Female mice of DDK strain are fully fertile in the intersubspecific crosses with Mus musculus molossinus and M. m. castaneus

The female mice of DDK strain are almost infertile when mated with males from other strains. This phenomenon is caused by the early death of F 1 embryos owing to the incompatibility system attributed to the ovum mutant (Om) locus on Chromosome (Chr) 11 and known as DDK syndrome. In the present study, DDK females were found to be fully fertile in the intersubspecific matings with the males of two wild mouse-derived strains, MOM (originated from Japanese wild mice, Mus musculus molossinus) and Cas (originated from Philippine wild mice, M. m. castaneus), indicating that no incompatibility exists between DDK oocytes and spermatozoa of MOM and Cas strains. Furthermore, this compatibility has been confirmed by the following two findings: (1) Normal fertility was shown by the two types of backcrosses, DDK females × F 1 (DDK♀ × MOMS) males and DDK females x F 1 (DDK♀ x Cas♂) males; and (2) the offspring from these backcrosses segregated equally into the homozygotes and heterozygotes as genotyped by the microsatellite markers closely linked to Om locus. MOM and Cas strains would be useful for further investigations on the Om locus. On the other hand, the litter size of F 1 [C57BL/6Cr (B6)♀ x Cas♂] females mated with B6 males was about half that of the mating with DDK males. It would be interesting to investigate whether this reduction in fertility is related to the Om locus or not.