Subpopulation Structure and Changes after Cryopreservation of Sperms from High and Low Fertility Water Buffalo

ABSTRACT
The aim of this study was to identify the sperm subpopulation structure in buffalo bulls with high and low fertility and to determine how sperm subpopulations change after semen cryopreservation. Semen was obtained from four bulls with high fertility (HF) and four bulls with low fertility (LF) and was cryopreserved. A total of 64 ejaculates were assessed for their sperm kinematics using computer assisted sperm analyzer (CASA). Ward’s Hierarchical Dendogram and K-Means clustering method were used to identify the subpopulations. In experiment 1, two significantly different (P≤0.05) sperm subpopulations were observed: Subpopulation 1 (SP1): sperms travel longer distances most rapidly and progressively, and Subpopulation 2 (SP2): sperms travel shorter distances slower but highly progressive. A higher percentage of SP1 was found in HF bulls (47.27); whereas, a higher percentage of SP2 was found in LF bulls (54.89). A low negative relationship (r=-0.18) was observed for the fertility level and sperm subpopulation structure. This implies that sperms that travel longer distances most rapidly and progressively (SP1) are most likely associated to high fertility, while sperms that travel shorter distances slower but highly progressive (SP2) are associated with low fertility. In experiment 2, based on the change in SP1 after cryopreservation, significantly higher sperm survival was observed in samples from HF bulls (29.97) as compared to those from LF bulls (31.64). During post thaw, there were more SP1 sperms in samples from HF bulls (27.52) than in those from LF bulls (26.74). Thus, semen containing higher proportion of SP1 sperms are more resistant to cryopreservation and have greater chances of obtaining high fertility. Overall, the identification of sperm heterogeneity in water buffaloes can be associated to sperm survival after cryopreservation and fertility. [...]