Study of the impact of breeding programs in dairy production on cryopreserved ram (Ovis aries) sperm parameters of Manchega sheep

Breeding programs in dairy sheep (Ovis aries) have significantly increased milk production but have led to a decline in other functional aspects like reproductive efficiency. While including functional traits in these programs has mitigated this decline, enhancing productivity remains the main goal due to its importance for sector sustainability. The present study aims to assess changes in reproductive physiology resulting from increased productivity, particularly focusing on the structural and functional characteristics of sperm, occurred over the last 25 years, which are closely linked to male fertility. Ejaculates from 145 mature rams belonging to Manchega dairy sheep breed from 7 different generations, along 2 decades, were collected using an artificial vagina and cryopreserved. After thawing, fertility-related parameters such as sperm motility, viability, and apoptosis-like changes, acrosomal status, mitochondrial activity, and membrane fluidity were assessed at 0 h and 2 h of incubation at 37°C. A regression analysis was carried out in which the changes in each of the sperm parameters were explained based on the generation to which each animal belongs and the animal itself. The genetic trend of these parameters was estimated using the pedigree information of the breed and the BLUPF90 suite. In addition, we analyzed potential alterations in allele and genotypic frequencies of genes associated with the assessed sperm traits by analyzing single nucleotide polymorphisms (SNPs) located within or in proximity to these genes across generations. Our results indicate minimal generational changes in the assessed sperm structural and functional parameters, despite a general positive trend toward more recent generations. No changes in the genetic trends of the evaluated parameters have been observed. Likewise, there were no significant changes in the allele and genotypic frequencies of the markers associated with the genes of interest. The observed changes across sperm parameters were subtle but detectable. Our results indicate that environmental alterations during the cryopreservation process, specifically the duration of storage, seem to be responsible for the differences between generations, as there were no genetic changes due to selection for the trait "milk production."