Genetic structure of a Japanese allotetraploid loach of the genus Cobitis (Osteichthyes, Cobitidae).

The Japanese allotetraploid spined loach of the genus Cobitis "yamato complex" sensu SAITOH et al. (2000), distributed in Western Japan, originated from hybridization between C. biwae on the maternal side and C. striata (Kyushu form) on the paternal side. Mitochondrial (mt) and nuclear DNA were analyzed in order to determine the genetic relationships among 15 populations spanning the entire range of the yamato complex. PCR-RFLP analysis of the ND1 mtDNA gene indicated that the yamato complex contains two divergent types of mtDNA: type A, consisting of one haplotype observed only in the Fukagawa River and type B consisting of 12 haplotypes found in the entire area. Phylogenetic analysis based on the cytochrome b mtDNA gene corroborated RFLP analysis, and indicated that type A was closely related to a different species, C. biwae (Kochi group) and C. striata (large race), rather than type B. The results of RAPD analysis on the Fukagawa River individuals, where types A and B sympatrically existed suggested that no reproductive isolation occurs between them. The existence of two distinct mtDNA types within the yamato complex suggest either multiple maternal origin at the speciation (tetraploidization) time or mtDNA introgression from other species afterwards.