This Week in Science

Science  16 Jan 2009:
Vol. 323, Issue 5912, pp. 307
  1. Galactic Dust

    Dust is detected in galaxies as far back as 870 million years after the Big Bang. How the universe became dusty at such early times is a matter of debate. Carbon stars are an important source of dust in the local universe, but their contribution is less clear in the early universe, where supernovae are thought to dominate the production of dust. Sloan et al. (p. 353) used data from the Spitzer Telescope to study a nearby star that resembles what carbon stars would look like in the early universe. Despite containing fewer heavy elements than the Sun, this star is producing dust, showing that its primordial composition is not an obstacle to dust production.

  2. Seeing Nothing on the Surface

    CREDIT: LIU ET AL.

    Transformation optics provides a recipe to manipulate Maxwell's equations, and, when used in conjunction with metamaterials, provides the possibility of having light do “tricks” that cannot be achieved with naturally available materials. One such application is cloaking, where light is coaxed around a certain region, with an object placed within that region being concealed from view. Liu et al. (p. 366) demonstrate production of a cloak for an object placed on a metal surface. The metamaterial cloak was constructed from thousands of individual units of anisotropic, nonresonant metamaterial elements, each designed and fabricated by an automated process. A successful cloak was fabricated that was operational over a broad range of frequencies.

  3. "Missing" Carbonate Source Found

    Most of the calcium carbonate production in the oceans usually is attributed to marine plankton, specifically the coccolithophores and foraminifera. Wilson et al. (p. 359; see the Perspective by Seibel and Dierssen) reveal that another very important source has been overlooked: teleost fish. These fish—the ones with bony skeletons—produce large quantities of calcium carbonate in their guts as an osmoregulatory response to their continuous ingestion of calcium- and magnesium-rich seawater, which they then excrete as waste. This calcium carbonate, richer in magnesium than that produced by plankton and thus more soluble, may constitute more than a quarter of the marine carbonate budget, and could be a missing piece in the ocean's carbonate budget that explains many of the puzzling aspects surrounding the distribution of carbonate in the upper 1000 meters of the ocean.

  4. Vaults in View

    CREDIT: T. TSUKIHARA/UNIVERSITY OF HYOGO

    An intriguing but poorly understood structure, broadly distributed in eukaryotic cells, is a large ribonucleo-protein particle termed a vault—so named because electron microscopy revealed a complex morphology resembling the multiple arches of cathedral vaults. Now Tanaka et al. (p. 384) report the structure of the rat liver vault that shows how the cagelike structure is assembled from 78 subunits with 39-fold dihedral symmetry. One domain has structural similarity to a lipid-raft binding domain, consistent with a previous finding that vaults are recruited to lipid rafts where they may play a role in pathogen clearance.

  5. Unshocked Apollo

    Although many of the rocks collected from the Moon during the Apollo missions have a magnetic signature, whether the magnetic fields were produced during impacts or are intrinsic to the Moon has been uncertain. Garrick-Bethell et al. (p. 356) have analyzed the oldest known unshocked Apollo sample, which dates to about 4.2 billion years ago. Their analysis required reconstructing the temperature history of the sample and associating specific remnant magnetic signatures in different minerals with that history. The data imply that there was a strong, persistent magnetic field at that time, which would most likely have been produced by a convecting iron core in the Moon.

  6. Forming Inorganic Biomorphs

    In the growth of crystals from a solution of barium or strontium carbonate in the presence of silica, there is a chemical coupling between the formation carbonate and the formation of silica. This coupling causes pH oscillations at the growth front and wavelike alternating precipitation of the two species. García-Ruiz et al. (p. 362; see the Perspective by Kunz and Kellermeier) track these oscillations and observe a variety of stunning curved morphologies, termed “biomorphs.” All of these complex inorganic forms emanate initially from a typical twinned pseudohexagonal witherite crystal that experiences noncrystallographic branching due to crystal growth poisoning by silica species. This observed complexity may mimic the growth of certain primitive organisms.

  7. Gene Genie

    Few speciation genes have been identified and those that have, appear to be involved in the reinforcement processes that occur during the later stages of speciation. Phadnis and Orr (p. 376, published online 11 December; see the Perspective by Willis) identify a speciation gene that causes reproductive isolation between populations of Drosophila pseudoobscura in the United States and Colombia. This gene, named Overdrive, causes hybrid sterility between two closely related subspecies of Drosophila and also causes hybrid segregation distortion—a bias in Mendelian segregation that favors transmission of the distorter gene to progeny. Thus, genetic conflict may be an important evolutionary force in speciation.

  8. Nematodes Behaving Badly

    Pathogenic and commensal microorganisms can trigger immune responses and control host behavior, revealing complex communication between host and microbe. Natural variations in pathogen susceptibility between different strains of the nematode worm Caenorhabditis elegans has been linked to a polymorphism in the npr-1 gene, which encodes a homolog of the mammalian neruopeptide Y receptor. Reddy et al. (p. 382) studied the mechanism of NPR-1-mediated pathogen susceptibility and found that behavioral changes in the nematodes in response to altered oxygen levels caused by the presence of bacteria resulted in an increased exposure to pathogen, thus causing increased sensitivity. These data underscore the important contribution of behavioral avoidance to nematode survival in the presence of pathogenic bacteria.

  9. Escape Artists

    CREDIT: SCHUMACHER ET AL.

    Bacterial multidrug tolerance (MDT) is an increasingly alarming phenomenon caused by the inability of antibiotics to eradicate infections completely. Most antibiotics target rapidly dividing cells. MDT is caused by a small population of bacterial cells, called persisters, which somehow become dormant and then switch back to growth phase after antibiotic removal, and resume the infection. The biochemical basis for persistence is unknown, but the Escherichia coli HipA (high persistence A) protein has been identified as a bona fide persistence factor. Schumacher et al. (p. 396) studied the structural mechanisms behind HipA function. HipA is a Ser/Thr kinase that phosphorylates the essential translation factor EF-Tu, potentially halting translation and leading to cell stasis. The DNA binding protein HipB, which neutralizes HipA, was found to do so by locking HipA into an inactive state and by sequestering it away from its EF-Tu target.

  10. Draxin and Axon Guidance

    The developing nervous system is assembled with the help of a variety of signals and morphogens that guide the trajectory of growing neurons. Islam et al. (p. 388) have identified an axon guidance protein which they call draxin. Experiments in chick and mouse indicate that draxin, which appears to inhibit neurite outgrowth, is required for neurons to successfully cross the midline and form the major commissures of the brain.

  11. Insider RNA Job

    Retroviruses are well known for their ability to acquire host cell-derived genetic sequences by accident after their integration into the host genome, but, because of the very different mechanisms of replication, nonretroviral RNA viruses, which lack a DNA-integrated phase, have not been thought to be able to hijack extraneous sequences so easily. Now Geuking et al. (p. 393) have found that a nonretroviral RNA virus of mice can join forces with an endogenous retrotransposon, which allows reverse transcription of the viral RNA and integration into the host genome. Lymphocytic choriomeningitis virus forms a recombinant with the endogenous intracisternal A-type particle to yield a complementary DNA molecule that is able to integrate into the mouse chromosomes together with the retrotransposon. These findings may have implications for the use of RNA virus vectors in gene therapy.

  12. Chromatin, DNA Repair, and Variation

    Genomic DNA is wrapped up in chromatin. The principle components of this packaging are the nucleosomes, consisting of histone protein complexes around which roughly 150 base pairs of duplex DNA can be wound. Chromatin can influence the access of effector proteins to the genome, such as transcription and DNA repair factors. Can chromatin then also influence genetic variation? Sasaki et al. (p. 401; published online 11 December; see the Perspective by Semple and Taylor) compare the genomes of two Japanese killifish (medaka) and show that the mutation rate around the start of transcription shows a periodic pattern of peaks and valleys correlating with nucleosome occupancy, with insertions and deletions at a maximum and substitutions at a minimum. It is likely that transcription-coupled repair contributes to this pattern of mutation.

  13. Keeping Polymers in Phase

    When the electrons in a molecule absorb light, they are said to become excited. The closest classical analogy to this quantum-mechanical phenomenon is oscillation up and down a radio antenna, and, in keeping with this analogy, the molecular electrons initially share a phase across the whole illuminated sample. However, random mutual interactions between the excited molecules, as well as solvent perturbations, quickly diminish this shared phase relationship, generally within tens of femtoseconds. Recent work has shown that certain photosynthetic proteins seem to have the capacity to preserve phase coherence between covalently connected domains, and thereby accelerate energy transfer from one to the other. Collini and Scholes (p. 369; see the Perspective by Brédas and Silbey) now show that a similar effect prevails in well-solvated conjugated polymers. Ultrafast anisotropy measurements uncover coherent energy transfer along individual polymer chains on a time scale longer than the expected dephasing time. Transfer between chains, however, does not occur coherently, suggesting an intramolecular phase preservation mechanism.

  14. Hybrid Sterility in Mice

    Two closely related plant or animal subspecies that, on mating, give rise to sterile offspring demonstrate the phenomenon of hybrid sterility, which represents an important isolating mechanism in the formation of new species. Mihola et al. (p. 373, published online 11 December; see the Perspective by Willis) have identified and isolated the gene hybrid sterility 1 (Hst1), which causes hybrid sterility between the mice subspecies Mus musculus musculus and Mus m. domesticus. Rescue of the hybrid sterility phenotype reveals that the gene PR domain 9 (Prdm9) is Hst1. Prdm9 encodes a histone H3 lysine 4 trimethyltransferase, and is expressed in germ cells of both testis (primary spermatocytes) and ovaries, where it regulates the expression of microrchidia 2b. Prdm9 may be a component of a Dobzhansky-Muller incompatibility that is part of an incipient speciation event.

  15. Myriad Mutation Accumulation

    The rate of mutation accumulation and its effects on the genome level has previously been characterized with inferences between distantly related organisms being used to determine evolutionary rates at the fine scale. Moran et al. (p. 379) sequenced the complete genome of seven strains of the bacteria Buchnera aphidicola, an endosymbiont of pea aphids, which revealed a high rate of nucleotide base substitution and allowed the resolution of individual stops underlying gene inactivation. In addition, net loss of DNA was found to be due to larger deletions, gene loss was identified, and the accumulation of adenine/thymine homopolymers was found to result in a high rate of small insertions and deletions, generating in frameshifts in protein-coding genes. These findings reveal a link between the biased base composition of reduced bacterial genomes and the continued loss of gene functions.