This Week in Science

Science  25 Sep 2009:
Vol. 325, Issue 5948, pp. 1596
  1. Butterfly Navigation


      Monarch butterflies migrate to Mexico from various parts of North America in the fall and navigate with the aid of Sun compass. This navigational mechanism, also employed by migratory birds, uses the circadian clock to compensate for the positional change of the Sun in the sky throughout the day. The mechanism behind time-compensated Sun compass orientation has remained obscure. Merlin et al. (p. 1700; see the Perspective by Kyriacou) now provide comprehensive data showing that the mechanism resides in the antennae of the butterflies, rather than the brain, as previously thought. The “antennal clocks” found in the monarchs probably provide the primary timing mechanism for Sun compass orientation. These findings reveal a further function for the antennae—a function that may extend widely to other insects that use this orientation mechanism.

    1. Facile Fluorination

        Fluorine atoms have become a useful substituent in pharmaceuticals. However, they remain challenging to introduce synthetically because present methods for carbon-fluorine bond formation require either corrosive conditions or somewhat exotic, and thus expensive, reagents. A sticking point has been the failure of traditional palladium catalysts to couple aryl groups with coordinated fluoride. Watson et al. (p. 1661, published online 13 August; see the Perspective by Gouverneur) show that pairing palladium with a well-designed phosphine ligand produces a versatile catalyst for aryl fluorination using simple fluoride salts. The method tolerates a range of functional groups and should facilitate efficient syntheses of multiple fluoroaromatic targets.

      1. Martian Impact

          CREDIT: BYRNE ET AL.

          Impact craters form frequently on Mars, exposing material that would otherwise remain hidden below the surface. Byrne et al. (p. 1674) identified mid-latitude craters that formed over the last few years, imaged them in great detail with a camera on board the Mars Reconnaissance Orbiter, and monitored subsequent changes. The craters excavated buried water ice, which was later seen sublimating away. In addition, some craters might have excavated completely through the ice. The observations are consistent with models and other observations that suggest water ice should be stable decimeters to about 1 meter below the martian surface at latitudes poleward of about 40°; and suggest that, in the recent past, Mars had a wetter atmosphere than at present.

        1. Polymer Photodetectors

            Optical sensing is used in a wide range of applications, such as low-light detection systems in cars and cameras. Most photodetectors have a limited spectral range and can only detect a narrow range of wavelengths. Gong et al. (p. 1665, published online 13 August) developed polymer photodetectors with extremely broad spectral response and exceptionally high sensitivity that can exceed the response of an inorganic semiconductor detector at liquid helium temperature. A key aspect in the device design is the inclusion of blocking layers to reduce significantly the dark current or noise in the devices.

          1. Tin Two-Step

              Doubly and triply bonded carbon compounds have a well-studied tendency to link up with one another and form rings. The rates of these reactions and their relative susceptibilities to acceleration by heat versus light are encapsulated in the decades-old Woodward-Hoffmann rules. More recently, alkene and alkyne analogs have been prepared with heavier elements such as silicon and tin substituted for carbon. Peng et al. (p. 1668; see the Perspective by Sita) have now discovered that two distannynes (compounds with triply bonded tins) react readily with ethylene to form cycloadducts, with tin-carbon σ bonds taking the place of C-C and Sn-Sn π bonds. These products, characterized spectroscopically and crystallographically, are only loosely bound at room temperature, easily reverting to their multiply bonded precursors on gentle heating.

            1. Togetherness

                Two of the most important questions in paleoclimatology are, how are the climates of the Northern and Southern Hemispheres linked, and what are the roles of the high latitudes and the tropics in driving and transmitting climate changes? Past investigations have concentrated on the study of large, rapid climate changes like deglaciations or the Younger Dryas because they are the easiest ones to see and to date. Licciardi et al. (p. 1677) expand the scope of these investigations by determining precise cosmogenic isotope ages for glacial moraines formed in the Peruvian Andes during the Holocene (the last 11,000 years). The precision of these data reveals a broad correlation between Peruvian glacial advances and climate in the North Atlantic region, revealing important climate linkages between the tropics and higher latitudes.

              1. Malaria Chloroquine Resistance Transporter

                  Malaria is one of the most deadly infectious diseases in the world today, and the emergence and spread of chloroquine-resistant parasites has been a disaster for world health. The Chloroquine Resistance Transporter (PfCRT) was originally identified because mutations in this protein confer chloroquine resistance in the human malaria parasite, Plasmodium falciparum. However, the mechanism by which they do so has been the subject of ongoing debate. Martin et al. (p. 1680) have now succeeded in expressing PfCRT at the surface of Xenopus laevis oocytes, establishing a robust and reproducible heterologous system for the study of this protein. The resistance-conferring form of the protein mediates the transport of chloroquine, whereas wild-type PfCRT does not. Thus, as suspected, chloroquine resistance in the malaria parasite indeed arises as a result of the transport of the drug via mutant PfCRT.

                1. Cataloging Kinase Targets

                    CREDIT: HOLT ET AL.

                    Protein phosphorylation is a central mechanism in the control of many biological processes (see the Perspective by Collins). It remains a challenge to determine the complete range of substrates and phosphorylation sites altered by a kinase like cyclin-dependent kinase 1 (Cdk1), which controls cell division in yeast. Holt et al. (p. 1682) engineered a strain of yeast to express a modified Cdk1 molecule that could be inhibited by a specific small-molecule inhibitor. The range of Cdk1-dependent phosphorylation was assessed by quantitative mass spectrometry, which revealed many previously uncharacterized substrates for Cdk1. In addition to phosphorylation on serine and threonine residues, which appears to be evolutionarily ancient, tyrosine phosphorylation occurs primarily in multicellular organisms. Tan et al. (p. 1686, published online 9 July) compared the overall presence of tyrosine residues in human proteins (which are frequently phosphorylated) and in yeast proteins (which are not). Loss of tyrosine residues has occurred during evolution, presumably to reduce adventitious tyrosine phosphorylation.

                  1. From Retrogene to Phenolic Metabolism

                      Metabolic plasticity, which involves the creation of new genes, is an essential feature of plant adaptation and speciation. Studying plants from the mustard family, Matsuno et al. (p. 1688) show that variants of the cytochrome P450 enzyme family were derived through retroposition, duplication, and subsequent mutaton. Evolutionary changes increased the volume of the substrate pocket altering with what sorts of substrates the enzymes could interact. The enzymes formed the basis for a new metabolic pathway, the products of which include constituents of pollen and of phenylpropanoid metabolism.

                    1. Character Transplant

                        When engineering bacteria, it can be advantageous to propagate the genomes in yeast. However, to be truly useful, one must be able to transplant the bacterial chromosome from yeast back into a recipient bacterial cell. But because yeast does not contain restriction-modification systems, such transplantation poses problems not encountered in transplantation from one bacterial cell to another. Bacterial genomes isolated after growth in yeast are likely to be susceptible to the restriction-modification system(s) of the recipient cell, as well as their own. Lartigue et al. (p. 1693, published online 20 August) describe multiple steps, including in vitro DNA methylation, developed to overcome such barriers. A Mycoplasma mycoides large-colony genome was propagated in yeast as a centromeric plasmid, engineered via yeast genetic systems, and, after specific methylation, transplanted into M. capricolum to produce a bacterial cell with the genotype and phenotype of the altered M. mycoides large-colony genome.

                      1. Rethinking Vaccine Distribution

                          The distribution of vaccines is a complex issue lying at the intersection of public health, economics, and ethics and it cannot be decided in hindsight as an epidemic unfolds. Thus, mathematical modeling can be valuable for guiding policy, and Medlock and Galvani (p. 1705, published online 20 August) present an analysis of how to distribute influenza vaccine among different age groups in a way that will minimize transmission. Scenarios were developed for different outcomes that tell us what happens, in terms of numbers of infections, mortality, and cost, when various cohorts are targeted for vaccination under different epidemic conditions, and compare 1918- and 1957-like epidemics. The scenarios could apply equally well to antiviral drug distribution. The conclusion is that the current recommendations for vaccine distribution from the U.S. Centers for Disease Control and Prevention may need to be revised to include age-related patterns of transmission to minimize the impact of epidemic influenza.

                        1. Bonding Oxides and Metals

                            The binding of noble metals that can act as catalysts to metal oxides that are reducible is assumed to occur at the exposed cation of the oxide. For nonreducable oxides such as aluminum oxide, it is not so obvious how the metal can bind strongly. Kwak et al. (p. 1670) used a combination of high-resolution transmission electron microscopy and solid-state magic-angle spinning nuclear magnetic resonance to study the anchoring of platinum at high and low loadings on alumina. At the surface, the Al3+ ions were penta-coordinated. Density functional calculations support a model in which the cation binds three oxygen atoms in the alumina and two from platinum oxide.

                          1. Correspondence Communications

                              Statistical physicists and social scientists have attempted to describe human activities, in terms of physical models, and look for universal principles. Correspondence patterns are thought to be driven primarily by the need to respond to other individuals with both e-mail and letter correspondence showing power-law distributions. Because there are different exponents for the two modes of correspondence, it has been suggested that human correspondence falls into one of two universality classes and that e-mail and letter correspondence are fundamentally different activities. Now Malmgren et al. (p. 1696) tested whether human correspondence patterns are instead driven by mechanisms such as circadian cycles, task repetition, and changing communication needs. Letter correspondence, like e-mail correspondence, was accurately modeled as a cascading nonhomogeneous Poisson process giving rise to non-Gaussian statistics, but not to power-law statistics. Instead, the correspondence patterns of each individual could be uniquely characterized by the parameters of the model; that is, the process was shown to be universal, but the parameters were not. Thus, an individual's affinity toward a particular life-style will affect communication patterns, which can be modeled as a complex system.

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