X-Y chromosome dissociation in wild derived Mus musculus subspecies, laboratory mice, and their F1 hybrids.

The frequencies of X and Y chromosome dissociation in primary spermatocytes of wild-caught specimens of six Mus musculus subspecies, fifteen inbred mouse strains, and their F1 hybrids were examined. Three different types (I, II, and III) of X-Y dissociation were identified. Type I was found in inter-subspecies hybrids (BALB/c X M. m. molossinus, M. m. bactrianus, M. m. castaneus, or M. m. urbanus), in which the X-Y dissociation was less than 20% in each subspecies but 60-80% in the f1 hybrids. Type II was seen in the DBA/2J and DDK strains, in which the dissociation frequency was about 30% in the homozygous condition and increased up to 50% in inter-strain hybrids (C57BL/6J X DBA/2J or DDK). Type III found in the WB/Re strain also showed high dissociation frequency (51.1%) in the homozygous condition but the value was remarkably lower (11.3%) in C57BL/6J X WB/Re)F1s. Some asian wild mice seem to have type II or III as well as type I.