Unlike the autosomes, recombination between the X chromosome and Y chromosome often thought to be constrained to two small pseudoautosomal regions (PARs) at the tips of each sex chromosome. The PAR1 spans the first 2.7 Mb of the proximal arm of the human sex chromosomes, while the much smaller PAR2 encompasses the distal 320 kb of the long arm of each sex chromosome. In addition to the PAR1 and PAR2, there is a human-specific X-transposed region that was duplicated from the X to the Y. The X-transposed region is often not excluded from X-specific analyses, unlike the PARs, because it is not thought to routinely recombine. Genetic diversity is expected to be higher in recombining regions than in non-recombining regions because recombination reduces the effect of linked selection. In this study, we investigate patterns of genetic diversity in noncoding regions across the entire X chromosome of a global sample of 26 unrelated genetic females. We observe that genetic diversity in the PAR1 is significantly greater than the non-recombining regions (nonPARs). However, rather than an abrupt drop in diversity at the pseudoautosomal boundary, there is a gradual reduction in diversity from the recombining through the non-recombining region, suggesting that recombination between the human sex chromosomes spans across the currently defined pseudoautosomal boundary. In contrast, diversity in the PAR2 is not significantly elevated compared to the nonPAR, suggesting that recombination is not obligatory in the PAR2. Finally, diversity in the X-transposed region is higher than the surrounding nonPAR regions, providing evidence that recombination may occur with some frequency between the X and Y in the XTR.

Human migration and human isolation serve as the driving forces of modern human civilization. Recent migrations of long isolated populations have resulted in genetically admixed populations. The history of population admixture is generally complex; however, understanding the admixture process is critical to both evolutionary and medical studies. Here, we utilized admixture induced linkage disequilibrium (LD) to infer occurrence of continuous admixture events, which is common for most existing admixed populations. Unlike previous studies, we expanded the typical continuous admixture model to a more general admixture scenario with isolation after a certain duration of continuous gene flow. Based on the extended models, we developed a method based on weighted LD to infer the admixture history considering continuous and complex demographic process of gene flow between populations. We evaluated the performance of the method by computer simulation and applied our method to real data analysis of a few well-known admixed populations.

A recent study conducted the first genome-wide scan for selection in Inuit from Greenland using SNP chip data. Here, we report that selection in the region with the second most extreme signal of positive selection in Greenlandic Inuit favored a deeply divergent haplotype introgressed from an archaic population most closely related to Denisovans. The region contains two genes, WARS2 and TBX15, and has previously been associated with body-fat distribution in humans. We show that the adaptively introgressed allele has been under selection in a much larger geographic region than just Greenland. Furthermore, it is associated with changes in expression of WARS2 and TBX15 in multiple tissues including the adrenal gland and subcutaneous adipose tissue, and is associated with BMI-adjusted waist circumference. We also find that in this region both the Denisovan and individuals homozygous for the introgressed allele display differential DNA methylation.

The effect of fluctuating environmental conditions (i.e. environmental stochasticity) on the evolution of virulence has been broadly overlooked, presumably due to a lack of connection between the fields of evolutionary epidemiology and insect ecology. Practitioners of the latter have known for a long time that stochastic environmental variations can impact the population dynamics of many insects, some of which are vectors of infectious diseases. Here we investigate whether environmental stochasticity affecting a vector’s life history can have an indirect impact on the evolutionarily expected virulence of the parasite, using Chagas disease as an example. We model the evolution of virulence using the adaptive dynamics framework, showing that parasite virulence should decrease when the vector’s dynamics randomly change in time. The decrease is even more pronounced when environmental variations are frequent and ample. This decrease in virulence can be viewed as a bet-hedging strategy: when a parasite is at a risk of not being transmitted (e.g. because vectors are scarce), its best option is to stay in the host longer – that is, to be less virulent. Lowering virulence is thus very similar to increasing iteroparity, a well-known risk-spreading strategy, and should be expected to evolve whenever parasite transmission varies randomly in time.

Understanding the causes of gene expression variation is of major importance for many areas of biology. While cis-regulatory changes have long been suggested to be particularly important for adaptation, our understanding of what determines cis-regulatory variation remains limited in most species. Here, we have investigated the prevalence, selective importance, and genomic correlates of cis-regulatory variation in the outcrossing crucifer species Capsella grandiflora. We identify genes with cis-regulatory variation through analyses of allele-specific expression (ASE) in deep transcriptome sequencing data from flower buds and leaves, and use population genomic analyses of high-coverage whole genome resequencing data from both a range-wide sample and a natural population to quantify the impact of positive and purifying selection on these genes. Our results show that in C. grandiflora, cis-regulatory variation is pervasive, affecting an average of 35% of genes within individual plants. Genes harboring cis-regulatory variation are (1) under weaker purifying selection, (2) significantly more likely to harbor nearby transposable element (TE) insertions, and (3) undergo lower rates of adaptive substitutions in comparison to other genes. Using a linear model, we identified ASE as the strongest factor contributing to purifying selection when considered alongside several other commonly used contributing factors. In turn, the main genomic correlates of cis-regulatory variation are presence of nearby TE insertions and gene expression level; notably, the signal of relaxed positive and purifying selection on genes with ASE remains after controlling for expression level. Our results suggest that variation in the intensity of selection across the genome is a major determinant of the presence of intraspecific cis-regulatory variation in this outcrossing plant species.

Measures of population differentiation, such as _FST, are traditionally derived from the ratio of genetic diversity within and between populations. However, the emergence of population clusters from multilocus analysis is a function of genetic structure (departures from panmixia) rather than diversity. If the populations are close to panmixia, slight differences between the mean pairwise distance within and between populations (low _FST) can manifest as strong separation between the populations, thus population clusters are often evident even when the vast majority of diversity is partitioned within populations rather than between them. Moreover, because _FST utilizes the mean rather than deviations from the mean, it does not directly reflect the strength of separation between population clusters. For any given _FST value, clusters can be tighter (more panmictic) or looser (more stratified), and in this respect higher _FST does not always imply stronger differentiation. In this study we propose substituting the mean in the _FST equation with the standard deviation, thereby deriving a novel measure of population separability, denoted _EST , which is more consistent with clustering and classification. To assess the utility of this metric, we ranked various human (HGDP) population pairs based on _FST and _EST and found substantial differences in ranking order. In some cases examined, most notably among isolated Amazonian tribes, _EST ranking seems more consistent with demographic, phylogeographic and linguistic measures of classification compared to _FST. Thus, _EST may at times outperform _FST in identifying evolutionarily significant differentiation.

Most spontaneous mutations affecting fitness are likely to be deleterious, but the strength of selection acting on them might be impacted by environmental stress. Such stress-dependent selection could expose hidden genetic variation, which in turn might increase the adaptive potential of stressed populations. On the other hand, this variation might represent a genetic load and thus lead to population extinction under stress. Previous studies to determine the link between stress and mutational effects on fitness, however, have produced inconsistent results. Here, we determined the net change in fitness in 29 genotypes of the green algae Chlamydomonas reinhardtii that accumulated mutations in the near absence of selection for approximately 1,000 generations across two stress gradients, increasing NaCl and decreasing phosphate. We found mutational effects to be magnified under extremely stressful conditions, but such effects were specific both to the type of stress as well as to the genetic background. The detection of stress-dependent fitness effects of mutations depended on accurately scaling relative fitness measures by generation times, thus offering an explanation for the inconsistencies among previous studies.

Population expansions trigger many biomedical and ecological transitions, from tumor growth to invasions of non-native species. Although population spreading often selects for more invasive phenotypes, we show that this outcome is far from inevitable. In cooperative populations, mutations reducing dispersal have a competitive advantage. Such mutations then steadily accumulate at the expansion front bringing invasion to a halt. Our findings are a rare example of evolution driving the population into an unfavorable state and could lead to new strategies to combat unwelcome invaders. In addition, we obtain an exact analytical expression for the fitness advantage of mutants with different dispersal rates.