Hybridization and mitochondrial genome introgression between Rana chensinensis and R. kukunoris

Mitochondrial genome (mito-genome) introgression among metazoans is commonplace, and several biological processes may promote such introgression. We examined two proposed processes for the mito-genome introgression between Rana chensinensis and R. kukunoris: natural hybridization and sex-biased dispersal. We sampled 477 individuals from 28 sites in the potential hybrid zone in the western Tsinling Mountains. Mitochondrial gene (cyt-b) trees were used to examine the introgression events. Microsatellite DNA loci, cyt-b and morphological data were used to identify hybrids and to examine the extent of natural hybridization. We detected rampant bidirectional introgressions, both ancient and recent, between the two species. Furthermore, we found a wide hybrid zone, and frequent and asymmetric hybridization. The hybrid zone cline analysis revealed a clear mitochondrial-nuclear discordance; while most nuclear markers displayed similar and steep clines, cyt-b had a displaced cline centre and a more gradual and wider cline. We also detected strong and asymmetric historical maternal gene flow across the hybrid zone. This widespread hybridization and detected low mito-nuclear conflicts may, at least partially, explain the high frequency of introgression. Lastly, microsatellite data and population genetic methods were used to assess sex-biased dispersal. A weak pattern of female-biased dispersal was detected in both species, suggesting it may not play an important role in the observed introgression. Our data are consistent with the hybridization hypothesis, but support for the sex-biased dispersal hypothesis is weak. We further suggest that selective advantages of the R. kukunoris-type mito-genome in thermal adaptation may also contribute to the introgression between the two species.