This Week in Science

Science  07 Jul 2006:
Vol. 313, Issue 5783, pp. 13
  1. Setting Global Priorities for Conservation


    At least nine major templates of global biodiversity conservation priority have been published during the past decade, but this literature has been largely disconnected from parallel advances in systematic conservation planning. Brooks et al. (p. 58) review how the different approaches relate to one another in the context of the broader conservation planning theory. Areas of agreement are identified between the approaches that lead to a clearer message on the need to protect particular regions.

  2. Bright Source of Photon Pairs

    Pairs of correlated photons can be generated via parametric down-conversion in nonlinear crystals, but this method has not provided enough pairs to realize application in areas such as quantum cryptography. Thompson et al. (p. 74) optically pumped a laser-cooled cloud of cesium atoms inside an optical cavity, and show that they can prepare 50,000 correlated pairs every second. The photons have linewidths sufficiently narrow that they should interact strongly with atoms and molecules.

  3. The Topology of Music

    In Western music, harmony—the selection of notes to create a chord—and counterpoint—how individual notes are connected in time to establish melodic voices—together form the basis for composition. The rules of harmony and counterpoint embody aesthetic norms but also represent constraints on the composer that can sometimes be difficult to reconcile. Tymoczko (p. 72; see the Perspective by Hook) discusses a mathematical system for organizing the 12 tones of the western scale that makes use of a topological structure called an orbifold, in which chords are points in the topological space and the segments connecting them indicate how chords progress. Examination of the geometry of these spaces provides an understanding of some long-standing puzzles in music theory.

  4. Standing Long and Tall

    Water alters the main anhydrous feldspar minerals of many rocks to clays. Because the hydrogen isotope composition of precipitation varies with altitude, the clay minerals that form from surface weathering preserved in ancient stream cobbles could be used to infer ancient elevations. Mulch et al. (p. 87) have applied this principle to infer the topography of the Sierra Nevada mountains 50 million years ago. The Sierra Nevada now stand about 2 kilometers above sea level, but when they arose has been widely debated. The hydrogen isotope data from ancient cobbles in a preserved river channel mimic those in present-day drainages, implying that the mountains have been high since the Eocene.

  5. Snagging Photogenerated Charges


    Biological photosystems use elaborate protein-chromophore structures to separate photoinduced charges so that, rather than recombine, they perform useful chemical reactions. Bhosale et al. (p. 84; see the Perspective by Kinbara and Aida) synthesized a photosystem that partitions electron and hole acceptors on the inside and outside of vesicles that react with the photoinduced charges before they recombine. Molecules containing face-to-face stacks of fluorophores self-assembled as tetramers in lipid vesicles. In the presence of visible light, these assemblies create long-lived electron-hole pairs that reduced quinones inside the vesicles and oxidized EDTA in the surrounding solution. The resulting pH gradient between the interior and exterior of the vesicle could be released by the addition of an intercalator that caused the photosystem to reassemble as a channel structure.

  6. The Well-Dressed Fly

    Drosophila melanogaster sports a set of forewings and a small pair of serially homologous structures, termed halteres, that help provide balance during flight. Crickmore and Mann (p. 63, published online 1 June; see the Perspective by Stern) used genetic analyses to examine the variation in size control between the wing and haltere during development. Ultrabithorax (Ubx), a homeotic selector gene, controls the size of the haltere primordium and, subsequently, the haltere itself, by restricting the expression and mobility of the morphogen, Decapentaplegic (Dpp), via the Dpp receptor, thickveins. Thus, selector genes can regulate organ size by regulating both growth factors and their receptors.

  7. Dicing with Viral Defense

    In RNA silencing, Dicer endonuclease cleaves double-stranded RNAs to generate small interfering (si)RNAs that target complementary RNA sequences. The plant Arabidopsis has four Dicerlike (DCL) enzymes (DCL1 to DCL4) that are involved in genesis of micro (mi)RNAs, natural antisense siRNAs, repeat-associated siRNAs, and trans-acting siRNAs, respectively. The role that the DCLs play in viral infection is less clearly understood. Deleris et al. (p. 68, published online 1 June; see the cover and the Perspective by Waterhouse and Fusaro) show that the Arabidopsis dicers have equally distinct roles in defending the plants from infection, with DCL4 and DCL2 having major and partially redundant roles in incorporating viral siRNAs into antiviral effector RISCs. DCL4 also prevents viral movement outside the plant vasculature.

  8. Resisting Antibiotic Stress

    An unwanted effect of antibiotic treatment is promotion of antibiotic resistance through the dissemination of resistance genes and the selection of resistant subpopulations of bacteria. Prudhomme et al. (p. 89) discovered that the stress of exposure to antibiotics triggers competence (“parasexuality”) in Streptococcus pneumoniae, not only enabling the acquisition of new antibiotic resistance genes but also permitting the acquisition of virulence determinants. Natural transformation is widespread—it has been observed in about 50 different species of bacteria. The discovery that it can be stimulated by antibiotics has significant implications for strategic antibiotic development.

  9. Of Mice and Mammoths


    What are the molecular genetic changes that underlie an organism's adaptation to a new environment? Beach mice (Peromyscus polionotus) living on coastal dunes of Florida's Gulf Coast barrier islands are prey for owls and hawks and subject to selection based on their ability to hide: Beach mice are lighter and have different coloration patterns compared to their mainland cousins. Hoekstra et al. (p. 101) show that a major determinant of this coat color variation is the melanocortin-1 receptor that is involved in the switch between production of dark and light pigments. The beach mice have a change in a single amino acid compared to the darker mainland mice, which correlates with changes in coat color. Curiously, Florida's Atlantic Coast beach mice do not have this allele, which suggests that their light color evolved independently. Römpler et al. (p. 62) looked at the gene for the melanocortin-1 receptor from woolly mammoths, and find that in this case as well, it may have been responsible for the existence of both dark- and light-haired individuals.

  10. Blood-Borne Prion Protein

    Prion proteins (PrP) are thought to represent the infectious agent responsible for a variety of progressive, fatal neurodegenerative disorders. Recently, three cases of variant Creutzfeldt-Jakob disease (the human form of “mad cow” disease, bovine spongiform encephalopathy) have been reported to have been transmitted by blood transfusion, so a presymptomatic test to allow the screening of blood donors is crucial. Saá et al. (p. 92) describe how an in vitro amplification procedure allowed the biochemical detection of PrP in the blood of experimentally infected animals long before the onset of clinical disease. Trifilo et al. (p. 94) examined scrapie infection in a transgenic mouse system in which the endogenous mouse PrP lacked its membrane anchor. The mice did not exhibit neurological problems, but instead developed heart disease. Furthermore, PrPres, the disease-associated folded PrP protein, could be detected in undiluted plasma without any amplification.

  11. Fine-Tuning of the Unfolded Protein Response

    The unfolded protein response (UPR) plays a central role in governing a cell's survival after exposure to stress. The UPR encompasses a diverse range of transcriptional and translational responses stimulated by the increase in levels of unfolded or misfolded proteins within the endoplasmic reticulum (ER). The UPR uses different strategies to provide a coherent response, which allows cells both to survive acute folding stresses and to fine-tune the ER folding capacity to meet the needs of a cell. Hollien and Weissman (p. 104; see the Perspective by Ron) systematically explored the outputs of various players in the UPR. They found that IRE1 (the transmembrane protein that monitors the folding status of the ER) mediates a rapid and robust down-regulation of a subset of ER-targeted messages by direct destabilization of ER-bound messenger RNAs.

  12. A Window on Self-Assembly

    In biological systems, noncovalent interactions among peptide, glycan, and lipid building blocks lead to the spontaneous formation of remarkably intricate aggregate structures. Recently, chemists have sought to apply similar self-assembly strategies to prepare synthetic structures in the nanometer-to-micrometer size regime. However, the aggregation mechanisms in synthetic systems remain largely unestablished. Jonkheijm et al. (p. 80; see the Perspective by Percec et al.) used spectroscopic probes to uncover the stages whereby a class of oligo(p-phenylenevinylene) (OPV) compounds forms helical stacks upon cooling in hydrocarbon solution. They find that preliminary nucleation of roughly 20 to 40 stacked OPV dimers gives rise to cooperative assembly of the helices. Moreover, the extent of aggregation correlates with the presence of an even number of carbons in the solvent chain, implying that the solvent molecules play a direct role in the process.

  13. A Thin Lid on a Metal Alloy Melt

    Premelting of surface layers has been widely observed, but the occurrence of frozen surface layers on bulk liquids has been observed mainly for complex liquids and liquid crystals, or for dilute metal alloys. Shpyrko et al. (p. 77) present x-ray diffraction evidence for the presence of a crystalline surface monolayer for the eutectic composition of gold-silicon alloys (Au82 Si18) above its melting point. The next seven to eight layers form ordered atomic layers that are still fluid in the lateral directions. This crystallization, which occurs even though the bulk Au-Si system exhibits only glassy phases, may arise in part from the silicon enrichment of the surface region.

  14. Trees Live and Let Live

    One hypothesis addressing species richness in tropical forest trees suggests that demographic variation (rates of recruitment and mortality) between species may be an important factor permitting the coexistence of many species. Most work on tree mortality in the tropics has not considered individual species, focusing instead on the entire forest stand as a unit. Large, regularly monitored sampling plots established in recent decades in various locations across the tropics are now yielding fruit. Condit et al. (p. 98, published online 8 June) considered species differences and quantified these differences across all species in entire communities. Contrary to predictions, the more diverse sites had less variation in mortality and thus fewer demographic niches, suggesting that demographic variation plays only a minor role in species coexistence.

  15. Cell Cycle Links to Cytokinesis

    After mitosis, cells must physically separate from one another in a process known as cytokinesis. Failure of cytokinesis can promote tumorigenesis. Yoshida et al. (p. 108, published online 8 June) examined how cell cycle signals are linked to the control of Rho-type GTPases and cytokinesis. Using budding yeast as a model, the authors identified a conserved signaling pathway whereby the mitotic polo kinase controls contractile ring assembly by directly regulating Rho1, the small guanosine triphosphatase that is required for cytokinesis in all eukaryotes.