Hierarchical chromatin polyvalency governs robust gene regulation and organogenesis

Precise temporal control of gene expression is fundamental for embryonic development, yet the epigenetic and chromatin basis governing transcriptional timing remain poorly understood. The bivalency model, characterized by coexistence of H3K4me3 and H3K27me3, has been proposed to mark a poised state ready for activation. However, this model has been challenged by lacking of rapid gene activation in response to H3K27me3 depletion, suggesting that the H3K27me3 mark is not responsible for the silencing. Here, through temporal epigenomic profiling of post-implantation mouse embryos and use of the protein degradation tag (dTAG) system, we demonstrate that H2Aub, but not H3K27me3, functions as the major repressor. We further reveal a hierarchical repression architecture in which H2Aub is responsible for transcriptional silencing, while H3K27me3 and H3K9me3 serve to reinforce the silencing state in post-implantation embryos. Functionally, disruption of this H2Aub-centered hierarchy perturbs temporal control of polyvalent gene activation, leading to severe organogenesis defects. Mechanistically, acute loss of H2Aub disrupts retinoic acid-FGF signaling pathway, causing somitogenesis arrest. Together, our findings establish chromatin polyvalency model as a multi-layered, hierarchical repression mechanism that governs temporal control of gene expression during embryogenesis.