This Week in Science

Science  19 Mar 2004:
Vol. 303, Issue 5665, pp. 1725
  1. Plant and Animal Diversity in the UK


    There has been a decline in the richness of grassland plant species in the United Kingdom during the past 25 years, especially in infertile grasslands (see the news story by Pennisi). Stevens et al. (p. 1876) conducted a transect of a grassland community across the UK. More than half of the variability in plant species number could be explained by the level of nitrogen deposition alone, such that current levels of nitrogen deposition are reducing plant species diversity, and may have already reduced acid grassland plant diversity by nearly 25%. Thomas et al. (p. 1879) analyzed data from surveys of all British native plant, butterfly, and bird species, from the past 40 years for the entire island. Unexpectedly, butterfly declines were greater than those of plants or birds. These results strengthen the hypothesis that the world is experiencing the sixth major extinction event in the history of life, and provide the strong evidence that insects (which constitute more than 50% of described species) may be at least as sensitive as more visible groups.

  2. Standard Optics

    Recent results with the generation of optical frequency combs spanning more than an octave of optical frequencies has shown great potential for applications in frequency standards and optical clocks. Ma et al. (p. 1843) compare several techniques for generating these optical combs and their subsequent stability in determining and synthesizing optical frequencies. Irrespective of the technique, they find that the precision with which the frequency can be determined can approach 1 part in 1019, suggesting that such a robust technique should find a wide range of applications in frequency metrology.

  3. Exit Strategies

    When molecules dissociate after photoexcitation, the fragments will have preferred recoil angles. Studies of unoriented molecules can provide insight into the recoil process if the fragments leave parallel or perpendicular to the molecular axis, but tend to be ambiguous for the more general case of intermediate angles. Rakitzis et al. (p. 1852) oriented OCS molecules in a hexapole trap and determined the sign and deflection angle of the CO fragment by imaging the its fragment. They show that CO is ejected preferentially in the direction of the laser's polarization, which is 135° away from the OCS molecule's permanent dipole (which points roughly from the carbon atom to the sulfur atom).

  4. Dust and Seas

    How were such dry conditions of the “Dust Bowl” drought of the 1930s maintained for so long over such an extensive area? Schubert et al. (p. 1855) use an atmospheric-land general circulation model to show that the root cause was the combination of anomalous tropical sea surface temperatures (SSTs)—warm in the Atlantic and cold in the Pacific—that prevailed at the time. Proxy records, mostly from tree rings, show that similarly severe droughts have occurred once or twice a century during the last 400 years. Whether we can expect a similar episode soon is unclear, say the authors, because climate models still cannot make detailed enough SST predictions for more than 1 year or so in advance.

  5. A Gang of Five

    Life on Earth is supported by the conversion of solar energy into chemical energy during photosynthesis. Photons are used to reduce carbon-containing compounds, such as CO2, to sugars by adding electrons that have been abstracted from H2O; as a result, oxygen is released as a by-product. (Later, energy is extracted from these sugars by “burning” them, generating CO2 and H2O, and completing the cycle.) Photosystem II is a large, membrane-embedded protein complex that catalyzes the fragmentation of H2O. At the heart of this enzyme is a metal cluster containing four Mn atoms and one Ca atom. Ferreira et al. (p. 1831; see the Perspective by Rutherford and Boussac) describe the structure of this cluster in which three of the Mn atoms and the Ca are arranged at nonadjacent corners of a cube, with the fourth Mn atom connected to the cube via one of the bridging oxy ligands.

  6. Moving Up, Down, and Around


    Molecular shuttles can be made in which a ring component moves between two recognition sites along a rod-shaped molecule in response to changes in condition such as pH. Badjic et al. (p. 1845) created a molecular platform that is moved up and down by molecular shuttles. Three rodlike molecules containing recognition sites extend like legs from a central core, and a molecule containing three macrocyclic rings fused to an aromatic core slips one ring over each leg to create the movable platform. Changing acid-base conditions moves the rings between two sets of recognition sites in a stepwise fashion. This shuttling between sites can be induced indefinitely. This movable platform extends only a few nanometers in each dimension and potentially could generate forces of up to 200 piconewtons, based on the energy of the acid-base reactions, as the platform moves 7 angstroms. In the pursuit of molecular control over rotary motion, Hawthorne et al. (p. 1849) reinvestigated a metal complex and show that it undergoes stepwise rotary motion that can be controlled by electron transfer or photoexcitation processes. A nickel atom is sandwiched between two dicarbollide ligands that each present a pentagonal face of three boron and two carbon atoms to the metal. The presence of the alkyl groups on the carbon atoms creates an energetic barrier that keeps the ligands from spinning freely and that limit their relative rotation to 144° (hence, there is no net rotation of the ligands over time). Theoretical calculations of the rotational barrier suggest that the energy of photoexcitation could be used to drive the motion of a larger structure.

  7. Structure of 1918 Flu Components

    At the end of World War I, an influenza pandemic caused the deaths of more than 20 million people. The first stages of virus infection—receptor binding, internalization, and intracellular membrane fusion—are mediated by one of the viral membrane glycoproteins, hemagglutinin (HA). Recently, the nucleotide sequences of segments of the 1918 influenza virus genome have been determined from archival material preserved in the Alaskan permafrost, and these sequences have allowed the preparation of 1918 HA protein. Gamblin et al. (p. 1838) now report the crystal structure of the HA from the 1918 virus and the structures of two related HAs in complex with receptor analogs. Stevens et al. (p. 1866) describe the structure of the uncleaved precursor of 1918 HA. The protein has structural features that have previously been observed primarily in avian influenza viruses. Notably, the receptor-binding site retains amino acids characteristic of avian virus but can interact with human receptors, which explains why the virus could be transmitted between humans (see the Perspective by Holmes).

  8. A Darker Shade of Pale

    Plumage variation in the form of light and dark morphs is present in many bird species. Its origin and evolution are of interest because the color variation is known to affect mate choice. Mundy et al. (p. 1870; see the Perspective by Hoekstra and Price) have found the genetic basis of the variation and find that it is the same in two unrelated species, snow geese and arctic skuas. They use the variation to date the origin of the plumage types and found that the trait has evolved relatively recently, with its evolution driven more by mutation than by selection. These results show that a single gene can generate different patterning effects.

  9. Seeing Supershear


    Supershear, where a rupture propagates faster than the shear wave velocity of the material, has been observed for large earthquakes and can enhance ground motions in an unanticipated manner. Xia et al. (p. 1859; see the Perspective by Ohnaka) have directly observed the spontaneous nucleation and propagation of supershear rupture in laboratory experiments on incoherent, frictional interfaces subjected to far-field stress loads that mimic a large earthquake. The experiments suggest that the transition to supershear depends more strongly on the uniaxial load than previous models have suggested. For earthquakes, the rupture length needs to be relatively long and the tectonic stress levels must be near the static fault strength to produce supershear.

  10. Counting Diversity in the Fly

    Much has been learned in recent years about the innate immune pathways that exist in invertebrates, in particular in the fruit fly, and these results have helped to unravel the emerging innate immune pathways in mammals. Aside from innate immunity, mammals possess an adaptive immune system that has evolved a high degree of polymorphism in some proteins and the ability to generate receptor diversity as a means of dealing with pathogens. Lazzaro et al. (p. 1873) observe that specific genes encoding proteins of the innate immune system in wild populations of Drosophila melanogaster are also polymorphic. In turn, this finding correlates with the heterogeneous ability of flies to cope with certain types of bacterial infection.

  11. Functional Gradients Revealed

    A variety of physiological processes—from cell motility to the partitioning of chromosomes during cell division—have been explained by the presumed existence of gradients of functionally active proteins. Niethammer et al. (p. 1862) have now observed the existence of steady-state phosphorylation gradients of the tubulin-binding protein stathmin. These gradients were present in the lamellipodia of motile cells during interphase and around condensed chromosomes during mitosis. In both of these cases, the gradients are likely to be the result of localized kinase and diffusible phosphatase activities and will contribute to the production of polarized microtubule structures.