Effects of freezing/thawing on motile sperm subpopulations of boar and donkey ejaculates

The main aim of this study is to assess the influence of freeze/thawing on motile sperm subpopulations in ejaculates from two phylogenetically different mammalian species, boar and donkey. Our results indicate that, whereas boar and donkey sperm respond very differently in their mean motion characteristics to freezing/thawing, this process did not change the existence of a 4-subpopulations structure in the ejaculates in either species when these subpopulations were defined by taking values of curvilinear velocity (VCL) as reference. Moreover, the freezing/thawing-linked changes in mean sperm-motion characteristics in both boar and donkey semen were especially due to changes in the proportion among each concrete subpopulation. In this way, the freezing/thawing-induced mean increase in motion characteristics observed in boar sperm was a result of the decrease in the percentage of sperm in Subpopulation 1 (from 53.9%+/-4.7% to 31.2%+/-3.9% after thawing) and a concomitant increase of sperm from Subpopulations 3 (from 13.3%+/-2.5% to 32.6%+/-3.9% after thawing) and 4 (from 3.4%+/-0.9% to 8.0%+/-1.1% after thawing). On the contrary, changes in mean motility of frozen/thawed donkey sperm were linked to an increase in the percentage of sperm in Subpopulation 1 (from 31.5%+/-4.3% to 58.8%+/-4.9% after thawing) and a concomitant decrease of sperm from Subpopulations 3 (from 32.4%+/-3.2% to 6.6%+/-1.8% after thawing) and 4 (from 12.2%+/-2.5% to 7.3%+/-1.9% after thawing). In conclusion, our results seem to indicate that motility changes induced by the freezing/thawing protocol are linked to concomitant changes in both the specific parameters and, more importantly, to the specific percentage of each of the motile sperm subpopulations. These changes did not affect the overall proportion of motile sperm present in both boar and donkey, which is conserved despite the detrimental effect caused by freezing/thawing in both species. Finally, the presence of some kind of motile sperm subpopulations structure has been described in mammalian species with a very great phylogenetic distance, thus suggesting that this structure could play some role in the maintenance of the overall function of mammalian ejaculates.