COVER The Interstellar Boundary Explorer (IBEX) spacecraft has returned the first global images of the interaction of our heliosphere with the local interstellar medium. IBEX observations show a ribbon of energetic neutral atoms (reds to greens), snaking between the positions of the two Voyager spacecraft (white dots). This ribbon marks the region where the galactic magnetic field (gray lines) wraps most tightly around the heliosphere’s boundary. See the series of Reports starting on page 959. Image: Patrick McPike/Adler Planetarium
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This Week in Science
Highlights of Research in This Issue
Volume 326, Number 5955, Issue of 13 November 2009
©2009 by The American Association for the Advancement of Science.
Sexual Blotch
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Sexual conflicts may provide selection pressures that facilitate invasion of new genetic sex determiners. Roberts et al. (p. 998, published online 1 October) describe data obtained from several species of sexually dimorphic cichlid fishes found in Lake Malawi that support this model. An orange-blotch color pattern has evolved among the females of rock-dwelling species because it provides effective camouflage against the algae-coated rocks. Blotched males (OB) are rare, possibly because the trait interferes with the normal male color pattern of blue stripes that are important cues for mate selection. The pigmentation pattern that creates this conflict between natural selection in females and sexual selection in males is caused by mutation in the cis-regulatory region of the Pax7 gene and is tightly linked to a dominant female sex determiner. When bred in the lab, the OB males inherit the intact maternal OB haplotype and their OB-carrying chromosome determines female sex in the offspring; the males seem to be sex-reversed by another mechanism.
Quantum Division
The notion of quantum mechanics is that variables are expressed as integer values. In quantum fluids, for instance, vortices are quantized in terms of the polarization and phase shifts observed as multiples of 2
, that is, full rotations of each variable around the vortex core. Theoretical work has predicted that in some instances there should be half-quantum vortices, where circumventing the vortex core is characterized by just rotations of the phase and polarization. By observing exciton-polariton condensates Lagoudakis et al. (p. 974) present evidence for the existence of these long-predicted half-quantum vortices.
Biffed into Shape
BiFeO3 is known to have a very large ferroelectric polarization. Although the bulk ground state is rhombohedral (with the electrical polarization along the [111] direction), in thin films and under epitaxial strain, the material can be tetragonally distorted (polarization along [001]). Zeches et al. (p. 977) show that under compressive strain, these films are monoclinic, a phase that is highly stable because it comprises the same symmetry as the monoclinic phase which has been reported at the so-called morphotropic phase boundaries in technologically important ferroelectrics. This work offers the possibility of obtaining large piezoelectric responses in lead-free systems.
GRACE and Movement Together
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Recent measurements of the rate of mass loss from the Greenland ice sheet vary approximately by a factor of three. Resolving these discrepancies is essential for determining the current mass balance of the ice sheet and to project sea level rise in the future. Van den Broeke et al. (p. 984) obtained consistent estimates from two independent methods, one based on observations of ice movement combined with model calculations and the other on remote gravity measurements made by the GRACE (Gravity Recovery and Climate Experiment) satellites. The combination of these approaches also resolves the separate contributions of surface processes and of ice dynamics, the two major routes of ice mass loss.
Outfoxing Immune Excess
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Immune responses are kept in check by Foxp3-expressing CD4+-regulatory T cells (Tregs) through a variety of mechanisms. Expression of specific transcription factors directs Treg responses into distinct T helper cell lineages; however, the transcription factors that regulate particular helper lineages have not been completely characterized. Chaudhry et al. (p. 986, published online 1 October) show that the transcription factor Stat3, that is required for the initial differentiation of TH17-effector T cells, is also required for Treg cell-mediated suppression of TH17-mediated immune responses. Mice carrying a Treg cell-specific deletion in Stat3 succumb to an intestinal inflammatory disease driven by uncontrolled TH17 responses. Thus, different classes of immune responses can result from the expression of helper lineage–specific transcription factors.
Sleep and Alzheimer’s Disease
Accumulation of amyloid-β (Aβ) in the brain is thought to be the initiating event in the pathogenesis of Alzheimer’s disease (AD). Aβ is a peptide secreted in a soluble monomeric form predominantly by neurons and its aggregation into toxic forms is concentration dependent. Synaptic activity regulates the release of Aβ in vivo. However, how physiological and environmental processes are involved in regulation of Aβ levels is not understood. Kang et al. (p. 1005, published online 24 September), by performing sleep-wake studies in freely behaving animals concomitant with in vivo microdialysis, found that brain interstitial fluid levels of Aβ were significantly correlated with wakefulness and negatively correlated with sleep. Furthermore, relatively short-term (3 weeks) sleep deprivation markedly accelerated amyloid plaque deposition in amyloid precursor protein transgenic mice. Thus, sleep-wake behavior is linked to Aβ levels and abnormal sleep may be linked to AD pathogenesis.
Shelterin’ the Ends
Linear chromosomes have two inherent problems: First, DNA polymerase cannot operate at the very end of a chromosome, and so genetic material could be lost at each cell division. Second, the ends of the chromosomes might be mistaken as broken DNA, activating damage signaling and repair pathways with potentially disastrous effects. De Lange (p. 948) reviews the solutions that cells have evolved for solving these potential problems by the evolution of chromosome end–capping telomeres and by avoiding inappropriate recognition of the telomeres. “End protection” is achieved by a combination of the shelterin complex and the formation of special t-loop structures in some species that disguise the chromosome end.
Life Histories to Suit
Nematode worms can profoundly manipulate their life histories in several ways. For example, Caenorhabditis elegans has two genders: males and hermaphrodites. Some clues for the evolution of this peculiar mating system have been revealed by Baldi et al. (p. 1002), who turned females of a related species, Caenorhabditis remanei, into hermaphrodites by modifying a gene involved in making sperm and another gene required for activating the spermatids. In most animals, the germ line is fully established during adulthood and a reproductive period is determined, at least in part, by aging of the germ line and the viability of oocytes. The reproductive longevity of hermaphrodite C. elegans can be increased at least 15-fold by starvation. Angelo and Van Gilst (p. 954, published online 27 August; see the Perspective by Ogawa and Sommer) found that in starved worms, the germline component of the reproductive system is actively killed, with the exception of a small set of preserved stem cells. When the worms are able to feed again, these cells regenerate into an entirely new and functional germ line. But this is not all. Kim et al. (p. 994, published online 1 October; see the Perspective by Ogawa and Sommer) show that subsets of the complex mixture of structurally related molecules in dauer pheromone act via distinct G protein–coupled receptors either to initiate long-term effects on development and physiology by modulating the neuroendocrine axis, or to trigger short-term acute effects on behavior by altering neuronal responses.
What’s Happening in the Heliosphere
The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959, published online 15 October), Fuselier et al. (p. 962, published online 15 October), Funsten et al. (p. 964, published online 15 October), and Möbius et al. (p. 969, published online 15 October) present data taken by NASA’s Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966, published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971, published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Sun’s travel through the galaxy as was previously thought.
Catching Glassy Crystallization
The initial steps for crystallization are difficult to study in atomic materials because they occur on very small length- and time scales. Measurements can be made on surfaces, or by using colloids as analogs, but ideally one would like to observe the ordering phenomena in atomic solids in bulk. Lee et al. (p. 980; see the Perspective by Gibson) use fluctuation transmission electron microscopy to explore the role of nucleation in propagating crystallization and discovered that formation of subcritical nuclei strongly influences the crystallization process.
BubR1 Broadens Its Remit
During mitosis in mammalian somatic cells, BubR1 is indispensable for spindle assembly checkpoint signaling and for establishing contacts between chromosomes and spindle microtubules. Homer et al. (p. 991) found that in mouse oocytes during meiosis I, BubR1 was not only required to sustain prophase I arrest but also for promoting the completion of meiosis I. Both effects converge on the Cdh1 coactivator of the multimeric ubiquitin ligase, known as the anaphase-promoting complex, and both functions are required for the production of fertilizable eggs.
Editors’ Choice
Volume 326, Number 5955, Issue of 13 November 2009
©2009 by The American Association for the Advancement of Science.
Evolution:
No b to Rule Them All
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The dependence of basal metabolic rate (BMR) on body mass (M) in mammals is generally expressed as BMR = aMb, but the value of the exponent b has long been disputed. Some researchers argue for geometric scaling (b = 0.67), as predicted by surface area–to–volume ratios. Others claim quarter-power scaling (b = 0.75), which has been supported by theoretical analyses of nutrient supply networks. However, most empirical “mouse-to-elephant” studies have not accounted for the shared evolutionary history of the sampled species. To do so, White et al. use two methods: phylogenetic generalized least squares and independent contrasts. They consider high-quality BMR data from 585 species (which include only measurements on inactive, fasted, nonreproductive adults in a thermoneutral environment) and a subset of 537 species that excludes lineages for which BMR may not be measurable. Their phylogenetically informed estimates of b fall between 0.67 and 0.75, and all of their statistically valid analyses reject both values. In addition, they find that b differs among lineages. Their results reinforce doubts as to the existence of a universal allometric relationship between mammalian BMR and body mass.
Evolution 63, 2658 (2009).
Chemistry:
Poison Probe
Angew. Chem. Int. Ed. 48, 8519 (2009).
Materials Science:
Up, Up and Away
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When hot vapor comes in contact with a cold surface, such as a shower wall, liquid droplets are created that quickly coalesce and form a film. This condensation process is ubiquitous in natural as well as artificial environments. In industrial settings, preventing film formation is generally desirable because liquid films are poor heat conductors. However, it can be challenging to remove the droplets more quickly than they coalesce, particularly when nonvertical sample orientations preclude help from gravity. Boreyko and Chen demonstrate the spontaneous elimination of droplets from a horizontal surface. They prepare a superhydrophobic substrate consisting of carbon nanotubes deposited on silicon micropillars (shown at right). Video imaging of the condensation of ambient moisture reveals that, after the droplets are formed, they initially coalesce without moving, then eventually reach a mobile phase where several droplets fuse and leave the surface of the sample in a dramatic out-of-plane jump. The energy for the jump is provided by the decrease in surface energy gained by coalescence; the average condensed droplet size is an order of magnitude smaller than that observed in gravitational removal. Interestingly, a similar mechanism is thought to be used by a type of mushroom to eject a spore from its sterigma.
Phys. Rev. Lett. 103, 184501 (2009).
Cell Biology:
As It Happens
. Putting together snapshots taken at 7.5-min intervals for 3 hours, they documented the waves of RNA synthesized by polymerase, with each molecule of enzyme jumping on at the transcription start site and hopping off over 100 kb away. During each round, upstream introns were excised as the polymerase moved downstream, often before the next intron was reached. Furthermore, it appeared that later rounds of transcription were begun and continued for a few kilobases before being aborted, as if the polymerases were colliding with a checkpoint that kept track of downstream occupancy of the gene. Each wave of transcription traveled at 
3 kb m–1, although the accumulation of polymerase at regions bound by the insulator proteins CTCF and cohesion, both of which are involved in chromosome looping, suggested that polymerase may be held up at specific sites.
Proc. Natl. Acad. Sci. U.S.A. 106, 18357 (2009).
