuously back to a few hundred million years ago. Older mountain belts and arc volcanoes clearly indicate that subduction events occurred 2 to 3 billion years ago. Has subduction necessarily occurred continuously since then? Silver and Behn (p. 85) argue that it may have ceased during times of supercontinent assembly several times in Earth's history, with consequences for cooling of the Earth.

  5. How Mosses Made Their Move

    Mosses represent an evolutionary intermediate between the aquatic algae and the dry-land angiosperms. Rensing et al. (p. 64; published online 13 December) present a draft genome sequence of the moss Physcomitrella patens, which gives insight into the evolution of plants from aquatic to terrestrial environments, the acquisition of certain hormone signaling pathways, and changes in life cycle.

  6. Search and Capture

    Cytokinesis, the physical process of separating two daughter cells, has resisted mechanistic understanding because it is not only complicated at the molecular level but it also involves the mechanical assembly of thousands of protein molecules into a regular structure. Vavylonis et al. (p. 97, published online 13 December) combined results from high-resolution observations of live cells and numerical simulations to suggest that the contractile ring assembles by a stochastic but remarkably reliable search-and-capture mechanism. Myosin motors in structures called nodes capture actin filaments growing randomly from other nodes. The motors pull on the attached node for a few seconds before releasing. These random movements condense the nodes into a continuous contractile ring around the equator of the cell.

  7. The Shapes of Things Past


    One measure of the diversity of life that is particularly useful in studying fossil assemblages is how organisms fill morphospace, which is a graphical representation of possible shapes or forms. This measure provides an approach that is complementary to taxonomic analyses. Shen et al. (p. 81) found that the early Ediacaran faunas, which represent Earth's first complex life, occupied all of the morphospace seen throughout the entire Edicaran, and that later Precambrian fossils assemblages show a decline. This pattern is similar to that seen for the later Cambrian Explosion.

  8. Small Brains, Big Decisions

    The sophisticated cognitive performance of primates is usually associated with highly specialized brain areas containing enormous numbers of neurons in high densities and a plethora of synaptic contacts. Schlegel and Schuster (p. 104) provide a rather surprising example of a minute network in a fish that drives a highly sophisticated and plastic decision. Archerfish decision-making not only involves pinpointing the sensory configuration in a huge four-dimensional space and mapping it precisely onto a fine-tuned two-dimensional motor space. It also involves judging which of two targets is more rewarding by selectively attending to this target and then deciding when a response is unlikely to succeed.

  9. A Little Excitement

    Chemical reactions most often proceed with a conserved degree of electronic excitation, though recent studies have pointed to an increasing range of simple systems in which excited-state reagents directly form ground-state products. Garand et al. (p. 72; see the Perspective by Bowman) probe the degree of this nonadiabatic behavior in the Cl + H2 reaction by advancing a variant of photoelectron spectroscopy to exceptionally high resolution. Their study accesses a key point on the reaction trajectory by gentle electron detachment from ClH2 and ClD2 anions. Comparison with sophisticated theoretical simulations implicates a small but significant nonadiabatic component of the overall reaction system.

  10. Stardust Noble Gases

    Materials trapped and preserved in icy comets date from the earliest history of the solar system, and thus constrain the composition and evolution of the pre-planetary solar nebula. Samples from comet Wild 2 were returned to Earth for laboratory study by the Stardust spacecraft. Marty et al. (p. 75) measured the helium and neon composition of the Stardust samples and found that their abundances are large consistent with ion irradiation. Together with Stardust mineralogic data, these results suggest that Wild 2 gases were acquired in an early, energetic nebular environment near the young Sun.

  11. Saturn's Polar Hexagons

    Like the other Giant planets, Saturn has a dynamic atmosphere. One peculiar feature, noted in 2005, was that its southern pole had a warm vortex that extended through most of the layers of the atmosphere. Fletcher et al. (p. 79) now report Cassini infrared images of that pole, which help resolve its temperature, and surprisingly, a similar warm vortex above the north pole, despite this region having been in darkness for some time. Both structures seem to be maintained by active downwelling and both form an unusual hexagonal pattern. The data imply that they are persistent features of Saturn's circulation and may be expected on the other giant planets.

  12. Why Oh Y

    Y chromosomes, found only in males, harbor very few genes with even fewer polymorphisms. Paradoxically, Y-linked polymorphisms in Drosophila have a number of important phenotypic effects. Lemos et al. (p. 91; see the Perspective by Rice and Friberg) looked at genome-wide variation in gene expression among lines of Drosophila that differed only in their Y chromosomes. A large number of X-linked and autosomal genes (and especially those more highly expressed in males) were differentially expressed between the lines, indicating polymorphic Y-linked regulatory variation. This regulatory variation may be due to polymorphisms in Y-chromosome repetitive sequences, which could have epigenetic effects on gene expression.

  13. Centromeres at the Ready

    Centromeres on eukaryotic chromosomes are formed from specialized regions of chromatin, containing the histone H3 variant centromere protein A (CENP-A). Centromeres play a vital role in the accurate segregation of the genome during cell division and act as the attachment site for the mitotic spindle microtubules that physically separate duplicated chromosomes during mitosis. Folco et al. (p. 94) show that formation of specialized CENP-A containing chromatin at centromeres requires the RNA interference-driven formation of neighboring heterochromatin. This neighboring heterochromatin is required for the establishment of the adjacent centromere but not its maintenance.

  14. Synaptic Metaplasticity

    N-methyl-D-aspartate (NMDA) receptor-dependent synaptic strengthening is the most common and best understood form of plasticity in the central nervous system. However, how NMDA receptors influence plasticity after the initiation of synaptic strengthening in vivo has not been well studied. Clem et al. (p. 101; see the Perspective by Brecht and Schmitz) examined the effect of cumulative sensory experience on synaptic strengthening at layer 4-2/3 synapses in the mouse barrel cortex. NMDA receptors opposed synaptic potentiation after a single-whisker experience and NMDA-receptor blockade restored pairing plasticity via a metabotropic glutamate receptor-dependent mechanism.