This Week in Science

Science  17 Feb 2006:
Vol. 311, Issue 5763, pp. 913
  1. Knowing Your Head from Your Toes


    Embryos of various organisms, such as insects and amphibians, establish head and tail ends at a very early stage because of the localization of cell fate determinants during oogenesis. Mammalian embryos have been thought to be different, with equipotent blastomeres in the early stages. Deb et al. (p. 992) show that early mouse embryos may also have localized determinants. In particular, Cdx2 messenger RNA is asymmetrically localized toward the vegetal pole of mouse oocytes, changes orientation after fertilization, and becomes concentrated in the late dividing blastomere of the two-cell-stage embryo. Thereafter, it marks the cell lineage leading to trophectoderm. Thus, specification of the trophectoderm is already pre-patterned in the mouse oocyte.

  2. Messy Moon Motions

    Two additional moons, named Mab and Cupid, and two outer rings have been discovered around Uranus by Showalter and Lissauer (p. 973, published online 22 December 2005; see the cover and the Perspective by Murray). These new members of the uranian system were spotted in images from the Hubble Space Telescope and traced in earlier pictures from Voyager 2. Substantial changes are seen in the passages of the moons and brightness of the rings since the Voyager 2 fly-by. Many of Uranus' moons do not follow simple keplerian orbits but exhibit complex dynamics, which suggest that the whole system is gravitationally unstable or chaotic.

  3. Martian Aurorae

    Aurorae occur when charged particles are accelerated along magnetic field lines into a planetary atmosphere. Lundin et al. (p. 980) have mapped the motions of ions and electrons flowing in arcs above Mars using the ASPERA-3 experiment on board the orbiting Mars Express spacecraft. The looped paths of charged particles in the martian atmosphere are associated with regions of strong magnetism on the planet's surface, where aurorae have also been seen. This formation mechanism for aurorae on Mars is similar to the one for Earth.

  4. Power to the People Movers

    Despite their high energy density, lithium batteries are not used in cars and other transportation applications because they cannot deliver power at a sufficiently high rate. Kang et al. (p. 977) report a combined theoretical and experimental exploration of a class of battery electrodes with a layered transition-metal structure that permits much faster lithium ion transport. The results suggest a general strategy for improving lithium-battery power delivery.

  5. Metallic Mantle Minerals


    In smaller terrestrial planets having an iron core, the main silicate mineral at depth is thought to be composed of MgSiO3, but its stability at higher pressures cannot yet be determined experimentally. Umemoto et al. (p. 983) used numerical calculations to infer its stability at extreme conditions that may be obtained in the giant outer planets or in newly found, large Earth-like planets in other solar systems. The results imply that MgSiO3 will dissociate to MgO and SiO2. The compression of electronic orbitals at high pressure will lead to more metal-like behavior of these compounds, which would affect their thermal properties and planetary heat flow.

  6. Going Faster

    How much meltwater the Greenland Ice Sheet may be contributing to global sea-level rise depends on the mass balance between the interior of the ice sheet and its margins. The present understanding is that the interior is gaining mass but the margins are eroding even more rapidly. Rignot and Kanagaratnam (p. 986; see the Perspective by Dowdeswell) present an ice velocity map of the entire Greenland Ice Sheet and estimate the rate of ice discharge around its entire margin. A comparison of their results to past data shows that there has been a widespread acceleration of ice flow since 1996, that mass loss has doubled in that time, and that ice dynamics, which are particularly dependent on warming, dominate the rapid retreat of Greenland's glaciers.

  7. Rethinking Sexual Selection

    Much that Darwin said about sexual selection in1871 is culturally and socially biased. His theory attempts to explain why males and females differ, often in ways that are contrary to expectations given natural selection. Roughgarden et al. (p. 965) offers an alternative model that presents social selection theory based on cooperative game theory. Thus, cooperation among individuals in sexual relations, as in other social relations, generates advantages such that groups of individuals that succeed in cooperation may have greater fitness vis-à-vis groups that fail to cooperate. Such differences could generate selection pressure toward individuals and groups that cooperate.

  8. Sex Pays Off

    Sex is expensive. For example, the daughters of an asexual female can reproduce at twice the rate of the progeny descended from a sexual female, assuming a sex ratio of one male to one female. So why is sex maintained despite this apparent disadvantage? One suggestion has been that the lack of meiotic recombination in asexual lineages results in the accumulation of mutations in a sexuals. Paland and Lynch (p. 990; see the Perspective by Nielsen) studied sexual and obligate asexual lineages of Daphnia (water fleas). Through a process of selective interference, the asexual lineages developed a fourfold greater number of mildly deleterious mutations in their mitochondrial genomes compared to the sexual lineages.

  9. Microbial Mobilization of Elemental Sulfur


    Microbial oxidation of elemental sulfur is important in the global sulfur cycle, but little is known about the mechanism of this reaction. Urich et al. (p. 996) have determined a 1.7 angstrom resolution structure of a sulfur oxygenase reductase from a thermoacidophilic archaeon. A spherical, positively charged reaction chamber forms from 24 monomers. Linear sulfur probably enters through apolar channels and is bound by a cysteine persulfide in one of the 24 active sites. This sulfane sulfur chain is the substrate of disproportionation and oxygenation at a nearby mononuclear nonheme iron.

  10. Revving Up the Circadian Clock

    In mammals, circadian rhythms regulate many aspects of behavior and physiology, including sleep-wake cycles and metabolism. Disruption of these rhythms is associated with certain psychiatric illnesses such as bipolar disorder. Yin et al. (p. 1002) describe a potential molecular link between circadian clock control and bipolar disorder. In cultured fibroblasts, a key negative regulator of clock gene expression, the Rev-erbα nuclear receptor, was rapidly degraded after exposure to lithium, which is used in treating bipolar disorder. This destabilization of Rev-erbα led to activation of clock genes.

  11. Don't Think Too Much

    We hope that thinking about a decision results in a good choice, and that the more complex the decision, the more time and effort were invested in thinking about it. Dijksterhuis et al. (p. 1005; see the news story by Miller) show that deliberate thinking about simple decisions (such as buying a shampoo) does yield choices that are judged to be more satisfying than those made with little thought, as expected. However, as the decisions become complex (more expensive items with many characteristics, such as cars), better decisions and happier ones come from not attending to the choices but allowing one's unconscious to sift through the many permutations for the optimal combination.

  12. Norepinephrine, Pleasure, and Reward

    Although norepinephrine is generally accepted to play a role in the adverse effects of opiate withdrawal, its role in mediating the rewarding and stimulatory effects of opiates remains controversial.Olson et al. (p. 1017) discovered that genetically engineered mice unable to synthesize norepinephrine, due to a targeted disruption of the dopamine β-hydroxylase (DBH) gene, appear totally blind to morphine reward, as measured in a conditioned place preference test. Importantly, sensitivity to morphine reward was completely rescued by restoration of DBH expression in a specific set of neurons.

  13. Rats Are Smarter Than We Think

    Although both human and nonhuman animals may use basic associative mechanisms to learn about causal relations, humans have a deeper understanding of causal relations that cannot be reduced to associative learning. In contrast, there is no definite proof that animals, including nonhuman primates, possess deep causal understanding. Blaisdell et al. (p. 1020) present evidence that rats can reason about the effects of their causal interventions. Rats correctly predicted that interventions on one effect of a common-cause model would not affect the other effect. Thus, rats can engage in more sophisticated causal reasoning than predicted by associative models.

  14. Power Laws and Scale Insects

    Power laws apply to many patterns in ecological systems, yet mechanistic understanding of the factors behind power law distributions in populations have proved largely elusive. Vandermeer and Perfecto (p. 1000) studied the biology and the spatial population dynamics of a green scale insect on 40 hectares of shade-grown coffee in Mexico. The scales have several natural enemies (parasitoid wasps and predatory leaf-beetles), but some groups are guarded from these enemies by honeydew-seeking Azteca ants. The frequency distribution of scale cluster sizes among coffee plants generally follows the power-function distribution expected for exponentially growing populations founded at random previous times. The basic power function is a result of exponential population growth operating at the individual bush level; deviations from the power function are the result of incomplete migration patterns of the scale and protection from its ant mutualist.

  15. Transcriptional Regulation of Synaptic Change

    Changes in synaptic activity of neurons can result in stable changes in neuronal function. Many such long-term changes are thought to be mediated by altered transcription of key target genes. Two studies have identified a role for the transcriptional regulator MEF2 (myocyte enhancer factor 2) in the control of synapse number (see the Perspective by Beg and Scheiffele). In cultured rat embryonic neurons, Flavell et al. (p. 1008) found that the calcium-dependent signaling that accompanied neuronal activity led to activation of MEF2 and decreased numbers of synapses. Depletion of MEF2A and MEF2D caused increased synapse formation and MEF2-activated transcription of genes known to reduce synapse formation. Shalizi et al. (p. 1012) also found regulation of synapse number by MEF2 in rat cerebellum and cerebellar slices. MEF2A appeared to act by repressing transcription of the gene encoding the transcription factor Nur77. MEF2A showed this repression activity when sumoylated (modified by covalent linking of small ubiquitin-related modifier protein, or SUMO). Sumoylation was, in turn, dependent on calcium-dependent dephosphorylation of MEF2A. Phosphorylation of MEF2A appeared to switch MEF2A between an acetylated and a sumoylated state.

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