Data from: 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 (Microtus 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 (mtDNA) demonstrated high levels of
diversity and a basal separation in two parapatric lineages. However,
mtDNA affiliation was at odds with morphological classification or
geographical distribution of the taxa. In contrast, statistical analyses
of microsatellites (nucDNA) 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 mtDNA 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
nucDNA. 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 Arvicoline rodents in the
future.