Hybrid origin of Japanese mice "Mus musculus molossinus": evidence from restriction analysis of mitochondrial DNA

The Japanese mouse, Mus musculus molossinus, has long been considered anindependent subspecies of the house mouse. A survey of restriction- sitehaplotypes of mitochondrial DNA (mtDNA) showed that Japanese mice have twomain maternal lineages. The most common haplotype is closely related to themtDNA of the European subspecies M. m. musculus. The other common haplotypeand two minor ones are closely related to each other and to the mtDNA of anAsiatic subspecies, M. m. castaneus. Two other rare variants are probablythe result of recent contamination by European M. m. domesticus. Themusculus type of mtDNA is found in the southern two-thirds of Japan,whereas the common castaneus type is found in the northern third and theminor variants are found sporadically throughout Japan. The castaneus mtDNAlineage had a few minor variants, whereas the musculus lineage wascompletely monomorphic. By contrast, the native population of M. m.castaneus and the Chinese and Korean musculus populations were highlypolymorphic. These results suggest that M. m. molossinus is a hybridbetween ancestral colonies, possibly very small, of M. m. musculus and M.m. castaneus, rather than an independent subspecies.