Extensive Introgression of Mitochondrial DNA Found between two Genetically Divergent Forms of Threespine Stickleback, Gasterosteus Aculeatus, around Japan

The genetic structure and phylogenetic relationships of Gasterosteus aculeatus around Japan were investigated for 15 populations of the Japan Sea and Pacific Ocean forms previously detected by allozyme analyses. PCR-RFLP analysis of mtDNA ND5/6 gene indicated two major divergent clades: (1) Alaskan population and (2) Russian and Japanese populations. Japanese populations were clearly subdivided into G. aculeatus populations and hariyo (G. aculeatus leiurus). However, the relationships among Japanese G. aculeatus populations did not support the genetic divergence between Japan Sea and Pacific Ocean forms shown by allozyme analyses. This suggests that mtDNA genetype similarity between the two forms could have resulted from complete replacement of mtDNA from one form to the other. MtDNA introgression is possible in two direction: from Japan Sea form to Pacific Ocean form and the reverse. The haplotype composition of each population observed in the present study suggests that the second direction is more likely. The migrant number of females per generation (Nm) between populations of the two forms was remarkablely large (9.083–infinite), suggesting that gene flow is taking place between the populations at the present time or might have occurred until recent years.