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Abstract BackgroundMorphologic sex differences between males and females typically emerge after the primordial germ cell migration and gonad formation, although sex is determined at fertilization based on chromosome composition. A key debated sexual difference is the embryonic developmental rate, within vitroproduced male embryos often developing faster. However, the molecular mechanisms driving early embryonic sex differences remain unclear. ResultsTo investigate the transcriptional sex difference during early development,in vitroproduced bovine blastocysts were collected and sexed by PCR. A significant male-biased development was observed in expanded blastocysts. Ultra-low input RNA-seq analysis identified 837 DEGs, with 231 upregulated and 606 downregulated in males. Functional enrichment analysis revealed male-biased DEGs were associated with metabolic regulation, whereas female-biased DEGs were related to female gonad development, sex differentiation, inflammatory pathways, and TGF-beta signaling. Comparing X chromosome and autosome expression ratio, we found that female-biased DEGs contributed to the higher X-linked gene dosage, a phenomenon not observed in male embryos. Moreover, we identified the sex-biased transcription factors and RNA-bind proteins, including pluripotent factors such asSOX21andPRDM14, and splicing factorsFMR1andHNRNPH2. Additionally, we revealed 1,555 significantly sex-biased differential alternative splicing (AS), predominantly skipped exons, mapped to 906 genes, with 59 overlapping with DEGs enriched in metabolic and autophagy pathways. By incorporating novel isoforms from long reads sequencing, we identified 1,151 sex-biased differentially expressed isoforms (DEIs) associated with 1,017 genes. Functional analysis showed that female-biased DEIs were involved in the negative regulation of transcriptional activity, while male-biased DEIs were related to energy metabolism. Furthermore, we identified sex-biased differential exon usage inDENND1B, DIS3L2, DOCK11, IL1RAPL2,andZRSR2Y,indicating their sex-specific regulation in early embryo development. ConclusionThis study provided a comprehensive analysis of transcriptome differences between male and female bovine blastocysts, integrating sex-biased gene expression, alternative splicing, and isoform dynamics. Our findings indicate that enriched metabolism processes in male embryos may contribute to the faster developmental pace, providing insights into sex-specific regulatory mechanisms during early embryogenesis. Plain English summaryMale and female early embryos develop at different speeds, with male embryos often developing faster than female embryos. However, the reasons behind these early differences remain unclear. In this study, we examined gene activity in bovine embryos to uncover the biological factors regulating these early sex differences. We collected in vitro-produced bovine blastocysts, examined their sex, and confirmed that male embryos develop faster. By analyzing global gene activity, including alternative splicing, which allows one gene to code for multiple RNA isoforms and proteins, we found distinct gene expression profiles between male and female embryos. Male embryos showed higher activity in genes related to metabolism and cellular functions, while female embryos had increased activity in genes associated with female-specific gonad development and gene expression regulation. We also examined differences in how genes on the X chromosome were expressed. Female embryos had higher X-linked gene expression, which may contribute to sex-specific developmental regulation. Additionally, we identified sex-specific transcription factors and RNA-binding proteins that regulate early embryo development, some of which are known to control pluripotency and gene splicing. Overall, our study provides new insights into how gene activity shapes early sex differences, suggesting that enhanced metabolism in male embryos may be a key driver of their faster developmental rate. HighlightsMale embryos develop faster due to increased gene expression in metabolism pathwaysFemale embryos exhibit higher X-linked gene expression, suggesting X-dosage compensation plays a role in early developmentSex-biased alternative splicing events contribute to embryonic metabolism, autophagy, and transcriptional regulation in embryosSex-biased isoform diversity contributes to distinct developmental regulation in male and female embryosKey pluripotency factors (SOX21, PRDM14) and splicing regulators (FMR1, HNRNPH2) drive sex-specific gene expression 

Summary Pluripotency, the ability of cells to self-renew and differentiate into all the cell types in an animal’s body, is vital for mammalian early development. This study presented a comprehensive comparative transcriptomic analysis of embryonic stem cells across multiple mammalian species, defining their progression through expanded/extended, naïve, formative, and primed pluripotency states. Our findings revealed both conserved and species-specific mechanisms underlying pluripotency regulation. We also emphasized the limitations of existing state-specific markers and their limited cross-species applicability, while identifyingde novopluripotency markers that can inform future research. Despite variability in gene expression dynamics, gene co-expression networks showed remarkable conservation across species. Among pluripotency states, the primed state demonstrated the highest conservation, evidenced by shared markers, preserved gene networks, and stronger selective pressures acting on its genes. These findings provide critical insights into the evolution and regulation of pluripotency, laying a foundation for refining stem cell models to enhance their translational potential in regenerative medicine, agriculture, and conservation biology.