This Week in Science

Science  24 Nov 2006:
Vol. 314, Issue 5803, pp. 1213
  1. Heat Flow Below


    Heat transfer across the core-mantle boundary (CMB) regulates not only the Earth's magnetic field through the geodynamo but also the style of mantle convection. Measuring heat transfer at such great depth is difficult, but mineral transitions within the mantle, which can be detected seismically, can provide insights. Post-perovskite (pPv) is the most extreme polymorph of perovskite, the primary mineral of the lower mantle, and may be abundant near the CMB. Lay et al. (p. 1272), have located a lens of material just a few hundred kilometers above the CMB beneath the Pacific Ocean that may be pPv. The heat flow in this region could be deduced by measuring the depth of the pPV lens seismically and by knowledge of pPv's mineral properties. Temperature gradients yield a heat flux comparable to the average at the Earth's surface as well as a lower limit to the heat flow.

  2. Lessons of the Past

    Conservation biology and practice are typically based on contemporary ecological information. Willis and Birks (p. 1261) review the need for a perspective that stretches further back in time, and discuss the potential contributions of paleoecological research to conservation biology.

  3. Complex Behavior in Ruthenate Superconductor

    The superconductor strontium ruthenate (Sr2RuO4) is a rather complex material with an unconventional (non-s-wave) pairing symmetry. Unlike other unconventional superconductors, such as the d-wave cuprates, theory suggested and experiments hinted at a p-wave symmetry and a pairing of triplet spins. Theorists also suggested the possibility of a complex p-wave symmetry that breaks time reversal symmetry. Kidwingiri et al. (p. 1267, published online 26 October; see the Perspective by Rice) use phase-sensitive Josephson junction interferometry to confirm the complex p-wave order parameter symmetry in Sr2RuO4, and also present direct evidence for the existence of coexisting chiral superconducting domains.

  4. Seeing Alpha and Beta

    Of the various binary objects in space, binary asteroids are the smallest, as well as the closest for observation. Ostro et al. (p. 1276, published online 12 October; see the cover) used radar to map the binary Earth-approaching asteroid (66391) 1999 KW4 and deduce its physical properties. Alpha, the main component, is an unconsolidated aggregate and spins on its axis every 2.8 hours. The smaller companion, Beta, is elongated and denser than Alpha. Scheeres et al. (p. 1280, published online 12 October) model the coupled orbital and rotational dynamics of the system. Alpha is spinning at a rate near its break-up speed, and the authors suggest that the system may have been put into its excited state by a close pass with the Sun or Earth. The binary asteroid may have ultimately originated from the disruption of a rubble-pile precursor.

  5. Reevaluating Greenland Ice Sheet Melting


    The rate at which Greenland Ice Sheet is melting appears to be accelerating. Luthcke et al. (p. 1286, published online 19 October; see the Perspective by Cazenave) report results from an analysis of data collected by GRACE (Gravity Recovery and Climate Experiment), the pair of satellites launched in 2002, that can follow melting by measuring tiny variations in gravity caused by the redistribution of Earth's mass. Like other recent studies, they find that Greenland is losing ice at an alarming rate, 101 ± 16 gigatons (Gt) of ice per year from 2003 to 2005, compared to the average of about 12 Gt of ice per year for the decade between 1992 and 2002, and they see that ice sheet appears to be losing mass along its southern edges and gaining slightly in its interior. However, the rate they have calculated is much less than other recent estimates, which are closer to 240 Gt of ice per year for the same period. Why the method used in this estimate is so much less than in other stories, and which estimate is correct, has yet to be resolved.

  6. Not Getting Any Younger

    Organic carbon in soils is the second largest active reservoir on Earth and exerts a key influence on the concentration of atmospheric CO2, and about half of soil organic carbon is refractory organic carbon. Smittenberg et al. (p. 1283) compare the radiocarbon ages of terrestrial vascular plant waxes found in marine sediments with those of the surrounding sediments, and find that they become increasingly older throughout the course of the Holocene. They conclude that in soils that have developed since the last deglaciation, accumulation of refractory organic has continued for the duration of the Holocene and is ongoing.

  7. Changes in the Deep

    It is becoming increasingly possible to describe the history of biodiversity in ecological as well as taxonomic terms. Wagner et al. (p. 1289; see the Perspective by Kiessling) provide evidence for a marked change in the ecological structure of marine benthic communities after the largest of the mass extinctions, the end Permian. Using data from a large, open-source repository of fossil occurrence data, they chart the shifts in relative abundances in fossil communities during the Phanerozoic. Before the mass extinction, communities were dominated by sessile, suspension-feeding organisms, whereas afterward, there was a shift to communities dominated by mobile creatures.

  8. A Deadly Complement

    So-called Dobzhansky-Muller genes interact to produce hybrid sterility. Brideau et al. (p. 1292; see the news story by Pennisi) have identified, cloned, and characterized the Lethal hybrid rescue (Lhr) gene in Drosophila simulans, which encodes a protein that localizes to heterochromatic regions of the genome. The proteins encoded by Lhr and Hybrid male rescue (Hmr) form a pair of Dobzhansky-Muller hybrid incompatibility genes, which appear to cause hybrid lethality only in a hybrid genetic background.

  9. Mobilizing Nutrients into Wheat

    Iron is a critical nutrient for plants as much as for the humans who eat them. In plants, iron is required for photosynthesis and respiration, but too much iron can be toxic (see the Perspective by Gitlin). Kim et al. (p. 1295, published online 2 November) provide insight into how plants collect and store iron while avoiding its toxic effects. Analysis of the vacuolar iron transport gene in Arabidopsis shows that the cellular vacuole is used for storage of iron. Uauy et al. (p. 1298) have identified the TaNAM gene, which regulates senescence, as well as the mobilization of nitrogen, zinc, and iron, from leaves to the developing grain. Cultivated wheat varieties have a nonfunctional copy of the TaNAM-B1 gene. Introduction of the functional allele increases grain protein, Zn, and Fe, potentially improving the nutritional content of wheat.

  10. Amateur Pathogen


    Typhoid fever is caused by Salmonella enterica serovar Typhi, which mostly affects impoverished populations in the Southern Hemisphere. Notoriously, it can be carried asymptomatically by individuals who shed large quantities of bacteria. Roumagnac et al. (p. 1301) analyzed 105 strains from around the world and discovered a population structure best explained by neutral genetic drift in which the pre-Neolithic ancestral strain and intervening mutations still exist. Various haplotypes were probably distributed globally during acute epidemics followed by prolonged persistence in the gall bladder of asymptomatic carriers.

  11. Brain Versus Brawn

    The clock genes that control circadian rhythms in mammals also contribute to other aspects of physiology, behavior, and health. One such clock gene, Bmal1, encodes a transcription factor whose inactivation in mice causes disturbances in circadian rhythms and alterations in activity level, body weight, and other physiological functions. By reexpressing the Bmal1 gene in selective tissues in Bmal1-deficient mice, McDearmon et al. (p. 1304) show that the transcription factor exerts distinct tissue-specific functions. Circadian rhythmicity in the mutant mice was normalized only when Bmal1 was expressed in the brain, whereas normalization of the animals' activity level and body weight required Bmal1 expression in muscle.

  12. Bacterial Assist for Chemotherapy

    A major challenge in cancer chemotherapy is delivering cytotoxic drugs to tumors in sufficient quantities to kill the malignant cells while sparing normal cells. One promising strategy for tumor-targeted drug delivery involves encapsulation of drugs within liposomes. Cheong et al. (p. 1308) find that they can markedly enhance the efficacy of liposomal doxorubicin in mouse tumor models by prior injection of the mice with spores of Clostridium novyi-NT, an anaerobic bacterium that selectively infects tumors. C. novyi-NT encodes a secreted protein, “liposomase,” that ruptures liposomes and promotes release of their cytotoxic cargo into the tumor.

  13. Predicting What Comes Next

    How does the brain make the perceptual decisions that lead to object recognition? Using functional magnetic resonance imaging, Summerfield et al. (p. 1311) observed predictive neural signals in the frontal cortex, which suggests that predictive coding accounts for perceptual inference. Moreover, direction-specific functional connectivity between the frontal and visual cortices was observed during perceptual decision-making.

  14. Regulatory RNAs

    Many small regulatory noncoding RNAs act by base-pairing with their target RNAs and interfering with their translation and/or affecting their stability, but not all such RNAs act in this way. The highly conserved prokaryotic 6S RNA interacts with and inhibits the Escherichia coli σ70-containing RNA polymerase (RNAP). 6S RNA contains a single-stranded RNA bulge structure that mimics a gene promoter “open complex” and suggests it might compete with promoters for RNAP. Wassarman and Saecker (p. 1601) show that the bulge does indeed bind in the active site of RNAP in a manner analogous to the open complex, which prevents RNAP binding to legitimate targets. In the presence of nucleotides, the RNAP can synthesize short-product RNAs from the 6S RNA template, which causes the RNAP to dissociate, freeing it to restart transcriptionduring outgrowth from stationary phase and in response to nutrient availability.

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