Three genetic lineages of the Japanese skink Plestiodon japonicus (Scincidae, Squamata) and the genetic composition of their contact zones

Geographic differentiation of the Japanese skink ( Plestiodon japonicus ) throughout the entire Japanese main islands was surveyed using two DNA markers, mitochondrial DNA (mtDNA) and an internal transcribed spacer of nuclear ribosomal genes (ITS-1). Both of the markers concordantly demonstrated geographically discontinuous differentiation between three genetic lineages: the northeastern, central and western lineages. Little to no gene flow occurs between the western and central lineages despite their parapatric distribution suggesting an existence of reproductive isolation despite no known morphological and ecological dissimilarities. On the other hand, the geographic ranges of the northeastern and central lineages are partially separated by a wide area occupied by genetically intermediate type having northeastern mtDNA and central ITS-1. The current data imply that the intermediate type was established by past secondary hybridization between the northeastern and central lineages. Although current gene flow status between the northeastern and central lineages should be elaborated by further data, at least the divergence between the central and western lineages should be maintained by reproductive isolation and therefore taxonomic revision of the species is desired.