Subpopulation distribution of motile sperm relative to activation medium in steelhead (Oncorhynchus mykiss)

In the present study, steelhead sperm were activated in artificial tap water, ovarian fluid, activating saline, or in combinations of these media, and motility characteristics were determined using computer-assisted sperm analysis. Motility characteristics of individual sperm were then assessed to test the hypothesis that motile sperm are distributed among discrete subpopulations and that their distribution is influenced by the activation medium. Analysis with k-means clustering detected three discrete motile sperm subpopulations in steelhead semen, regardless of the activation medium. Based on multivariate analysis of variance, proportions of these subpopulations did not differ between sperm activated with ovarian fluid and activating saline, or any combination of these two media. However, subpopulation distributions for sperm activated with either ovarian fluid or activating saline were influenced by the level of dilution of these media in artificial tap water. There was an increase in the number of sperm in high velocity (curvilinear), high straightness, and high wobble subpopulation with increased levels of ovarian fluid or activating saline. The change in sperm motility characteristics with a change in activation medium may play a role in normal fertilization, as discharged sperm pass from seminal plasma and water through ovarian fluid en route to the egg.