Porcine sperm fertilizing potential in relationship to sperm functional capacities

Fertile males vary in the fertilization efficiency of their sperm, but these differences are not readily detected with the currently routine in vitro semen analyses. Inseminating females with mixed semen containing equal numbers of sperm from two males, heterospermic insemination, is an efficient means to detect these differences in vivo. In this study, heterospermic insemination procedure was used to estimate relative in vivo fertility of the nine pairs of ejaculates from 12 fertile boars. In vitro assays that model various steps in the fertilization process were used to estimate the contribution of various steps in the fertilization process to the variation in sperm fertilizing potential. These assays included sperm binding to the porcine zona pellucida (ZP), acrosomal status in conjunction with viability before and after capacitation, interaction with the hamster oocyte plasma membrane and characteristics of sperm motion. The ejaculates exhibited significant variation in sperm fertilizing potential as assessed by the heterospermic insemination procedure. The ability to interact with the oocyte plasma membrane was significantly correlated with relative in vivo fertility (R2 = 0.80; P < 0.005); the other functional characteristics of these samples were not correlated with in vivo fertility. The lack of correlation between in vivo sperm fertilizing potential and some of the functional characteristics such as motility is likely a reflection of the very limited variation in these characteristics in fertile males. The high correlation between in vivo fertility and ability of porcine sperm to interact with hamster oocyte plasma membrane suggests assessment of this interaction may improve the in vitro assessment of sperm fertilizing potential.