Abstract
The X chromosome, being hemizygous in males, is exposed one third of the time increasing the visibility of new mutations to natural selection, potentially leading to different evolutionary dynamics than autosomes. Recently, we found an enrichment of hard selective sweeps over soft selective sweeps on the X chromosome relative to the autosomes in a North American population of Drosophila melanogaster. To understand whether this enrichment is a universal feature of evolution on the X chromosome, we analyze diversity patterns across six commonly studied Drosophila species. We find an increased proportion of regions with steep reductions in diversity on the X chromosome compared to autosomes, accompanied by elevated homozygosity and an increased proportion of singletons. To assess if these signatures are consistent with positive selection, we simulate a wide variety of evolutionary scenarios spanning variations in demography, mutation rate, recombination rate, background selection, hard sweeps, and soft sweeps, and find that the diversity patterns observed on the X are most consistent with hard sweeps. Our findings highlight the importance of sex chromosomes in driving evolutionary processes and suggest that hard sweeps have played a significant role in shaping diversity patterns on the X chromosome across multiple Drosophila species.
Significance Understanding the unique evolutionary dynamics of the X chromosome is a longstanding question in evolutionary biology given its central role in speciation, fertility, and sexual dimorphism. Recently, we found that hard sweeps, which are indicative of gradual adaptation, are more common on the X chromosome than on the autosomes in a North American population of Drosophila melanogaster. This is because new mutations on the X chromosome are more visible to natural selection due to male hemizygosity. Here, we analyze multiple whole genome sequences from six Drosophila species and find that this pattern is common across all species. Our study demonstrates the power of comparative population genetics in unraveling the forces shaping genetic variation within and between species.
Competing Interest Statement
The authors have declared no competing interest.
