Zebrafish Polycomb repressive complex-2 critical roles are largely Ezh2- over Ezh1-driven and concentrate during early embryogenesis

Abstract

Polycomb repressive complex-2 (PRC2) methylation of histone H3 lysine-27 (H3K27me) is associated with stable transcriptional repression. PRC2 famously silences Hox genes to maintain anterior-posterior segment identities but also enables early cell fate specification, restrains progenitor cell differentiation, and canalizes cell identities. Zebrafish PRC2 genetic studies have focused on ezh2, which, with its paralog ezh1, encodes the H3K27 methyltransferase component. ezh2 loss-of-function mutants reinforce essential vertebrate PRC2 functions during early embryogenesis albeit with limited contributions to body plan establishment. However, redundancy with ezh1 and the lethality of maternal-zygotic homozygous ezh2 nulls could obscure additional early developmental and organogenesis roles of PRC2. Here, we combine new and existing zebrafish ezh1 and ezh2 alleles to show collective maternal/zygotic ezh2 exclusively provides earliest embryonic PRC2 H3K27me3 activity. Zygotic ezh1, which becomes progressively expressed as ezh2 levels dissipate, has minor redundant and noncompensatory larval roles but itself is not required for viability or fertility. Zygotic Ezh2/PRC2 promotes correct craniofacial bone shape and size by maintaining proliferative pre-osteoblast pools. An ezh2 allelic series including disrupted maternal ezh2 uncovers axial skeleton homeotic transformations and pleiotropic organogenesis defects. Further, once past a critical early window, we show zebrafish can develop near normally with minimal bulk H3K27me3. Our results suggest Ezh2-containing PRC2 stabilizes rather than instructs early developmental decisions while broadly contributing to organ size and embellishment.