Solar sail capture trajectories at Mercury

Mercury is an ideal environment for future planetary exploration by solar sail since it has proved difficult to reach with conventional propulsion and hence remains largely unexplored. In addition, its proximity to the Sun provides a solar sail acceleration of order ten times the sail characteristic acceleration at 1 AU. Conventional capture techniques are shown to be unsuitable for solar sails and a new method is presented. It is shown that capture is bound by upper and lower limits on the orbital elements of the approach orbit and that failure to be within limits results in a catastrophic collision with the planet. These limits are presented for a range of capture inclinations and sail characteristic accelerations. It is found that sail hyperbolic excess velocity is a critical parameter during capture at Mercury, with only a narrow allowed band in order to avoid collision with the planet. The new capture methodis demonstrated for a Mercury sample return mission

Guppy Y chromosome integrity maintained by incomplete recombination suppression

The loss of recombination triggers divergence between the sex chromosomes and promotes degeneration of the sex-limited chromosome. Several livebearers within the genus Poecilia share a male-heterogametic sex chromosome system that is roughly 20 million years old, with extreme variation in the degree of Y chromosome divergence. In P. picta, the Y is highly degenerate and associated with complete X chromosome dosage compensation. In contrast, although recombination is restricted across almost the entire length of the sex chromosomes in P. reticulata and P wingei, divergence between the X and the Y chromosome is very low. This clade therefore offers a unique opportunity to study the forces that accelerate or hinder sex chromosome divergence. We used RNA-seq data from multiple families of both P. reticulata and P. wingei, the species with low levels of sex chromosome divergence, to differentiate X and Y coding sequence based on sex-limited SNP inheritance. Phylogenetic tree analyses reveal that occasional recombination has persisted between the sex chromosomes for much of their length, as X- and Y-linked sequences cluster by species instead of by gametolog. This incomplete recombination suppression maintains the extensive homomorphy observed in these systems. In addition, we see differences between the previously identified strata in the phylogenetic clustering of X-Y orthologs, with those that cluster by chromosome located in the older stratum, the region previously associated with the sex-determining locus. However, recombination arrest appears to have expanded throughout the sex chromosomes more gradually instead of through a stepwise process associated with inversions

Sex-specific selection drives the evolution of alternative splicing in birds

Males and females of the same species share the majority of their genomes, yet they are frequently exposed to conflicting selection pressures. Gene regulation is widely assumed to resolve these conflicting sex-specific selection pressures, and although there has been considerable focus on elucidating the role of gene expression level in sex-specific adaptation, other regulatory mechanisms have been overlooked. Alternative splicing enables different transcripts to be generated from the same gene, meaning that exons which have sex-specific beneficial effects can in theory be retained in the gene product, while exons with detrimental effects can be skipped. However, at present, little is known about how sex-specific selection acts on broad patterns of alternative splicing. Here we investigate alternative splicing across males and females of multiple bird species. We identify hundreds of genes that have sex-specific patterns of splicing, and establish that sex differences in splicing are correlated with phenotypic sex differences. Additionally, we find that alternatively spliced genes have evolved rapidly as a result of sex-specific selection, and suggest that sex differences in splicing offer another route to sex-specific adaptation when gene expression level changes are limited by functional constraints. Overall, our results shed light on how a diverse transcriptional framework can give rise to the evolution of phenotypic sexual dimorphism

Testing the extraction of past seawater Nd isotopic composition from North Atlantic deep sea sediments and foraminifera

Neodymium isotopes provide a paleoceanographic proxy for past deep water circulation and local weathering changes and have been measured on various authigenic marine sediment components, including fish teeth, ferromanganese oxides extracted by acid-reductive leaching, cleaned foraminifera, and foraminifera with Fe-Mn oxide coatings. Here we compare Nd isotopic measurements obtained from ferromanganese oxides leached from bulk sediments and planktonic foraminifera, as well as from oxidatively-reductively cleaned foraminiferal shells from sediment cores in the North Atlantic. Sedimentary volcanic ash contributes a significant fraction of the Nd when the ferro-manganese (Fe-Mn) oxide coatings are leached from bulk sediments. Reductive leachates of marine sediments from North Atlantic core tops near Iceland, or directly downstream from Iceland-Scotland Overflow Waters, record ɛNd values that are significantly higher than seawater, indicating that volcanic material is easily leached by acid-reductive methods. The ɛNd values from sites more distal to Iceland are similar to modern seawater values, showing little contamination from Iceland-derived volcanogenic material. In all comparisons, core top planktonic foraminifera ɛNd values more closely approximate modern deep seawater than the bulk sediment reductive leached value suggesting that the foraminifera provide a route toward quantifying the Nd isotopic signature of deep North Atlantic water masses

ngsLD: evaluating linkage disequilibrium using genotype likelihoods

MOTIVATION: Linkage disequilibrium measures the correlation between genetic loci and is highly informative for association mapping and population genetics. As many studies rely on called genotypes for estimating linkage disequilibrium, their results can be affected by data uncertainty, especially when employing a low read depth sequencing strategy. Furthermore, there is a manifest lack of tools for the analysis of large-scale, low-depth and short-read sequencing data from non-model organisms with limited sample sizes. RESULTS: ngsLD addresses these issues by estimating linkage disequilibrium directly from genotype likelihoods in a fast, reliable and user-friendly implementation. This method makes use of the full information available from sequencing data and provides accurate estimates of linkage disequilibrium patterns compared to approaches based on genotype calling. We conducted a case study to investigate how linkage disequilibrium decays over physical distance in two avian species. AVAILABILITY: The methods presented in this work were implemented in C/C and are freely available for non-commercial use from https://github.com/fgvieira/ngsLD

Effects of glycerol and sire breed on growth and carcass traits of finishing wether lambs

Objectives: To evaluate crude glycerin as an energy source for finishing lambs and to determine the effect of sire breed on finishing lamb growth performance and carcass characteristics. In light of previous research, the hypothesis for this experiment was that glycerol would have an energy value similar to that of corn when fed in high concentrate diets to finishing lambs

Shared and species-specific patterns of nascent Y chromosome evolution in two guppy species

Sex chromosomes form once recombination is halted around the sex-determining locus between a homologous pair of chromosomes, resulting in a male-limited Y chromosome. We recently characterized the nascent sex chromosome system in the Trinidadian guppy (Poeciliareticulata). The guppy Y is one of the youngest animal sex chromosomes yet identified, and therefore offers a unique window into the early evolutionary forces shaping sex chromosome formation, particularly the rate of accumulation of repetitive elements and Y-specific sequence. We used comparisons between male and female genomes in P. reticulata and its sister species, Endler’s guppy (P. wingei), which share an ancestral sex chromosome, to identify male-specific sequences and to characterize the degree of differentiation between the X and Y chromosomes. We identified male-specific sequence shared between P. reticulata and P. wingei consistent with a small ancestral non-recombining region. Our assembly of this Y-specific sequence shows substantial homology to the X chromosome, and appears to be significantly enriched for genes implicated in pigmentation. We also found two plausible candidates that may be involved in sex determination. Furthermore, we found that the P. wingei Y chromosome exhibits a greater signature of repetitive element accumulation than the P. reticulata Y chromosome. This suggests that Y chromosome divergence does not necessarily correlate with the time since recombination suppression. Overall, our results reveal the early stages of Y chromosome divergence in the guppy

Evolution of dosage compensation under sexual selection differs between X and Z chromosomes.

Complete sex chromosome dosage compensation has more often been observed in XY than ZW species. In this study, using a population genetic model and the chicken transcriptome, we assess whether sexual conflict can account for this difference. Sexual conflict over expression is inevitable when mutation effects are correlated across the sexes, as compensatory mutations in the heterogametic sex lead to hyperexpression in the homogametic sex. Coupled with stronger selection and greater reproductive variance in males, this results in slower and less complete evolution of Z compared with X dosage compensation. Using expression variance as a measure of selection strength, we find that, as predicted by the model, dosage compensation in the chicken is most pronounced in genes that are under strong selection biased towards females. Our study explains the pattern of weak dosage compensation in ZW systems, and suggests that sexual selection plays a major role in shaping sex chromosome dosage compensation

Phenotypic sexual dimorphism is associated with genomic signatures of resolved sexual conflict

Intra‐locus sexual conflict, where an allele benefits one sex at the expense of the other, has an important role in shaping genetic diversity of populations through balancing selection. However, the potential for mating systems to exert balancing selection through sexual conflict on the genome remains unclear. Furthermore, the nature and potential for resolution of sexual conflict across the genome has been hotly debated. To address this, we analysed de novo transcriptomes from six avian species, chosen to reflect the full range of sexual dimorphism and mating systems. Our analyses combine expression and population genomic statistics across reproductive and somatic tissue, with measures of sperm competition and promiscuity. Our results reveal that balancing selection is weakest in the gonad, consistent with the resolution of sexual conflict and evolutionary theory that phenotypic sex differences are associated with lower levels of ongoing conflict. We also demonstrate a clear link between variation in sexual conflict and levels of genetic variation across phylogenetic space in a comparative framework. Our observations suggest that this conflict is short‐lived, and is resolved via the decoupling of male and female gene expression patterns, with important implications for the role of sexual selection in adaptive potential and role of dimorphism in facilitating sex‐specific fitness optima