The far-ultraviolet signature of the 'missing' baryons in the Local Group of galaxies

The number of baryons detected in the low-redshift (z < 1) Universe is far smaller than the number detected in corresponding volumes at higher redshifts. Simulations of the formation of structure in the Universe show that up to two-thirds of the 'missing' baryons may have escaped detection because of their high temperature and low density. One of the few ways to detect this matter directly is to look for its signature in the form of ultraviolet absorption lines in the spectra of background sources such as quasars. Here we show that the amplitude of the average velocity vector of 'high velocity' O vi (O5+) absorption clouds detected in a survey of ultraviolet emission from active galactic nuclei decreases significantly when the vector is transformed to the frames of the Galactic Standard of Rest and the Local Group of galaxies. At least 82 per cent of these absorbers are not associated with any 'high velocity' atomic hydrogen complex in our Galaxy, and are therefore likely to result from a primordial warm-hot intergalactic medium pervading an extended corona around the Milky Way or the Local Group. The total mass of baryons in this medium is estimated to be up to approximately 10(12) solar masses, which is of the order of the mass required to dynamically stabilize the Local Group.     

A regulatory mutation in IGF2 causes a major QTL effect on muscle growth in the pig

Most traits and disorders have a multifactorial background indicating that they are controlled by environmental factors as well as an unknown number of quantitative trait loci (QTLs). The identification of mutations underlying QTLs is a challenge because each locus explains only a fraction of the phenotypic variation. A paternally expressed QTL affecting muscle growth, fat deposition and size of the heart in pigs maps to the IGF2 (insulin-like growth factor 2) region. Here we show that this QTL is caused by a nucleotide substitution in intron 3 of IGF2. The mutation occurs in an evolutionarily conserved CpG island that is hypomethylated in skeletal muscle. The mutation abrogates in vitro interaction with a nuclear factor, probably a repressor, and pigs inheriting the mutation from their sire have a threefold increase in IGF2 messenger RNA expression in postnatal muscle. Our study establishes a causal relationship between a single-base-pair substitution in a non-coding region and a QTL effect. The result supports the long-held view that regulatory mutations are important for controlling phenotypic variation.     

Extreme reversed sexual size dimorphism in the extinct New Zealand moa Dinornis

The ratite moa (Aves; Dinornithiformes) were massive graviportal browsers weighing up to 250 kg (ref. 1) that dominated the New Zealand biota until their extinction approximately 500 yr ago. Despite an extensive Quaternary fossil record, moa taxonomy remains problematic and currently 11 species are recognized. Three Dinornis species were found throughout New Zealand and differed markedly in size (1-2 m height at back) and mass (from approximately 34 to 242 kg). Surprisingly, ancient mitochondrial DNA sequences show that the three species were genetically indistinguishable within each island, but formed separate North and South Island clades. Here we show, using the first sex-linked nuclear sequences from an extinct species, that on each island the three morphological forms actually represent just one species, whose size varied markedly according to sex and habitat. The largest females in this example of extreme reversed sexual size dimorphism were about 280% the weight and 150% the height of the largest males, which is unprecedented among birds and terrestrial mammals. The combination of molecular and palaeontological data highlights the difficulties of analysing extinct groups, even those with detailed fossil records.     

High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils

Solar luminosity on the early Earth was significantly lower than today. Therefore, solar luminosity models suggest that, in the atmosphere of the early Earth, the concentration of greenhouse gases such as carbon dioxide and methane must have been much higher. However, empirical estimates of Proterozoic levels of atmospheric carbon dioxide concentrations have not hitherto been available. Here we present ion microprobe analyses of the carbon isotopes in individual organic-walled microfossils extracted from a Proterozoic ( approximately 1.4-gigayear-old) shale in North China. Calculated magnitudes of the carbon isotope fractionation in these large, morphologically complex microfossils suggest elevated levels of carbon dioxide in the ancient atmosphere--between 10 and 200 times the present atmospheric level. Our results indicate that carbon dioxide was an important greenhouse gas during periods of lower solar luminosity, probably dominating over methane after the atmosphere and hydrosphere became pervasively oxygenated between 2 and 2.2 gigayears ago.     

Epoxidation of polybutadiene by a topologically linked catalyst

Nature has evolved complex enzyme architectures that facilitate the synthesis and manipulation of the biopolymers DNA and RNA, including enzymes capable of attaching to the biopolymer substrate and performing several rounds of catalysis before dissociating. Many of these 'processive' enzymes have a toroidal shape and completely enclose the biopolymer while moving along its chain, as exemplified by the DNA enzymes T4 DNA polymerase holoenzyme and lambda-exonucleoase. The overall architecture of these systems resembles that of rotaxanes, in which a long molecule or polymer is threaded through a macrocycle. Here we describe a rotaxane that mimics the ability of processive enzymes to catalyse multiple rounds of reaction while the polymer substrate stays bound. The catalyst consists of a substrate binding cavity incorporating a manganese(III) porphyrin complex that oxidizes alkenes within the toroid cavity, provided a ligand has been attached to the outer face of the toroid to both activate the porphyrin complex and shield it from being able to oxidize alkenes outside the cavity. We find that when threaded onto a polybutadiene polymer strand, this catalyst epoxidizes the double bonds of the polymer, thereby acting as a simple analogue of the enzyme systems.     

The evolution of comets in the Oort cloud and Kuiper belt

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.     

Germline mutations and sequence variants of the macrophage scavenger receptor 1 gene are associated with prostate cancer risk

Deletions on human chromosome 8p22-23 in prostate cancer cells and linkage studies in families affected with hereditary prostate cancer (HPC) have implicated this region in the development of prostate cancer. The macrophage scavenger receptor 1 gene (MSR1, also known as SR-A) is located at 8p22 and functions in several processes proposed to be relevant to prostate carcinogenesis. Here we report the results of genetic analyses that indicate that mutations in MSR1 may be associated with risk of prostate cancer. Among families affected with HPC, we identified six rare missense mutations and one nonsense mutation in MSR1. A family-based linkage and association test indicated that these mutations co-segregate with prostate cancer (P = 0.0007). In addition, among men of European descent, MSR1 mutations were detected in 4.4% of individuals affected with non-HPC as compared with 0.8% of unaffected men (P = 0.009). Among African American men, these values were 12.5% and 1.8%, respectively (P = 0.01). These results show that MSR1 may be important in susceptibility to prostate cancer in men of both African American and European descent.     

Observational evidence for the accretion-disk origin for a radio jet in an active galaxy

Accretion of gas onto black holes is thought to power the relativistic jets of material ejected from active galactic nuclei (AGN) and the 'microquasars' located in our Galaxy. In microquasars, superluminal radio-emitting features appear and propagate along the jet shortly after sudden decreases in the X-ray fluxes. This establishes a direct observational link between the black hole and the jet: the X-ray dip is probably caused by the disappearance of a section of the inner accretion disk as it falls past the event horizon, while the remainder of the disk section is ejected into the jet, creating the appearance of a superluminal bright spot. No such connection has hitherto been established for AGN, because of insufficient multi-frequency data. Here we report the results of three years of monitoring the X-ray and radio emission of the galaxy 3C120. As has been observed for microquasars, we find that dips in the X-ray emission are followed by ejections of bright superluminal knots in the radio jet. The mean time between X-ray dips appears to scale roughly with the mass of the black hole, although there are at present only a few data points.