Kinematic Sub-Populations in Bull Spermatozoa: A Comparison of Classical and Bayesian Approaches

The ejaculate is heterogenous and sperm sub-populations with different kinematic patterns can be identified in various species. Nevertheless, although these sub-populations are statistically well defined, the statistical differences are not always relevant. The aim of the present study was to characterize kinematic sub-populations in sperm from two bovine species, and diluted with different commercial extenders, and to determine the statistical relevance of sub-populations through Bayesian analysis. Semen from 10 bulls was evaluated after thawing. An ISAS(R)v1 computer-assisted sperm analysis (CASA)-Mot system was employed with an image acquisition rate of 50 Hz and ISAS(R)D4C20 counting chambers. Sub-populations of motile spermatozoa were characterized using multivariate procedures such as principal components (PCs) analysis and clustering methods (k-meansmodel). Four different sperm sub-populations were identified from three PCs that involved progressiveness, velocity, and cell undulatory movement. The proportions of the different sperm sub-populations varied with the extender used and in the two species. Despite a statistical difference (p <0.05) between extenders, the Bayesian analysis confirmed that only one of them (Triladyl(R)) presented relevant differences in kinematic patterns when compared with Tris-EY and OptiXcell(R). Extenders differed in the proportion of sperm cells in each of the kinematic sub-populations. Similar patterns were identified inBos taurusandBos indicus. Bayesian results indicate that sub-populations SP1, SP2, and SP(3)were different for PC criteria and these differences were relevant. For velocity, linearity, and progressiveness, the SP(4)did not show a relevant difference regarding the other sperm sub-populations. The classical approach of clustering or sperm subpopulation thus may not have a direct biological meaning. Therefore, the biological relevance of sperm sub-populations needs to be reevaluated.