Karyology and biogeography of Dugesia japonica and Dugesia ryukyuensis in Kyushu, southern Japan

In southern Japan, two dugesiid species of freshwater planarians are known: Dugesia japonica Ichikawa et Kawakatsu, 1964, and Dugesia ryukyuensis Kawa katsu, 1976. D. japonica is a common and polymorphic species widely distrib uted in the Far East (karyotypes: n=8, 2x=16, 3x=24). D. ryukyuensis is a s pecies recorded only from the Southwest Islands of Japan (Nansei Shoto) (ka ryotypes: n=7, 2x=14, 3x=21). Recently, small populations of D. ryukyuensis were found in the lowland areas in Kyushu on the East China Sea (the Nishi sonogi Peninsula, the Shimabara Peninsula, the Goto Islands, the Satsuma Pe ninsula, and the Osumi Peninsula). The current geographical distribution of D. japonica and D. ryukyuensis in southern Japan can be explained from geo logical and faunal viewpoints, as follows: (1) two separate inversions by t he ancestor of D. japonica, one in the Miocene and one after early Quaterna ry; (2) only one expansion of its domain by the ancestor of D. ryukyuensis in the Miocene.