Historical biogeography and interspecific mtDNA introgression in Euhadra peliomphala (the Japanese land snail)

We assess variation in mitochondrial DNA (mtDNA) using partial sequences (5 60 bp) of the COI gene among populations of Japanese land snails, Euhadra p eliomphala. Phylogeographical analysis reveals five primary clades that cor respond basically to five discrete areas: the Bose Peninsula (B), the Kanto area (K), around Lake Ashinoko (A), the Northern Izu Peninsula (N), and th e Southern Int Peninsula (S). Although there are no current geographical ba rriers separating these five subpopulations, the borders between each area correspond to palaeogeographical events during the Pleistocene, such as vol canic activities and the disappearance of the landbridge between the Southe rn Bose area and the mainland of Japan. In addition, our analysis of isolat ion-by-distance and the distribution of pairwise sequence differences indic ate that haplotypes of the B and Ii lineages have recently increased their population size exponentially. We have also discovered interspecific mtDNA introgression between E. peliomphala and the parapatric species E. brandtii at the contact zone in the Northern Kanto area. Unexpectedly, the E. brand tii-type mtDNA was also found in the populations of E. peliomphala in the S outhern Bose area. Because recent contact between both species is unlikely, the introgressed mtDNA of E. brandtii could be a remnant of past hybridiza tion, when E. peliomphala colonized this area via the landbridge that exist ed in the early Pleistocene.