Living at a high altitude involves many environmental challenges. The combined effects of hypoxia and cold stress impose severe physiological challenges on endothermic animals. The yak is integral to the livelihood of the people occupying the vast, inhospitable Qinghai–Tibetan plateau and the surrounding mountainous region. Due to long-term selection, the yak exhibits stable and unique genetic characteristics which enable physiological, biochemical, and morphological adaptations to a high altitude. Thus, the yak is a representative model for mammalian plateau-adaptability studies. Understanding coping mechanisms provides unique insights into adaptive evolution, thus informing the breeding of domestic yaks. This review provides an overview of genetic adaptations in Bos grunniens to high-altitude environmental stress. Combined genomics and theoretical advances have informed the genetic basis of high-altitude adaptations.

The change in the feeding system can greatly improve the growth performance of the yak (Bos grunniens), an important livestock species in the plateau region. Here, we comprehensively compared the effects of different feeding systems on the growth performance and ruminal development of yaks, and investigated the effects of ruminal microorganisms and metabolites using the 16S rRNA gene sequencing and liquid chromatograph–mass spectrometer (LC-MS) technologies. We found that compared to traditional grazing feeding, house feeding significantly improved the growth performance (such as average daily gain and net meat weight) and rumen development of the yaks. At the genus level, the abundance of Rikenellaceae RC9 Gut group, Christensenellaceae R-7 group, Lachnospiraceae NK3A20 group, Ruminococcaceae UCG-014, and Prevotellaceae UCG-003 showed significant differences and was closely related to rumen development in the two distinct feeding systems. Also, metabolomics revealed that the change in the feeding system significantly affected the concentration and metabolic pathways of the related rumen metabolites. The metabolites with significant differences were significantly enriched in purine metabolism (xanthine, adenine, inosine, etc.), tyrosine metabolism (L-tyrosine, dopaquinone, etc.), phenylalanine metabolism (dihydro-3-caumaric acid, hippuric acid, etc.), and cAMP signaling pathway [acetylcholine, (-)-epinephrine, etc.]. This study scientifically support the house fattening feeding system for yaks. Also, our results provide new insights into the composition and function of microbial communities that promote ruminal development and in general growth of the yaks.

Cattle-yak, as the hybrid offspring of cattle (Bos taurus) and yak (Bos grunniens), demonstrates obvious heterosis in production performance. Male hybrid sterility has been focused on for a long time; however, the mRNAs and non-coding RNAs related to muscle development as well as their regulatory networks remain unclear. The phenotypic data showed that the production performance (i.e., body weight, withers height, body length, and chest girth) of cattle-yak was significantly better than that of the yak, and the economic benefits of the cattle-yak were higher under the same feeding conditions. Then, we detected the expression profiles of the longissimus dorsi muscle of cattle-yak and yak to systematically reveal the molecular basis using the high-throughput sequencing technology. Here, 7,126 mRNAs, 791 lncRNAs, and 1,057 circRNAs were identified to be differentially expressed between cattle-yaks and yaks in the longissimus dorsi muscle. These mRNAs, lncRNA targeted genes, and circRNA host genes were significantly enriched in myoblast differentiation and some signaling pathways related to muscle development (such as HIF-1 signaling pathway and PI3K-Akt signaling pathway). We constructed a competing endogenous RNA (ceRNA) network and found that some non-coding RNAs differentially expressed may be involved in the regulation of muscle traits. Taken together, this study may be used as a reference tool to provide the molecular basis for studying muscle development.

This study aimed to investigate the nutritional properties of yak milk in various areas of Gannan. The milk composition analyzer, automatic amino acid analyzer, and flavor analyzer were used to detect the conventional nutrients, amino acids, and volatile flavor substances of 249 yak milks in Meiren grassland, Xiahe grassland, and Maqu grassland (hereinafter referred to as Meiren yak, Xiahe yak, and Maqu yak) in the Gannan area. The results showed that the fat content of Meiren yak milk was significantly higher than that of Maqu yak and Xiahe yak (p < 0.05). The protein content of Meiren yak milk was significantly higher than that of Xiahe yak (p < 0.05), but not significantly different from that of Maqu yak (p > 0.05). The casein content in the milk of Maqu yak was significantly higher than that of Meiren yak and Xiahe yak (p < 0.05). There was no significant difference in the lactose content of yak milk in the three regions (p > 0.05). The content of glutamic acid in the milk of Meiren yak, Xiahe yak, and Maqu yak was noticeably high, which was 1.03 g/100 g, 1.07 g/100 g, and 1.10 g/100 g, respectively. The total amino acid (TAA) content was 4.78 g/100 g, 4.87 g/100 g, and 5.0 g/100 g, respectively. The ratios of essential amino acids (EAA) and total amino acids (TAA) in the milk of Meiren yak, Xiahe yak, and Maqu yak were 42.26%, 41.27%, and 41.39%, respectively, and the ratios of essential amino acids (EAA) and nonessential amino acids (NEAA) were 73.19%, 70.28%, and 70.61%, respectively. In the yak milk samples collected from three different regions, a total of 34 volatile flavor compounds were detected, including 10 aldehydes, five esters, six ketones, four alcohols, two acids, and seven others. The main flavor substances qualitatively obtained from Meiren yak milk were ethyl acetate, n-valeraldehyde, acetic acid, heptanal, and n-hexanal. Xiahe yak milk mainly contains ethyl acetate, isoamyl alcohol, n-valeraldehyde, heptanal, and ethyl butyrate. Maqu yak milk mainly contains ethyl acetate, n-valeraldehyde, isoamyl alcohol, heptanal, ethyl butyrate, and n-hexanal. Principal component analysis showed that the flavor difference between Xiahe yak and Maqu yak was small, while the flavor difference between Xiahe yak, Maqu yak, and Meiren yak was large. The findings of this research can serve as a foundation for the future advancement and application of yak milk.

Yak reproductive characteristics have received extensive attention, though the molecular regulation mechanism of its ovarian activity remains to be explored. Therefore, this study initially conducted a comparative analysis of yak ovarian activities in anestrus, estrus, and pregnancy regarding their morphology and histology, followed by implementing RNA sequencing (RNA-seq) technology to detect the overall gene expression and biological mechanism in different reproductive stages. H&E staining showed that there were more growing follicles and mature follicles in ovarian tissue sections during estrus than ovarian tissues during non-estrus. The RNA-seq analysis of yak ovary tissues in three periods showed that DEGs related to follicular development and hormone metabolism were screened in the three comparison groups, such as COL1A2, NR4A1, THBS2, PTGS2, SCARB1, STAR, and WNT2B. Bioinformatics analysis showed that these DEGs are involved in ion binding, cell development, metabolic processes, enriched in ECM–receptor interactions, steroid biosynthesis, together with aldosterone generation/discharge and Wnt/PI3K-Akt signaling pathways. In addition, we speculate alternate splice development events to have important role/s in regulating ovarian functional genomic expression profiles. These results provide essential knowledge aimed at scrutinizing pivotal biomarkers for yak ovarian activity, together with paving the way for enhancing researchers’ focus on improving yak reproductive performance.

Spermatogenesis is a complex process that involves proliferation and differentiation of diploid male germ cells into haploid flagellated sperm and requires intricate interactions between testicular somatic cells and germ cells. The cellular heterogeneity of this process presents a challenge in analyzing the different cell types at various developmental stages. Single-cell RNA sequencing (scRNA-seq) provides a useful tool for exploring cellular heterogeneity. In this study, we performed a comprehensive and unbiased single-cell transcriptomic study of spermatogenesis in sexually mature 4-year-old yak using 10× Genomics scRNA-seq. Our scRNA-seq analysis identified six somatic cell types and various germ cells, including spermatogonial stem cells, spermatogonia, early-spermatocytes, late-spermatocytes, and spermatids in yak testis. Pseudo-timing analysis showed that Leydig and myoid cells originated from common progenitor cells in yaks. Moreover, functional enrichment analysis demonstrated that the top expressed genes in yak testicular somatic cells were significantly enriched in the cAMP signaling pathway, PI3K-Akt signaling pathway, MAPK signaling pathway, and ECM receptor interactions. Throughout the spermatogenesis process, genes related to spermatogenesis, cell differentiation, DNA binding, and ATP binding were expressed. Using immunohistochemical techniques, we identified candidate marker genes for spermatogonial stem cells and Sertoli cells. Our research provides new insights into yak spermatogenesis and the development of various types of cells in the testis, and presents more reliable marker proteins for in vitro culture and identification of yak spermatogonial stem cells in the later stage.