The objective of this study was to determine the effect of season on sperm quality variables, expression of the fertility-related protein SP22 and selected mRNA transcripts in fresh and cryopreserved stallion sperm. Four stallions were collected in each of the four seasons: summer, fall, winter and spring. Ejaculates were divided and then evaluated for motility, morphology, SP22 staining and expression of selected mRNAs as either fresh semen samples or cryopreserved samples. A significant interaction between season and cryopreservation status was found for total and progressive sperm motility. RNA yield from sperm was not affected by any variable examined. There was no effect of season or cryopreservation on the relative amounts of mRNA for PGK2, TPX1, TIMP3 or ACTB. There was a tendency (P=0.1) for an effect of stallion on the relative amount of ACTB mRNA. The proportion of sperm immunostained for SP22 over the equatorial segment was affected (P<0.05) by stallion. In addition, there was an interaction (P<0.05) between season and cryopreservation status on the percentage of sperm staining for SP22 on the equatorial segment. The correlation among total motility, progressive motility and SP22 immunostaining was much greater (P<0.05) during the breeding season (March and June) than during the non-breeding season (September and December). Based on data analyzed, semen collected in the Northern Hemisphere between March and June may be best suited for cryopreservation.
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