Sperm head phenotype and male fertility in ram semen

Although there is ample evidence for the effects of sperm head shape on sperm function, its impact on fertility has not been explored in detail at the intraspecific level in mammals. Here, we assess the relationship between sperm head shape and male fertility in a large-scale study in Manchega sheep (Ovis aries), which have not undergone any selection for fertility. Semen was collected from 83 mature rams, and before insemination, head shapes were measured for five parameters: area, perimeter, length, width, and p2a (perimeter2/2×π×area) using a computer-assisted sperm morphometric analysis. In addition, a cluster analysis using sperm head length and p2a factor was performed to determine sperm subpopulations (SPs) structure. Our results show the existence of four sperm SPs, which present different sperm head phenotype: SP1 (large and round), SP2 (short and elongated), SP3 (shortest and round), and SP4 (large and the most elongated). No relationships were found between males' fertility rates and average values of sperm head dimensions. However, differences in fertility rates between rams were strongly associated to the proportion of spermatozoa in an ejaculate SP with short and elongated heads (P < 0.001). These findings show how the heterogeneity in sperm head shape of the ejaculate has an effect on reproductive success, and highlight the important role of modulation of the ejaculate at the intraspecific level.