Experimental demonstration that pre- and post-conceptional mechanisms influence sex ratio in mouse embryos.

Previously we have demonstrated in two monotocous species (bovine and sheep), a relationship between time of insemination, moment of ovulation, and embryo sex ratio. Here, we have analyzed in a polytocous specie (mice) if in addition to pre-conceptional mechanisms, also post-conceptional ones affect the offspring sex ratio. To verify this hypothesis we carried out two experiments. In the first experiment, we analyzed the effect of mating dynamics on the sex ratio of mice with synchronic male and female embryo development. Females were mated before and after ovulation and sacrificed 13 days later for sex determination of embryos and reabsorptions. A decreased litter size, and an increased offspring sex ratio in matings occurring later in oestrus, supported the view that a biased sex ratio may occur as the result of behavioral differences between the populations of X- or Y-bearing spermatozoa. In the second experiment, embryos developmentally synchronic and asynchronic with the recipient female endometrium were transferred, and again, 13 days later, females were sacrificed for sex determination of embryos and reabsorptions. The male proportion per litter found, indicated that our developmentally asynchronic transfers favored a sex ratio disbalance at birth. When combined, these results become the first experimental evidence supporting the view that both pre- and post-conceptional mechanisms of sex ratio distortion in polytocous species are not mutually exclusive and both may explain, under different conditions, sex ratio deviations at birth. Copyright 2003 Wiley-Liss, Inc.