29 Increase of genomic inbreeding at chromosome level affects differently the sperm head morphometrics shape in frozen–thawed stallion sperm

Inbreeding depression (a phenotypic decrease due to an increase in homozygosity resulting from mating between related individuals) has been demonstrated in various species and productive traits. It has been shown that inbreeding can significantly impact the fertility of stallions, affecting both sperm motility and longevity. The use of medium-density genotyping has enabled the identification of genomic regions associated with inbreeding depression in various species and productive traits. In this study, we aimed to analyse the effect of increased inbreeding on stallion sperm morphology at the chromosome level. Fifty-three Pura Raza Español stallions were morphometrically assessed using the MORP module of SCA sperm analyzer (Microptics). We analysed one frozen–thawed sperm sample per individual. Measurements of sperm head area, perimeter, width, length, as well as five additional composite measures (ellipticity, rugosity, regularity, elongation, and perimeter-to-area percentage) were determined in at least 50 spermatozoa per sample. Additionally, genotyping was performed using a medium-density SNP array chip (GGP, Neogen), providing allele composition for ~68 000 autosomal markers per individual. Inbreeding values for each individual were estimated using an F_ROH-based approach implemented in the sliding windows methodology of PLINK, both for the entire genome and per chromosome. The results revealed positive correlations between homozygosity increase and sperm width (r = 0.24*), and negative correlations with ellipticity (r = −0.27*) and elongation (r = −0.26*). Pearson correlation was used. Conversely, area and perimeter were less affected by the increase in homozygosity. Intriguingly, the increase in homozygosity at specific genomic regions, such as ECA22 and ECA23, significantly amplified sperm width (r ~ 0.34), while an increase in homozygosity at ECA24 decreased sperm width (r² = −0.23). On the contrary, the variation of homozygosity values on ECA10, ECA13, ECA25, and ECA31 did not affected sperm width (r ~ 0). Interestingly, this variability in the effect of homozygosity was also observed in the rest of the traits analysed. Overall, our study highlights that increased homozygosity in certain genomic regions differentially influences the average shape of stallion sperm. Further investigations are required to associate these regions with candidate genes and markers, enhancing our understanding of the genetic factors underlying sperm morphological variation in this species.