Pressure-induced insulator conductor transition in a photoconducting organic liquid-crystal film

Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance. Metal insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (approximately 100 microm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10(-13) A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.     

Identification of the T-cell and Ia contact residues of a T-cell antigenic epitope

The precise molecular structure of the antigenic determinant recognized by the T-cell receptor of the CD4-positive cell has not been completely resolved. A major advance in our understanding of this issue has been made by our demonstration of a direct association between an immunogenic peptide and a purified Ia molecule. The most likely and economical hypothesis is that antigen binds directly to an Ia molecule creating the antigenic determinant and that this antigen-Ia complex is recognized by the T-cell receptor. We examined in detail a determinant of hen egg-white lysozyme (HEL) contained in the tryptic fragment HEL(46-61), recognized by T cells in H-2k strains of mice. This peptide binds with a Kd of approximately 3 microM to I-Ak molecules. We have already ascertained that (1) the 10-mer HEL(52-61) is the shortest stimulatory peptide; (2) the Leu56 residue, the only residue different from mouse lysozyme, is responsible for the immunogenicity; (3) the Leu56 and Tyr53 residues are critical for recognition by the T-cell receptor and (4) HEL(46-61) generates multiple determinants when it associated with the I-Ak molecule. If antigen and Ia interact, the antigen must have two features: it must bind to an Ia molecule and also interact with the T-cell receptor. The two sites do not appear to be laterally separable in this peptide and are therefore probably composed of distinct but interspersed amino-acid residues. We have now identified the three residues of HEL(52-61) that contact the T-cell receptor and three other residues that contact the I-Ak molecule. From modelling studies we also propose that HEL(52-61) assumes an alpha-helical conformation as it is bound to I-Ak and recognized by the T-cell receptor.     

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.     

Migratory neural crest-like cells form body pigmentation in a urochordate embryo

The neural crest, a source of many different cell types in vertebrate embryos, has not been identified in other chordates. Current opinion therefore holds that neural crest cells were a vertebrate innovation. Here we describe a migratory cell population resembling neural crest cells in the ascidian urochordate Ecteinascidia turbinata. Labelling of embryos and larvae with the vital lipophilic dye DiI enabled us to detect cells that emerge from the neural tube, migrate into the body wall and siphon primordia, and subsequently differentiate as pigment cells. These cells express HNK-1 antigen and Zic gene markers of vertebrate neural crest cells. The results suggest that migratory cells with some of the features of neural crest cells are present in the urochordates. Thus, we propose a hypothesis for neural crest evolution beginning with the release of migratory cells from the CNS to produce body pigmentation in the common ancestor of the urochordates and vertebrates. These cells may have gained additional functions or were joined by other cell types to generate the variety of derivatives typical of the vertebrate neural crest.     

Community structure and metabolism through reconstruction of microbial genomes from the environment

Microbial communities are vital in the functioning of all ecosystems; however, most microorganisms are uncultivated, and their roles in natural systems are unclear. Here, using random shotgun sequencing of DNA from a natural acidophilic biofilm, we report reconstruction of near-complete genomes of Leptospirillum group II and Ferroplasma type II, and partial recovery of three other genomes. This was possible because the biofilm was dominated by a small number of species populations and the frequency of genomic rearrangements and gene insertions or deletions was relatively low. Because each sequence read came from a different individual, we could determine that single-nucleotide polymorphisms are the predominant form of heterogeneity at the strain level. The Leptospirillum group II genome had remarkably few nucleotide polymorphisms, despite the existence of low-abundance variants. The Ferroplasma type II genome seems to be a composite from three ancestral strains that have undergone homologous recombination to form a large population of mosaic genomes. Analysis of the gene complement for each organism revealed the pathways for carbon and nitrogen fixation and energy generation, and provided insights into survival strategies in an extreme environment.     

Same-sex mating and the origin of the Vancouver Island Cryptococcus gattii outbreak

Genealogy can illuminate the evolutionary path of important human pathogens. In some microbes, strict clonal reproduction predominates, as with the worldwide dissemination of Mycobacterium leprae, the cause of leprosy. In other pathogens, sexual reproduction yields clones with novel attributes, for example, enabling the efficient, oral transmission of the parasite Toxoplasma gondii. However, the roles of clonal or sexual propagation in the origins of many other microbial pathogen outbreaks remain unknown, like the recent fungal meningoencephalitis outbreak on Vancouver Island, Canada, caused by Cryptococcus gattii. Here we show that the C. gattii outbreak isolates comprise two distinct genotypes. The majority of isolates are hypervirulent and have an identical genotype that is unique to the Pacific Northwest. A minority of the isolates are significantly less virulent and share an identical genotype with fertile isolates from an Australian recombining population. Genotypic analysis reveals evidence of sexual reproduction, in which the majority genotype is the predicted offspring. However, instead of the classic a-alpha sexual cycle, the majority outbreak clone appears to have descended from two alpha mating-type parents. Analysis of nuclear content revealed a diploid environmental isolate homozygous for the major genotype, an intermediate produced during same-sex mating. These studies demonstrate how cryptic same-sex reproduction can enable expansion of a human pathogen to a new geographical niche and contribute to the ongoing production of infectious spores. This has implications for the emergence of other microbial pathogens and inbreeding in host range expansion in the fungal and other kingdoms.     

Adaptive evolution drives divergence of a hybrid inviability gene between two species of Drosophila

Speciation--the splitting of one species into two--occurs by the evolution of any of several forms of reproductive isolation between taxa, including the intrinsic sterility and inviability of hybrids. Abundant evidence shows that these hybrid fitness problems are caused by incompatible interactions between loci: new alleles that become established in one species are sometimes functionally incompatible with alleles at interacting loci from another species. However, almost nothing is known about the genes involved in such hybrid incompatibilities or the evolutionary forces that drive their divergence. Here we identify a gene that causes epistatic inviability in hybrids between two fruitfly species, Drosophila melanogaster and D. simulans. Our population genetic analysis reveals that this gene--which encodes a nuclear pore protein--evolved by positive natural selection in both species' lineages. These results show that a lethal hybrid incompatibility has evolved as a by-product of adaptive protein evolution.     

Control of leaf morphogenesis by microRNAs

Plants with altered microRNA metabolism have pleiotropic developmental defects, but direct evidence for microRNAs regulating specific aspects of plant morphogenesis has been lacking. In a genetic screen, we identified the JAW locus, which produces a microRNA that can guide messenger RNA cleavage of several TCP genes controlling leaf development. MicroRNA-guided cleavage of TCP4 mRNA is necessary to prevent aberrant activity of the TCP4 gene expressed from its native promoter. In addition, overexpression of wild-type and microRNA-resistant TCP variants demonstrates that mRNA cleavage is largely sufficient to restrict TCP function to its normal domain of activity. TCP genes with microRNA target sequences are found in a wide range of species, indicating that microRNA-mediated control of leaf morphogenesis is conserved in plants with very different leaf forms.