Phylogeographic analysis of mitochondrial gene flow and introgression in the salamander, Plethodon shermani

Plethodon shermani comprises a series of geographically disjunct populations occupying high-elevation mountain isolates. These populations hybridize at their borders with salamanders of the Plethodon glutinosus species complex, and past range expansions inferred from Pleistocene climatic cycles may have increased the possible genetic interactions between P. shermani and species of the P. glutinosus complex. Because mitochondrial DNA haplotypes often show introgression across species borders, we survey mtDNA variation for evidence of past and ongoing genetic interactions between P. shermani, its close relative Plethodon cheoah, and species of the P. glutinosus complex. Ongoing hybridization with the P. glutinosus-complex species Plethodon teyahalee is accompanied by extensive mitochondrial introgression in some Unicoi populations of P. shermani, but it has little genetic impact on P. shermani populations outside hybrid zones at three other isolates (Tusquitee, Wayah Bald, Standing Indian). Some Unicoi populations of P. shermani exhibit mtDNA evidence of past hybridization with diverse lineages from P. aureolus and P. glutinosus. The Tusquitee isolate of P. shermani is also characterized by mtDNA haplotypes most closely related to Plethodon aureolus and P. glutinosus, presumably introduced by past genetic contact with these species or with introgressed populations of Unicoi P. shermani. The mtDNA variation in sampled populations of the Wayah Bald and Standing Indian isolates of P. shermani appears largely unaffected by ongoing hybridization. Principal components analyses of allozymic data indicate that P. shermani isolates collectively form a genetically homogeneous unit clearly demarcated from species with which they have had current or past genetic interactions. Rapid mtDNA introgression associated with transient contacts between P. shermani and other species permits a fine-level resolution of evolutionary lineages not evident from allozymic data.