Mechanisms of selective fertilization in the rabbit: sperm transport and viability.

on the proportion of colour-marked offspring. Labelling of living spermatozoa with fluorescein isothiocyanate was used to establish the identity of spermatozoa recovered from the female tract and/or associated with eggs following mixed inseminations. Little difference in the proportion of the two sperm populations present in the female tract was apparent 6 hr after insemination but, by 13 hr, the spermatozoa of the superior buck predominated in all segments of the tract. Moreover, a higher proportion of the spermatozoa found in association with the eggs was contributed by the superior buck following either intravaginal or intratubal mixed insemination 10 hr before ovulation. Although one factor in selective fertilization is a differential survival of the two sperm populations, compensation for this factor by intratubal deposition of equivalent numbers of active uterine spermatozoa did not raise the level of fertilization achieved by the inferior buck. It