The fertility of DDK mice was investigated in reciprocal crosses between DDK and C57BL/6J strains of mice. When mated to C57BL males, DDK females were semi-sterile. By contrast, when C57BL females were mated to DDK males, they showed better fertility than in the intra-strain mating. Embryological study has revealed that the low fertility of DDK females is caused by embryonic death at the morula-blastocyst or pre-egg cylinder stage, 3 to 5 days after copulation. A defect in trophoblast formation was a common feature of the dead embryos.
Experiments involving ovary transplantation have shown that the cause of embryonic death in DDK females mated to C57BL males is not to be found in the interrelationship between the F1 embryos and the uterine environment of DDK females, but in the fertilized eggs themselves. Possible mechanisms leading to the embryonic death are discussed.