This Week in Science

Science  16 Mar 2007:
Vol. 315, Issue 5818, pp. 1461
  1. From Ecology to Evolution


    Although the time scales of ecological and evolutionary processes can be quite different, the opportunities for the interplay of the two are increasingly evident. Pelletier et al. (p. 1571) show how the feedback between ecological variation and evolutionary change can be estimated using individual contributions to population growth. In a long-term detailed study of a population of Soay sheep living on the remote Scottish island of St. Kilda, variation in size-related traits of individual animals influenced population growth and fluctuates with the environment. It was also possible to estimate the contribution of additive genetic variation to population growth, which provides a measure of how evolutionary processes influence ecological change. Finally, an assessment could be made of how ecological variation influences selection pressures.

  2. Insights into Ice Stream Discharge

    How quickly sea level will rise as climate warms depends mainly on how much the ocean expands from warming, how fast the polar ice sheets melt, and how fast the ice sheets discharge frozen ice into the ocean. This third process is by far the most poorly constrained, but in recent years large and rapid increases have occurred in the discharge rates of some of these outlet glaciers—as much as doubling in less than 1 year (see the Perspectives by Vaughan and Arthern and by Truffer and Fahnestock). Fricker et al. (p. 1544, published online 15 February) analyzed ice-surface elevations obtained from satellite laser altimetry in the vicinity of two important Antarctic ice streams and found rapid, local changes in the height of the ice on annual time scales. They interpret these results as the signatures of subglacial water movement between lakes at the base of the ice sheet. Howat et al. (p. 1559, published online 8 February) show that glacial discharge from ice streams in Greenland can decrease as suddenly as it can increase. Their findings illustrate the difficulty of extrapolating short-term trends in ice mass balance to longer intervals.

  3. Resonantly Depinning Domain Walls

    In conventional magnetic-storage media, changes in magnetization of localized regions are produced with a magnetized head. In efforts to decrease the bit size, reduce power consumption, and develop new active magnetoelectronic technology, the possibility of using electrical pulses to directly manipulate magnetization is being explored. The injection of a sufficiently large current pulse through a domain wall (which separates regions of different polarity) is known to cause domain walls to move. Thomas et al. (p. 1553) now show that a train of well-timed current pulses can also depin the domain wall, but at much lower pulse amplitudes. The subthreshold depinning, which is explained in terms of a resonant amplification of the domain-wall motion within its confining potential, could have implications in addressing magnetoelectronic devices.

  4. Energy Management


    Chemical reactions are often modeled with reference to an energy landscape comprised of activation barriers and potential wells and, in general, any excess energy in the system is expected to spread evenly across this landscape like an overflowing stream. Kim et al. (p. 1561) observe a surprising exception to this trend. Through a combination of theory and experiments, they find that two conformations of the propanal cation, separated by ∼1 kilocalorie per mole, dissociate to form very different product distributions upon absorption of light energy ∼100 times greater than for the small barrier to their interconversion. The calculations suggest that molecular rearrangements in the excited electronic state funnel each distinctly configured structure toward an isolated portion of the ground state surface, after which dissociation outpaces conformational equilibration.

  5. Following Fluctuations

    Near a second-order, or continuous, phase transition, fluctuations of the order parameter (such as for magnetization or superfluidity) completely govern the behavior of the system on all length scales and exhibit a universal scaling behavior that can be characterized by critical exponents. However, probing the actual phase transition at the critical region itself and extracting these critical exponents has proven experimentally challenging. Donner et al. (p. 1556; see the Perspective by Altman) looked at a cloud of cold atoms (bosons) near the onset of Bose-Einstein condensation and probed the spatial correlations between the atoms as the temperature was varied around the critical point. As these results can carry over to a multitude of other systems, they should provide an important testing ground for the general theory of second-order phase transitions.

  6. Warm Currents

    Most studies of electron transport through molecules have focused on currents generated by applied voltages, but many details about the electronic structure of molecular junctions can be gleaned from measuring voltage changes when there is a temperature differential between the two electrodes. For example, the sign of the corresponding Seebeck coefficient S will reflect the position of the Fermi levels relative to the highest occupied and lowest unoccupied orbitals of the molecules. Reddy et al. (p. 1568, published online 15 February) measured S values for several conjugated organic dithiols on gold surfaces. The molecules were in contact with a gold scanning tunneling microscope tip that was kept at constant temperature; the substrate was then heated. The positive sign of S indicates that these molecules are hole conductors.

  7. New Look at an Old Problem

    A ribozyme that can catalyze RNA assembly is central to the RNA-world hypothesis. No known existing ribozyme catalyzes the required template-dependent 5′ to 3′ phosphodiester bond ligation, but proof of principle has been provided by laboratory-created nucleotide triphosphate ribozymes. Robertson and Scott (p. 1549; see the Perspective by Joyce) have determined the structure of such a ligase ribozyme at 2.6 angstrom resolution. The structure of the active site suggests that the ligase ribozyme uses strategies of transition-state stabilization and acid-base catalysis well known in natural ribozymes and protein enzymes.

  8. Attention in Fruit Flies


    Insect brains compare favorably with vertebrate brains in their levels of sophistication. However, can insects like the fruit fly show selective attention? Using local field potential recordings during visual fixation, van Swinderen (p. 1590) demonstrated attention-like processes in Drosophila brain activity. The author also examined the effect of the short-term learning mutants dunce and rutabaga on novelty-evoked responses and found that these mutations attenuate selective attention and delay its onset.

  9. Not-So-Hot Tropical Diversification

    What causes the latitudinal gradient in species diversity, with greater species richness in the tropics? Weir and Schluter (p. 1574) present data and simulations that together point to high speciation rates, not in the tropics as often assumed, but rather at temperate latitudes and low extinction rates in the tropics. This finding contradicts the hypothesis that the tropics have an elevated speciation rate relative to the temperate zones, as previously suggested.

  10. From MicroRNA to Carcinogenesis

    Misregulation of microRNA (miRNA) function has been implicated in cancer. However, the precise role of miRNAs in tumorigenesis has been unclear. High Mobility Group A2 protein (Hmga2) is a small, nonhistone, chromatin-associated protein found in a number of benign and malignant tumors, where the gene is often truncated at the 3′ end. Mayr et al. (p. 1576, published online 22 February) now show that it is the loss of the noncoding 3′ untranslated region of the Hmga2 messenger RNA, and specifically regulator sites for the let-7 miRNA, which cause the overexpression of Hmga2, and that this overexpression contributes to the progression of carcinogenesis both in a tissue culture assay and in nude mice.

  11. HIV Evolution: Host or Virus?

    During infection, the human immunodeficiency virus (HIV) is under pressure to mutate in order to escape immune detection. A population-level study has suggested that polymorphisms in genes that encode the major histocompatibility complex (MHC) proteins responsible for presenting viral antigens to cytotoxic T cells have a strong influence on how the virus evolves. However, Bhattacharya et al. (p. 1583; see the Perspective by Klenerman and McMichael) now present an analysis that takes into account other confounding effects of viral phylogeny and reveals that the majority of such associations result from effects of viral lineages, rather than immune escape. Although MHC polymorphism is still likely to have some influence on viral evolution, this effect could be significantly less than previously suggested.

  12. Measurable Models

    Cytochrome c oxidase (CcO) reduces oxygen to water to power its proton pump, and does so with little generation of deleterious partial oxidation products such as peroxide or superoxide. This selectivity is likely the result of rapid delivery of four electrons to O2 in a hydrophobic site by a heme Fe, a Cu center, and a Tyr residue. The delivery of the electrons that reduce these sites after reaction is actually quite slow, but rebinding of fresh O2 apparently does not occur until these steps are complete. Verifying this scenario in the enzyme is difficult, however, and model compounds mounted directly on electrode surfaces receive electrons so quickly that the effect of individual redox centers cannot be distinguished. Collman et al. (p. 1565) show that by attaching model compounds to electrodes via a long alkyl tether, they can make electron transfer the rate-limiting step and observe decreases in the formation of partially oxidized products when the Cu and Tyr mimic sites are included.

  13. Keeping Things Quiet

    Among its many functions, RNA silencing provides broad cellular defense against viruses in many plant and animal species. To perform defense functions, microRNAs (miRNAs) arm an RNA-induced silencing complex (RISC), which either degrades messenger RNA or inhibits its translation. Triboulet et al. (p. 1579, published online 22 February) show that proteins responsible for generating miRNA potently inhibit human immunodeficiency virus-type 1 (HIV-1) in naturally infected cells and that HIV-1 counters this by down-modulating the expression of selected cellular miRNAs to favor replication. Targeting the miRNA processing pathway may offer a strategy for activating latent viral reservoirs, which remain barriers to eliminating chronic HIV-1 infection.

  14. Genome Ranger

    Protecting the germline genome from the ravages of transposons and other parasitic or repetitive DNA is vital for the continued well-being of all multicellular organisms. A class of small RNAs, called repeat-associated small interfering (rasi) RNAs in Drosophila and Piwi (pi) RNAs in mammals, has been implicated in the defense of germline DNA. They are found associated with the Piwi class of Argonaute (Ago) proteins. Gunawardane et al. (p. 1587, published online 22 February) studied the rasi/piRNAs bound to the Drosophila Piwi proteins AGO3, which, they found, have “slicer” activity. AGO3 rasiRNAs were complementary to the first 10 nucleotides of rasiRNAs bound to another Piwi protein, Aubergine. Thus, the slicer activity of Piwi proteins may be involved in piRNA production.

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