Mouse sperm genotype and the rate of egg penetration in vitro. 
(C57BL/10 X CBA)F1 mouse eggs were incubated in vitro with either F1 or outbred TO sperm for 15 minutes-6 hours.  Upon removal from the sperm suspensions some eggs were treated with pronase to remove zonae and then cultured, while the remainder were simply cultured to allow a comparison of the fertilization rates in the two groups.  Although a brief incubation was sufficient for a high rate of fertilization when the zonae remained intact, results from the zona-free eggs indicated that penetration, i.e., sperm fusion with the vitellus, had not been achieved in all eggs during the shorter incubations.  Results from the latter groups of eggs indicated that a 1 hour incubation with TO sperm and a 2 hour incubation with F1 sperm were needed to obtain a mean fertilization rate of 50%.  Fertilization was complete within 1.5 hours using TO sperm and 2.5 hours using F1 sperm, thus indicating that under similar environmental conditions sperm genotype can significantly affect the rate of penetration.