A maternal Polycomb imprint is maintained in early Drosophila development through lower-order methylation states

Polycomb group (PcG) proteins regulate embryonic epigenetic states by facilitating mono-, di-, and tri-methylation of H3 lysine 27 (H3K27me1/2/3). For the zygote to inherit a maternal H3K27 methylation state, PcG modifications established during oogenesis must survive until zygotic genome activation. Whether parental PcG states persist through Drosophila embryogenesis remains unclear. Here, we combine stochastic modeling with in vivo experiments to define the fate of maternal H3K27 methylation. We find that the inherited PcG state consists of broad, non-canonical H3K27me2/3 domains and that H3K27me3 is not maintained through cleavage divisions but lost and later re-established within canonical PcG domains. In contrast, H3K27me2 persists in cis on maternal chromatin. Modeling and genetic analyses indicate that this retention is consistent with Polycomb repressive complex 2 (PRC2) allostery-dependent maintenance rather than non-specific, nucleation-independent methylation. These findings suggest that the early embryo inherits a maternal H3K27 landscape that persists primarily as H3K27me2 and is later remodeled into canonical H3K27me3 domains.