Deep mitochondrial introgression and hybridization among ecologically divergent vole species

The completion of speciation is typically difficult to ascertain in rapidly diverging taxa but the amount of hybridization and gene flow in sympatry or parapatry contains important information about the level of reproductive isolation achieved. Here, we examined the progress in speciation between the Mediterranean ( M icrotus duodecimcostatus ) and the Lusitanian pine vole ( M. lusitanicus ), which are part of the most rapid radiation of species known in mammals. These two Iberian pine voles are classified as separate species because of differences in morphology and ecology, but relatively many ambiguous individuals can be found in sympatric conditions. Our phylogenetic analyses of rangewide data from the mitochondrial cytochrome b gene (mt DNA ) demonstrated high levels of diversity and a basal separation in two parapatric lineages. However, mt DNA affiliation was at odds with morphological classification or geographical distribution of the taxa. In contrast, statistical analyses of microsatellites (nuc DNA ) showed two clear genetic clusters in allopatry and sympatry generally matching morphological classification. This cytonuclear discordance over a large geographic area suggests historical introgression of mt DNA from M . duodecimcostatus to M . lusitanicus . There was statistical evidence for at least two recent hybrids in the sympatry zone but gene flow is apparently low given clear‐cut differences in nuc DNA . Our results indicate a relatively advanced speciation process in these Iberian pine voles without fully established reproductive isolation. This situation enables use of combined population genomic and experimental approaches for the separation of patterns and mechanisms in the ongoing explosive diversification of these and other A rvicoline rodents in the future.