Spermatogenesis is directed by germ cell intrinsic factors and extrinsic signals from somatic cells. The development of spermatogenic cells is a hierarchal differentiation process that includes spermatogonia proliferation, meiosis of spermatocytes and spermiogenesis [1]. Spermatogonial lineages originate from primordial germ cell-derived gonocytes (also termed prespermatogonia) that resume proliferation and transition to become undifferentiated spermatogonia or directly enter differentiation during the early stage of development for both mouse and yak [2,3]. A subpopulation of gonocytes form the foundational spermatogonial stem cell (SSC) pool that sustains continued spermatogenesis during adult life. In cattle, gonocytes migrate to the basement membrane and develop into undifferentiated spermatogonia and differentiating spermatogonia approximately 3–5 months after birth [4]. Miscues in developmental events during the gonocyte to spermatogonia transition, including migration, proliferation, and differentiation, can cause severe problems in adult spermatogenesis.

Sertoli cells play central roles in mammalian spermatogenesis by providing germ cells with essential structural, nutrient, and regulatory support [5]. Sertoli cells have fundamental roles in regulating gonocyte survival, fate decisions and proliferation in mice [6,7]. In mice, RA synthesized by Sertoli cells stimulates spermatogonial differentiation and induces the expression of Stra8 (stimulated by retinoic acid gene 8) and c-Kit [8]. In neonatal murine testes, RA stimulates the proliferation of gonocytes and the expression of differentiation markers [9]. The SSC niche factor GDNF also plays a key role in promoting the gonocyte to spermatogonia transition. The GFRA1/RET receptor complex is present in mouse gonocytes, and its depletion severely affects proliferation and SSC content after transplantation [10]. In culture, GDNF supplementation enhances porcine gonocyte proliferation and colony expansion [11]. Sertoli cells are the main source of FGFs (fibroblast growth factors), PDGF (platelet-derived growth factor), and TGFbeta superfamily ligands [[12], [13], [14]]. These factors work in concert to control gonocyte proliferation and the expression of genes required for obtaining spermatogonial fates in mice [14,15]. The function of Sertoli cells and the potential roles of Sertoli cell-derived factors in contributing to spermatogenetic arrest in hybrid animals are underexplored.

The hybrid offspring of yak (Bos grunniens) and taurine cattle (Bos taurus) is an excellent model to decipher the molecular control of interspecies hybrid sterility in large animals. Cattle-yak females exhibit normal fertility, while males are sterile due to impaired spermatogenesis [16]. Spermatogenesis in cattle-yak is arrested in the meiotic phase, and ectopic apoptosis occurs in spermatocytes of adult cattle-yak [17,18]. Cattle and yak have the same number of chromosomes (2n = 60) and no significant difference in morphology [19]. Meiotic defects are not the only problem that hinders spermatogenesis and causes spermatogenic failure; the formation and maintenance of the spermatogonial lineage are impaired in cattle-yak [20,21]. In particular, spermatogonial differentiation, which relies heavily on signals from Sertoli cells, is partially arrested in cattle-yaks [22]. Levels of DNA methylation and H3K9 acetylation were different in all testicular cells, including Sertoli cells, of adult cattle-yak compared to the age-matched yak [23]. Sertoli cells of cattle-yak contained significantly enriched amounts of H3K27me3 and H4K20me3 [17]. Together, these findings indicate that aberrant gene expression in Sertoli cells potentially adversely affects spermatogonial formation and differentiation in cattle-yak.

Based on this evidence, the present study was designed to examine differences in the gene expression profiles of Sertoli cells between yak and cattle-yak. Because the spermatogonial population arises from gonocytes during the neonatal period of development in mice [24] and this event occurs from 3 to 5 months in cattle, yak and cattle-yak [2,4,17], this study examined gene expression in Sertoli cells from 3-month-old yak and cattle-yak testes using RNA sequencing (RNA-seq) and screened candidate genes that have potential roles in regulating the process of gonocyte to spermatogonia transition. The outcomes of these experiments provided insights into the molecular interactions between Sertoli cells and germ cells and highlighted the roles of Sertoli cells in hybrid sterility in large animals.