Genome divergence and gene flow between Drosophila simulans and D. mauritiana
Sarah B. Kingan, Anthony J. Geneva, Jeffrey P. Vedanayagam, Daniel Garrigan
The fruit fly Drosophila simulans and its sister species D. mauritiana are a model system for studying the genetic basis of reproductive isolation, primarily because interspecific crosses produce sterile hybrid males and their phylogenetic proximity to D. melanogaster. We present an analysis of whole-genome patterns of polymorphism and divergence that shows, on average, the genomes of the two species differ at slightly more than 1% of nucleotide positions and an estimated 40% of autosomal and 60% of X linked loci are reciprocally monophyletic. However, the analysis also identifies 21 major genomic regions, comprising ~1% of the genome, in which one species is segregating for haplotypes that are more similar to haplotypes from the other species than expected, given the levels of sequence divergence in that genomic region. This disjoint distribution of interspecific coalescence times is consistent with recent introgression between the cosmopolitan D. simulans and the island endemic D. mauritiana. We find that the putatively introgressed regions are more likely to have significantly higher rates of crossing-over and are enriched for genes with significantly slower rates of protein evolution. We also uncover instances in which genes experiencing lineage-specific positive selection closely interact with genes experiencing introgression. Finally, we find that a large introgressing region on the X chromosome has experienced a strong selective sweep in D. mauritiana and also has high levels of homozygosity in D. simulans. A detailed analysis reveals that the introgressing X chromosome haplotypes are closely associated with the presence of the MDox locus, which is the progenitor of the Winters sex-ratio meiotic drive genes. These results highlight how genetic systems that evolve rapidly in allopatry, including selfish meiotic drive elements, remain robust in natural hybrid genotypes and do not systematically promote reproductive isolation.