A boundary between occurrences of mitochondrial cytochrome b haplotypes as well as mito-nuclear discordance in the house mouse Mus musculus from the northern area of Tochigi Prefecture, Japan

The author examined the occurrence pattern of mitochondrial DNA (mtDNA) haplotypes of house mouse (Mus musculus) subspecies musculus and castaneus using 61 individuals caught mainly in the northern area of Tochigi Prefecture, and previously unclarified genealogical discordance on the Pacific Ocean side of the Tohoku region (eastern Tohoku) was characterized. The musculus and castaneus mtDNA haplotypes were found to be widespread, respectively, in the southern and northern areas of the study area, and their geographic ranges are separated by a narrow boundary in Nasushiobara. On the northern side of the boundary, all 27 individuals carrying the castaneus mtDNA haplotypes proved to be homozygous for musculus-type alleles of nuclear genes Tcf25 and Dbndd1, certainly indicating ‘mito-nuclear discordance’ in eastern Tohoku. All 34 individuals on the boundary’s southern side concordantly carry the musculus mtDNA haplotype and are homozygous for the same nuclear musculus-type alleles as is the population on the northern side of the boundary. The occurrence pattern of the mtDNA haplotypes differed from that seen in a hybrid zone (their mixture, i.e., around the Yangtze River in South China), suggesting that extensive introgression across the boundary is unlikely. As an alternative hypothesis, it is explained that the mito-nuclear discordance has arisen before the migration to eastern Tohoku. One individual carries a domesticus-type allele of the nuclear genes, suggesting that the mouse has been affected by introgression of the domesticus genome. These findings should help to clarify some issues in the colonization history of M. musculus on the Japanese islands.