23 Sperm quality of Pure Spanish stallions is affected by inbreeding coefficient and age

Abstract

Inbreeding depression, a genetic condition produced by the mating of close-related individuals, has been associated with a reduction of fertility in several species. However, a loss in sperm quality was also associated with age. In horses, the few existing reports have described a tendency of both parameters to produce a negative effect on sperm quality. However, those reports were performed using a subjective evaluation of sperm motility. In the present study, a total of 692 ejaculates from 86 Pure Spanish stallions (PRE), aged between 3 and 22 years, were evaluated using a computer-assisted methodology to determine the effect of inbreeding in four semen parameters: free-gel volume (V), sperm concentration (C, by haemocytometer), and total (TM) and progressive (PM) sperm motility (by Spermvision sperm class analyser; Minitube). The inbreeding coefficient (F) was estimated using 300 000 PRE pedigree records approximately (minimum pedigree depth, eight equivalent complete generations; range, between 1 and 30.1%). Stallion, age, ejaculate, and season of semen collection were the variables included in the statistical model (general linear model), with ejaculate and season being the variables with a major effect (by variance components analysis). Our results showed that sperm concentration (r = −0.18; P < 0.0001) and volume (to a lesser extent) were reduced with advancing age, both showing a major decline after 15 years of age. To the contrary, sperm motility was not affected by age of the stallion. We also found a negative correlation between the inbreeding coefficient and ejaculate volume (r = −0.14; P < 0.001), with a marked decrease seen when F was between 7 and 20%. Also, a negative correlation was observed in PM (r = −0.08; P < 0.05), although to a lower extent. Conversely, C and TM were not affected by inbreeding depression (P > 0.05). In conclusion, our results demonstrated that high levels of inbreeding can compromise severely the sperm quality of the PRE stallion, which, subsequently, may have a negative influence on fertility. Ongoing studies using genomic data will help to detect genetic variants associated with stallion semen quality and how it is influenced by inbreeding in specific genomic regions.