Dynamic regulation of spermatogenesis and hybrid sterility revealed by single-cell analysis in yak and cattle

Spermatogenesis is a highly orchestrated germ cell differentiation process involving the dynamic regulation of cell fate transitions. Dissecting the molecular landscapes of spermatogenic cell types is crucial for identifying fertility-related problems and improving the reproductive performance of farm animals. Here, we conducted transcriptomic and chromosome spreading across meiotic stages of testicular cells from taurine cattle (Bos taurus), yak (Bos grunniens), and their hybrid progenies to describe the transcriptional landscape of normal spermatogenesis and identify potential regulators that are involved in hybrid sterility. The results revealed seven types of spermatogonia, ten spermatocytes and ten types of spermatids in the cattle or yak testes. In sharp contrast, the testes of the cattle-yak hybrids contained only seven spermatogonial subtypes and six types of spermatocytes. Notably, the arrest of spermatocytes at the diplotene-to-diakinesis transition was accompanied by defects in double-strand break repair. In the testes of backcrossed offspring, spermatogenic arrest was partially rescued, and round spermatozoa were produced. By performing joint analysis, we identified 115 genes that exhibited differential protein abundance in spermatocytes of cattle-yak. Among them, 24 genes carrying genomic structural variations were differentially expressed in spermatocytes of cattle-yak but recovered in those of backcrossed offspring. This work provides important insights into spermatogenesis in large animals and serves as a valuable resource for identifying the factors determining reproductive isolation.