Low fertility in vivo resulting from female factors causes small litter size in 129 inbred mice.

Purpose: 129 inbred mice show poor reproductive ability, as evidenced by small litters; however, the exact cause of this is unknown. In the present in vivo study we examined fertility and subsequent post-implantation development in an attempt to clarify the cause of small litter size in 129 mice.

Methods: 129 or C57BL/6J females that displayed vaginal plugs 1 day after mating with males of the same strain were examined for the presence of fertilized eggs. Reciprocal matings were also performed between 129 and C57BL/6J mice. Subsequent post-implantation development of fertilized eggs was examined by dissecting females 18-19 days after the vaginal plugs were found.

Results: Mean numbers of recovered eggs were 7.9 and 8.0 in 129 and C57BL/6J mice, respectively. Half of the recovered eggs were unfertilized in 129 mice, whereas all were fertilized in C57BL/6J mice. Mean numbers of live fetuses 18-19 days after mating were significantly lower in 129 mice (4.7) than in C57BL/6J mice (7.3). In different types of pairings using both strains of mice, the fertility was significantly lower whenever 129 females were used.

Conclusions: The small litter size in 129 mice is caused by low fertility resulting from female factors.