β-Defensin gene copy number variation in cattle

β-Defensins are peptides with antimicrobial roles, characterized by a conserved tertiary structure. Beyond antimicrobial functions, they exhibit diverse roles in both the immune response and fertility, including involvement in sperm maturation and function. Copy number variation (CNV) of β-defensin genes is extensive across mammals, including cattle, with possible implications for reproductive traits and disease resistance. In this study, we comprehensively catalogue 55 β-defensin genes in cattle. By constructing a phylogenetic tree to identify human orthologues and lineage-specific expansions, we identify 1 : 1 human orthologues for 35 bovine β-defensins. We also discover extensive β-defensin gene CNV across breeds, with <i>DEFB103,</i> in particular, showing extensive multi-allelic CNV. By comparing β-defensin expression levels in testis from calves and adult bulls, we find that 14 β-defensins, including <i>DEFB103</i>, increase in expression during sexual maturation. Analysis of β-defensin gene expression levels in the caput of adult bull epididymis, and β-defensin gene copy number, in 94 matched samples shows expression levels of four β-defensins are correlated with genomic copy numbers, including <i>DEFB103</i>. We therefore demonstrate extensive CNV in bovine β-defensin genes, in particular <i>DEFB103</i>, with potential functional consequences for fertility.