The DDK strain （Mus musculus domesticus） of inbred mouse has a unique peculiarity known as DDK syndrome. The DDK females are mostly infertile when crossed with males of other inbred strains, while DDK males exhibit normal fertility in the reciprocal crosses, as intrastrain matings. This DDK syndrome has been demonstrated to be caused by an incompatibility system between DDK ooplasmic factor and the sperm gene of other strains owing to the ovum mutant （Ore） locus on mouse Chromosome 11. Recently, it was reported that DDK females are fully fertile when crossed to males of MOM （M. m. molossinus） and CASP （M. m. castaneus） strains, indicating that no incompatibilities exist between DDK ooplasmic factor and sperm gene of MOM or CASP males. In the present study, DDK females were found to be also fully fertile when crossed to the males of PWK wild-derived inbred strain （originated from Czech Republic wild mice, M. m. musculus）. The crosses of DDK females × F1 （DDK9 × PWK♀） males also resulted in normal fertility. Furthermore, the transmission ratios of Om alleles from these Fl males to their backcross N2 offspring are 50%：50% as genotyped by microsatellite markers closely linked to Om locus. Moreover, it was demonstrated that PWK females are also fully fertile when crossed to DDK males. All above results indicated that no incompatibility exists between ooplasmic factor and sperm gene in the intersubspecific crosses with DDK and PWK strains. PWK strain would also be useful for further investigations on the DDK syndrome, and DDK strain can be used more widely for various studies in the mouse.
