Single-cell analysis identifies critical regulators of spermatogonial development and differentiation in cattle-yak bulls.

Spermatogenesis is a continuous process in which functional sperm are produced through a series of mitotic and meiotic divisions and morphological changes in germ cells. The aberrant development and fate transitions of spermatogenic cells cause hybrid sterility in mammals. Cattle-yak, a hybrid animal between taurine cattle (Bos taurus) and yak (Bos grunniens), exhibits male-specific sterility due to spermatogenic failure. In the present study, we performed single-cell RNA sequencing analysis to identify differences in testicular cell composition and the developmental trajectory of spermatogenic cells between yak and cattle-yak. The composition and molecular signatures of spermatogonial subtypes were dramatically different between these 2 animals, and the expression of genes associated with stem cell maintenance, cell differentiation and meiotic entry was altered in cattle-yak, indicating the impairment of undifferentiated spermatogonial fate decisions. Cell communication analysis revealed that signaling within different spermatogenic cell subpopulations was weakened, and progenitor spermatogonia were unable or delayed receiving and sending signals for transformation to the next stage in cattle-yak. Simultaneously, the communication between niche cells and germ cells was also abnormal. Collectively, we obtained the expression profiles of transcriptome signatures of different germ cells and testicular somatic cell populations at the single-cell level and identified critical regulators of spermatogonial differentiation and meiosis in yak and sterile cattle-yak. The findings of this study shed light on the genetic mechanisms that lead to hybrid sterility and speciation in bovid species.

Comparative proteomic analysis identifies differentially expressed proteins associated with meiotic arrest in cattle-yak hybrids.

Cattle-yak, the interspecific hybrid between yak and taurine cattle, exhibits male-specific sterility. Massive loss of spermatogenic cells, especially spermatocytes, results in azoospermia in these animals. Currently, the mechanisms underlying meiosis block and defects in spermatocyte development remain elusive. The present study was designed to investigate the differences in the protein composition of spermatocytes isolated from 12-month-old yak and cattle-yak testes. Histological analysis confirmed that spermatocytes were the most advanced germ cells in the testes of yak and cattle-yak at this developmental stage. Comparative proteomic analysis identified a total of 452 differentially abundant proteins (DAPs) in the fluorescence-activated cell sorting (FACS) isolated spermatocytes from cattle-yak and yak. A total of 291 proteins were only present in yak spermatocytes. Gene Ontology analysis revealed that the downregulated DAPs were mostly enriched in the cellular response to DNA damage stimulus and double-strand breaks (DSBs) repair via break-induced replication, while the proteins specific for yak were related to cell division and cycle, spermatogenesis, and negative regulation of the extrinsic apoptotic signaling pathway. Ultimately, these DAPs were related to the critical process for spermatocyte meiotic events, including DSBs, homologous recombination, synapsis, crossover formation, and germ cell apoptosis. The database composed of proteins associated with spermatogenesis, including KPNA2, HTATSF1, TRIP12, STIP1, LZTFL1, LARP7, MTCH2, STK31, ROMO1, CDK5AP2, DNMT1, RBM44, and CHRAC1, is the focus of further research on male hybrid sterility. In total, these results provide insight into the molecular mechanisms underlying failed meiotic processes and male infertility in cattle-yak.

Long read genome assemblies complemented by single cell RNA-sequencing reveal genetic and cellular mechanisms underlying the adaptive evolution of yak.

Wild yak (Bos mutus) and domestic yak (Bos grunniens) are adapted to high altitude environment and have ecological, economic, and cultural significances on the Qinghai-Tibetan Plateau (QTP). Currently, the genetic and cellular bases underlying adaptations of yak to extreme conditions remains elusive. In the present study, we assembled two chromosome-level genomes, one each for wild yak and domestic yak, and screened structural variants (SVs) through the long-read data of yak and taurine cattle. The results revealed that 6733 genes contained high-FST SVs. 127 genes carrying special type of SVs were differentially expressed in lungs of the taurine cattle and yak. We then constructed the first single-cell gene expression atlas of yak and taurine cattle lung tissues and identified a yak-specific endothelial cell subtype. By integrating SVs and single-cell transcriptome data, we revealed that the endothelial cells expressed the highest proportion of marker genes carrying high-FST SVs in taurine cattle lungs. Furthermore, we identified pathways which were related to the medial thickness and formation of elastic fibers in yak lungs. These findings provide new insights into the high-altitude adaptation of yak and have important implications for understanding the physiological and pathological responses of large mammals and humans to hypoxia.