100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures

Saturn's main rings are composed predominantly of water-ice particles ranging between about 1 centimetre and 10 metres in radius. Above this size range, the number of particles drops sharply, according to the interpretation of spacecraft and stellar occultations. Other than the gap moons Pan and Daphnis (the provisional name of S/2005 S1), which have sizes of several kilometres, no individual bodies in the rings have been directly observed, and the population of ring particles larger than ten metres has been essentially unknown. Here we report the observation of four longitudinal double-streaks in an otherwise bland part of the mid-A ring. We infer that these 'propeller'-shaped perturbations arise from the effects of embedded moonlets approximately 40 to 120 m in diameter. Direct observation of this phenomenon validates models of proto-planetary disks in which similar processes are posited. A population of moonlets, as implied by the size distribution that we find, could help explain gaps in the more tenuous regions of the Cassini division and the C ring. The existence of such large embedded moonlets is most naturally compatible with a ring originating in the break-up of a larger body, but accretion from a circumplanetary disk is also plausible if subsequent growth onto large particles occurs after the primary accretion phase has concluded.     

Spin-imbalance in a one-dimensional Fermi gas

Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) proposed an exotic pairing mechanism in which magnetism is accommodated by the formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive. Unlike the three-dimensional (3D) case, theories predict that in one dimension (1D) a state with FFLO correlations occupies a major part of the phase diagram. Here we report experimental measurements of density profiles of a two-spin mixture of ultracold (6)Li atoms trapped in an array of 1D tubes (a system analogous to electrons in 1D wires). At finite spin imbalance, the system phase separates with an inverted phase profile, as compared to the 3D case. In 1D, we find a partially polarized core surrounded by wings which, depending on the degree of polarization, are composed of either a completely paired or a fully polarized Fermi gas. Our work paves the way to direct observation and characterization of FFLO pairing.     

Positional identification of TNFSF4, encoding OX40 ligand, as a gene that influences atherosclerosis susceptibility

Ath1 is a quantitative trait locus on mouse chromosome 1 that renders C57BL/6 mice susceptible and C3H/He mice resistant to diet-induced atherosclerosis. The quantitative trait locus region encompasses 11 known genes, including Tnfsf4 (also called Ox40l or Cd134l), which encodes OX40 ligand. Here we report that mice with targeted mutations of Tnfsf4 had significantly (P 相似文献   

Repetitive transient rises in cytoplasmic free calcium in hormone-stimulated hepatocytes

In the stressed animal, the vasoactive hormones vasopressin and angiotensin-II and the neurotransmitter noradrenaline induce liver cells to release glucose from glycogen. The intracellular signal that links the cell-surface receptors for noradrenaline (alpha 1) and vasoactive peptides to activation of glycogenolysis is known to be a rise in the cytoplasmic concentration of free calcium ions (free Ca). The receptors for these agonists induce the hydrolysis of phosphatidylinositol 4,5-bisphosphate, a minor plasmalemma lipid, to produce inositol trisphosphate and diacylglycerol. Inositol trisphosphate has been shown to mobilize intracellular calcium in hepatocytes. We show here, by means of aequorin measurements in single, isolated rat hepatocytes, that the free Ca response to these agonists consists of a series of transients. Each transient rose within 3 s to a peak free Ca of at least 600 nM and had a duration of approximately 7 s. The transients were repeated at intervals of 0.3-4 min, depending on agonist concentration. Between transients, free Ca returned to the resting level of approximately 200 nM. Clearly, the mechanisms controlling free Ca in hepatocytes are more complex than hitherto suspected.     

How sleep affects the developmental learning of bird song

Sleep affects learning and development in humans and other animals, but the role of sleep in developmental learning has never been examined. Here we show the effects of night-sleep on song development in the zebra finch by recording and analysing the entire song ontogeny. During periods of rapid learning we observed a pronounced deterioration in song structure after night-sleep. The song regained structure after intense morning singing. Daily improvement in similarity to the tutored song occurred during the late phase of this morning recovery; little further improvement occurred thereafter. Furthermore, birds that showed stronger post-sleep deterioration during development achieved a better final imitation. The effect diminished with age. Our experiments showed that these oscillations were not a result of sleep inertia or lack of practice, indicating the possible involvement of an active process, perhaps neural song-replay during sleep. We suggest that these oscillations correspond to competing demands of plasticity and consolidation during learning, creating repeated opportunities to reshape previously learned motor skills.     

Conventional taxonomy obscures deep divergence between Pacific and Atlantic corals

Only 17% of 111 reef-building coral genera and none of the 18 coral families with reef-builders are considered endemic to the Atlantic, whereas the corresponding percentages for the Indo-west Pacific are 76% and 39%. These figures depend on the assumption that genera and families spanning the two provinces belong to the same lineages (that is, they are monophyletic). Here we show that this assumption is incorrect on the basis of analyses of mitochondrial and nuclear genes. Pervasive morphological convergence at the family level has obscured the evolutionary distinctiveness of Atlantic corals. Some Atlantic genera conventionally assigned to different families are more closely related to each other than they are to their respective Pacific 'congeners'. Nine of the 27 genera of reef-building Atlantic corals belong to this previously unrecognized lineage, which probably diverged over 34 million years ago. Although Pacific reefs have larger numbers of more narrowly distributed species, and therefore rank higher in biodiversity hotspot analyses, the deep evolutionary distinctiveness of many Atlantic corals should also be considered when setting conservation priorities.     

Biodiversity conservation: climate change and extinction risk

Thomas et al. have carried out a useful analysis of the extinction risk from climate warming. Their overall conclusion, that a large fraction of extant species could be driven to extinction by expected climate trends over the next 50 years, is compelling: it adds to the many other reasons why new energy policies are needed to reduce the pace of warming.     

A previously unidentified MECP2 open reading frame defines a new protein isoform relevant to Rett syndrome

Rett syndrome is caused by mutations in the gene MECP2 in approximately 80% of affected individuals. We describe a previously unknown MeCP2 isoform. Mutations unique to this isoform and the absence, until now, of identified mutations specific to the previously recognized protein indicate an important role for the newly discovered molecule in the pathogenesis of Rett syndrome.     

How Prometheus creates structure in Saturn's F ring

Images of Saturn's narrow and contorted F ring returned by the Cassini spacecraft have revealed phenomena not previously detected in any planetary ring system. The perturbing effect of the inner shepherding satellite, Prometheus, seems to introduce channels through the F ring and a 'streamer'--a line of particles that link the ring to the satellite. The detailed mechanism for the formation of these features has been lacking an explanation. Here we show that these phenomena can be understood in terms of a simple gravitational interaction as Prometheus approaches and recedes from the F ring every 14.7 hours. Our numerical models show that as Prometheus recedes from its closest approach to the F ring, it draws out ring material; one orbital period later, this affected region has undergone keplerian shear and is visible as a channel, in excellent agreement with structures seen in the Cassini images. Prometheus' periodic disruption of the F ring will become more pronounced as the two orbits approach their minimum separation in 2009. The model predicts that the appearance of streamers and the associated channels will vary in a regular fashion on a timescale of one orbital period.     

Chromatin remodelling at a DNA double-strand break site in Saccharomyces cerevisiae

The repair of DNA double-strand breaks (DSBs) is crucial for maintaining genome stability. Eukaryotic cells repair DSBs by both non-homologous end joining and homologous recombination. How chromatin structure is altered in response to DSBs and how such alterations influence DSB repair processes are important issues. In vertebrates, phosphorylation of the histone variant H2A.X occurs rapidly after DSB formation, spreads over megabase chromatin domains, and is required for stable accumulation of repair proteins at damage foci. In Saccharomyces cerevisiae, phosphorylation of the two principal H2A species is also signalled by DSB formation, which spreads approximately 40 kb in either direction from the DSB. Here we show that near a DSB phosphorylation of H2A is followed by loss of histones H2B and H3 and increased sensitivity of chromatin to digestion by micrococcal nuclease; however, phosphorylation of H2A and nucleosome loss occur independently. The DNA damage sensor MRX is required for histone loss, which also depends on INO80, a nucleosome remodelling complex. The repair protein Rad51 (ref. 6) shows delayed recruitment to DSBs in the absence of histone loss, suggesting that MRX-dependent nucleosome remodelling regulates the accessibility of factors directly involved in DNA repair by homologous recombination. Thus, MRX may regulate two pathways of chromatin changes: nucleosome displacement for efficient recruitment of homologous recombination proteins; and phosphorylation of H2A, which modulates checkpoint responses to DNA damage.