Diversification and introgression in four chromosomal taxa of the Pearson's horseshoe bat (Rhinolophus pearsoni) group.

Chromosomal variation among closely related taxa is common in both plants and animals, and can reduce rates of introgression as well as promote reproductive isolation and speciation. In mammals, studies relating introgression to chromosomal variation have tended to focus on a few model systems and typically characterized levels of introgression using small numbers of loci. Here we took a genome-wide approach to examine how introgression rates vary among four closely related horseshoe bats (Rhinolophus pearsoni group) that possess different diploid chromosome numbers (2n = 42, 44, 46, and 60) resulting from Robertsonian (Rb) changes (fissions/fusions). Using a sequence capture we obtained orthologous loci for thousands of nuclear loci, as well as mitogenomes, and performed phylogenetic and population genetic analyses. We found that the taxon with 2n = 60 was the first to diverge in this group, and that the relationships among the three other taxa (2n = 42, 44 and 46) showed discordance across our different analyses. Our results revealed signatures of multiple ancient introgression events between the four taxa, with evidence of mitonuclar discordance in phylogenetic trees and reticulation events in their evolutionary history. Despite this, we found no evidence of recent and/or ongoing introgression between taxa. Overall, our results indicate that the effects of Rb changes on the reduction of introgression are complicated and that these may contribute to reproductive isolation and speciation in concert with other factors (e.g. phenotypic and genic divergence).