Evolutionary basis of mitonuclear discordance between sister species of mole salamanders ( Ambystoma sp.).

Distinct genetic markers should show similar patterns of differentiation between species reflecting their common evolutionary histories, yet there are increasing examples of differences in the biogeographic distribution of species-specific nuclear (nu DNA) and mitochondrial DNA (mt DNA) variants within and between species. Identifying the evolutionary processes that underlie these anomalous patterns of genetic differentiation is an important goal. Here, we analyse the putative mitonuclear discordance observed between sister species of mole salamanders ( Ambystoma barbouri and A. texanum) in which A. barbouri-specific mt DNA is found in animals located within the range of A. texanum. We test three hypotheses for this discordance (undetected range expansion, mt DNA introgression, and hybridization) using nu DNA and mt DNA data analysed with methods that varied in the parameters estimated and the timescales measured. Results from a Bayesian clustering technique ( structure), bidirectional estimates of gene flow ( migrate- n and IMa2) and phylogeny-based methods (* beast, bucky) all support the conclusion that the discordance is due to geographically restricted mt DNA introgression from A. barbouri into A. texanum. Limited data on species-specific tooth morphology match this conclusion. Significant differences in environmental conditions exist between sites where A. texanum with and without A. barbouri-like mt DNA occur, suggesting a possible role for selection in the process of introgression. Overall, our study provides a general example of the value of using complimentary analyses to make inferences of the directionality, timescale, and source of mt DNA introgression in animals. [ABSTRACT FROM AUTHOR]