Multi-omics identification of ZFPM2 and CD44 as key candidates for testicular size in sheep

Testicular size is a key determinant of ram fertility, yet its genetic architecture in sheep remains poorly understood. Crossbred sheep exhibit substantial testicular developmental variation and serve as ideal models for screening fertility-related genes. Here, we performed whole-genome sequencing (WGS)-based genome-wide association study (GWAS) on 115 rams, including seven crossbred populations (each derived from a distinct sire breed crossed with Hu sheep) and a purebred Hu sheep population. We identified two variants within ZFPM2, including an intronic single nucleotide polymorphism (SNP) rs421073404 and a missense SNP rs1086332841, both significantly correlated with testis weight, length, and width. Importantly, these variants exerted testis-specific effects, consistent with their weak correlation with body weight (r < 0.2). Genomic selection scans (F_ST and π-ratio) between rams with large (>150 g/side) and small (<75 g/side) testes revealed multiple divergent genomic regions, with prominent haplotype differentiation at the CD44 locus on chromosome 15. Notably, two linked missense variants (rs160734053 and rs414318703) in CD44 exerted opposing effects on testicular size, suggesting allelic heterogeneity at this locus. Integrative RNA-seq and ATAC-seq across 0-12 months further uncovered prepubertal stage-specific expression and chromatin accessibility patterns of ZFPM2 and CD44, providing mechanistic insights into their regulatory roles. Collectively, this study provides candidate SNPs for sheep marker-assisted selection and multi-omic evidence for dissecting the genetic basis of testicular development in ovine breeding.