Editors' Choice

Science  27 Jan 2006:
Vol. 311, Issue 5760, pp. 437
  1. ECOLOGY/EVOLUTION

    Taking In the Welcome Mat

    1. Andrew M. Sugden

    Ants are ubiquitous in tropical forests, and they exhibit a wide variety of nesting and foraging behaviors that have fascinated naturalists and ecologists ever since the pioneering of scientific exploration in the tropics. Despite many decades of intense study and the high visibility of ants, Longino has managed to unearth previously unreported nesting habits in two endemic Costa Rican ant species in the genus Stenamma. These ants build nests in the vertical clay banks of streams, and the entrance to the nest is formed by a tunnel through the center of a shallow dish sitting atop a pedestal of clay or through a similarly shaped disk of soil lying on a mound of small stones. In both types of dwelling, a spherical pebble near the entrance can be retrieved and used to plug the doorway in times of danger. Each ant colony maintains several such nests, but occupies only one at a time. Because of their colonial habit, ants attract predators, and much of their nesting repertoire revolves around defense. Hence, it appears that the elaborate constructions of Stenamma may minimize the chances of attack by marauding hordes of army ants, which are one of the dominant forces shaping tropical forest ecosystems. — AMS

    Biotropica 37, 670 (2005).

  2. PSYCHOLOGY

    Post-Testanic Potentiation

    1. Gilbert J. Chin

    Tests are an inescapable part of schooling, though generally less prevalent now than in days of yore. Two reasons for administering tests are (i) to assess student achievement and aptitude, and (ii) to impel students to study, and presumably to learn, the subject matter.

    Roediger and Karpicke demonstrate that the actual taking of a test, as opposed to simply preparing to take it, has beneficial consequences. After being allowed to study a reading comprehension passage (preparation material for the Test of English as a Foreign Language), students either were tested for retention of the ideas or allowed a second study session; students in both groups were then tested 5 min, 2 days, or 1 week later. The study-study (SS) group performed better at first but did not score as well as the study-test (ST) group on the later test dates. An expanded protocol confirmed this pattern, with SSSS students outperforming SSST and STTT students when tested right away but with the rankings reversed after 1 week. Notably, the repeated-study students had read the text four times more than the repeated-test students had, yet they retained significantly less of the information. — GJC

    Psychol. Sci. 17, 249 (2006).

  3. CHEMISTRY

    Using Silver to Sugarcoat DNA

    1. Phil D. Szuromi

    One strategy for wiring nanodevices together is to make the desired connections with DNA strands, which can then be metallized. For molecular electronics, it would be useful to create metal-free gaps in these wires, and for the chemical reduction of silver with aldehydemodified DNA, such gaps can be created by binding large proteins to the DNA, which then act as a resist.

    Burley et al. describe an alternative approach in which modified DNA molecules are synthesized using Pwo polymerase with modified dTTPs bearing acetylenic groups. A protected aldehyde, in the form of a galactose that has been modified with an azide group, can then react with the acetylenic side chains via “click” chemistry. Treatment of a 318-base pair modified DNA with silver salt solution (Tollens' reagent) and then with a developer solution deposited silver nanoparticles on the DNA, which was confirmed by atomic force microscopy. — PDS

    J. Am. Chem. Soc. 10.1021/ja055517v (2006).

  4. VIROLOGY

    HIV Hijacks Exosomes

    1. Stella M. Hurtley

    Understanding the mechanisms by which HIV infects cells is a key step in developing effective treatments. Wiley and Gummuluru describe how immature dendritic cells of the immune system can capture HIV particles and, soon after internalization, transmit them to T cells without themselves becoming infected.

    Dendritic cells are one of the first immune cell types encountered by incoming virus particles in the mucosa. HIV particles bind to dendritic cells and are internalized, ending up in multivesicular endosomes. Dendritic cells constitutively release some of the internal vesicles from multivesicular endosomes—so-called exosomes—into the extracellular milieu. For dendritic cells that have recently internalized HIV, it appears that the exosomes contain intact infectious HIV particles, which can then infect CD4+ target T cells. Indeed, the exosome-associated virus particles are up to 10-fold more infectious per particle than are cell-free virus preparations. The remaining dendritic cell-associated virus is transported from multivesicular endosomes to lysosomes and degraded. This exosomal pathway may explain how HIV can evade immune destruction even after having entered the wrong target cell of the immune system. The relative importance of this pathway—in comparison to the so-called infectious synapse, wherein dendritic cells directly pass HIV on to target T cells—remains to be established. — SMH

    Proc. Natl. Acad. Sci. U.S.A. 103, 738 (2006).

  5. CHEMISTRY

    A Guide to Achieving Closure

    1. Jake S. Yeston

    Epoxides are versatile intermediates in both enzymatic and laboratory syntheses of complex organic compounds. These three-membered rings, composed of an oxygen and two substituted carbon atoms, are strained and can be opened readily by scission of a C-O bond. Moreover, the liberated oxygen can attack another epoxide in the same molecule, forming a larger and more stable cyclic ether in the process.

    In general, an epoxide can be opened via attack at either carbon, and substituents introduced to favor one path over the other can prove difficult to remove from the desired product. Simpson et al. have found that the tetracyclic core common to the ladder polyethers (marine natural products associated with red tides) can be prepared efficiently via a base-catalyzed epoxide-opening cascade that is guided by trimethylsilyl substituents. Attack by the oxygen is favored at the silyl-substituted carbon of the adjacent epoxide, yielding the naturally occurring six-membered rings over the kinetically favored five-membered ones. Furthermore, including a fluoride salt in the reaction mixture has the happy consequence of eliminating the pendant trimethylsilyl group after each ring closes. — JSY

    J. Am. Chem. Soc., 10.1021/ja057973p (2006).

  6. MICROBIOLOGY

    A Stomach Full

    1. Caroline Ash

    Until hints to the contrary in several recent studies, the stomach was conventionally thought of as being almost as uninhabitable as Mars. Upon analysis of small-subunit 16S ribosomal RNA libraries prepared from endoscopy samples collected from 23 individuals, Bik et al. discovered, living in the human stomach, a zoo of microorganisms of which a significant proportion had been identified previously as residing in the mouth and 10% were previously unsuspected denizens. Indeed, a member of the genus that includes the notoriously radiation-resistant Deinococcus radiodurans was found, perhaps reflecting the tough physicochemical environment of the stomach. Nineteen of the people were found to be positive for Helicobacter pylori but otherwise showed significant variation in their gastric ecosystems. In all, 128 phylotypes were discovered, with Streptococcus and Prevotella spp. being the most abundant after H. pylori. The authors proffer the suggestion that there are multiple ecological niches in the stomach, each with its own demographic, although currently we can only guess at the roles these organisms play in health and disease. — CA

    Proc. Natl. Acad. Sci. U.S.A. 103, 732 (2006).

  7. APPLIED PHYSICS

    Gated Spin Control in Carbon Nanotubes

    1. Ian S. Osborne

    The growing fields of molecular electronics and spin electronics offer the future possibility of high-density electronic devices, but with the advantage of avoiding the problem of how to dissipate the heat that builds up in such densely packed structures. Nagabhirava et al. have combined the two approaches using a carbon single-walled nanotube (c-SWNT) to bridge the gap between the ferromagnetic source and drain contacts. With the gap reduced to around 10 nm in order to reduce spin-scattering events along the nanotube, they show that the magnitude and sign of the magnetoresistance, a measure of the flow of polarized electrons through the carbon nanotube in response to an external magnetic field, can be reproducibly modified from +10% to −15% by application of a bias on a back gate. The results provide strong evidence for spin transport through c-SWNTs and promise for the spin transistor, a device in which a gate bias controls the flow of spin-polarized current between the source and drain contacts. — ISO