Identification of sperm subpopulations with specific motility characteristics in stallion ejaculates

The aim of this study was to test the presence of separate sperm subpopulations, with specific motility characteristics, in stallion ejaculates by using a computer-assisted semen motility analysis (CASA) system. Motility data were analyzed with a hierarchical clustering of variables based on a correlation or covariance matrix to select like parameters of sperm motility descriptors that better explain the kinetics of spermatozoa. The statistical analyses clustered the whole motile sperm population in both fresh and 24 h stored ejaculates into four separate groups. There were significant differences in the distribution of the four subpopulations (P < 0.001) as well as in the total sperm number and the percentage of total motility (P < 0.01) in fresh semen among the five stallions tested. Our results show that separate subpopulations of spermatozoa with different motility characteristics coexist in stallion ejaculates. These subpopulations were maintained, although with a less-progressive motion pattern, after 24 h of storage. The study of subpopulations in ejaculates that have confirmed fertilizing capacity showed that the majority of the motile spermatozoa in these ejaculates are included in a subpopulation with high progressive motility and low linearity, and the ejaculates with proven fertility that have a total sperm count > or = 20 x 10(9) spermatozoa/ejaculate show all of their motile sperm included in this subpopulation. Our results show that the use of the CASA system is a relatively simple approach to the study of sperm subpopulation patterns in equine ejaculates.