Yu Wang

As the hybrid offspring of cattle and yak, cattle–yaks suffer from male sterility, manifesting as cascading spermatogenic failure. Despite the Y chromosome’s pivotal role in spermatogenesis, the absence of a high-quality yak Y assembly has long impeded mechanistic understandings from this perspective. Here, a near-complete 42.4 Mb yak Y chromosome is constructed through a multi-stage assembly strategy that integrates de novo assembly with pangenome graph construction and Hi-C guided refinement. By developing a rigorously standardized gene annotation pipeline for precise cross-species comparison, we find that yaks have undergone a greater expansion of Y-linked ampliconic genes than cattle. Integrating this ampliconic landscape with short-read and full-length transcriptomics further demonstrates that yaks exhibit a drastic 2-to-4-fold increase in transcriptionally active copies of spermatogenesis-related ampliconic genes (including TSPY1, ZNF280BY, HSFY and PRAMEY) relative to cattle. Given negligible homology outside the pseudoautosomal region and conservation of key meiotic proteins, we propose a ‘cis-trans regulatory mismatch’ model driven by divergent Y-linked amplification as a working hypothesis to explain the primary genetic mechanism of cattle–yak male sterility. Together, these findings offer critical insights for addressing cattle–yak male sterility and establish the Y chromosome as an active driver of reproductive isolation beyond its traditional degenerate characterization.

In bacteriophage T7, the primase synthesizes primer at a specific site, 5′-(G/T)₂GTC-3′. However, the pentanucleotide alone cannot define the activity of the primase. In this study, we demonstrated that the 10-nt sequence flanking the 3′ end of pentanucleotide sites made considerable contributions to the interactions between T7 primase and single-strand DNA (ssDNA). Approximately 26 template sequences with multiple features were screened out from the T7 genome, which exhibited strong binding affinity to T7 primase and high priming activity, thus supporting genome replication. Notably, a dinucleotide in the 3′ flank of the pentanucleotide site was found to be instrumental in T7 primase binding to ssDNA, which might be recognized by the zinc-binding domain of T7 primase. As a result, a multiple-site recognition model for T7 primase to select priming sites was proposed. These results shed light on how T7 primase selects priming sites, a process that may be shared by its bacterial counterparts. Furthermore, our study provides novel methodologies for investigating the interactions between prokaryotic primases and their ssDNA templates, thereby laying the groundwork for the development of novel inhibitors.

Efficient nutritional assimilation and energy metabolism in the stomachs of yaks contribute to their adaption to harsh environments. Accurate gene expression profile analysis will help further reveal the molecular mechanism of nutrient and energy metabolism in the yak stomach. RT-qPCR is regarded as an accurate and dependable method for analyzing gene expression. The selection of reference genes is essential to obtain meaningful RT-qPCR results, especially in longitudinal gene expression studies of tissues and organs. Our objective was to select and validate optimal reference genes from across the transcriptome as internal controls for longitudinal gene expression studies in the yak stomach. In this study, 15 candidate reference genes (CRGs) were determined according to transcriptome sequencing (RNA-seq) results and the previous literature. The expression levels of these 15 CRGs were quantified using RT-qPCR in the yak stomach, including the rumen, reticulum, omasum and abomasum at five stages: 0 days, 20 days, 60 days, 15 months and three years old (adult). Subsequently, the expression stabilities of these 15 CRGs were evaluated via four algorithms: geNorm, NormFinder, BestKeeper and the comparative C_T method. Furthermore, RefFinder was employed to obtain a comprehensive ranking of the stability of CRGs. The analysis results indicate that RPS15, MRPL39 and RPS23 are the most stable genes in the yak stomach throughout the growth cycle. In addition, to verify the reliability of the selected CRGs, the relative expression levels of HMGCS2 were quantified via RT-qPCR using the three most stable or the three least stable CRGs. Overall, we recommend combining RPS15, MRPL39 and RPS23 as reference genes for the normalization of RT-qPCR data in the yak stomach throughout the growth cycle.