Intraspecific invasion occurring in geographically isolated populations of the Japanese cyprinid fish Zacco platypus

Geographically isolated populations of freshwater fish have been introduced into conspecific native populations. One of the most representative cases is the pale chub Zacco platypus. The pale chub of Lake Biwa in western Japan has been inadvertently introduced to other waters with the release of game fish. Because they are not clearly discriminated from other populations by their morphological characteristics, the morphological detection of the colonization is not possible. Hence, a genetic analysis of fish collected from rivers of the Kanto Plain in eastern Japan, where the introduction was made, was conducted. The haplotype variety of the mitochondrial gene cytochrome b of the fish revealed the presence of two major groups of haplotypes. We concluded that one of the groups was introduced from western Japan, especially Lake Biwa, and the other was native to the Kanto Plain. The haplotypes from western Japan were established in all of the rivers studied and occurred together with the native haplotypes in the rivers within the original distribution range of the species. The allelic composition of microsatellite loci indicated that fish from the two different sources were well admixed through hybridization in the populations of the rivers of the Kanto Plain.