Mejia-Trujillo, R.; Zhao, Q.; Rahman, A.; Sarinay Cenik, E.

Oogenesis requires extensive and dynamic chromatin remodeling that primes gene promoters for later transcriptional activation during embryonic development. Here, we uncover a pivotal, non-canonical role for RNA Polymerase I (Pol I) in driving these chromatin state transitions during Caenorhabditis elegans oogenesis. Using the auxin-inducible degron system to selectively deplete either a Pol I-specific catalytic subunit or a ribosome assembly factor, we disentangle the consequences of impaired nucleolar integrity from reductions in ribosome biogenesis. Strikingly, although disrupting ribosome assembly caused minimal effects on oocyte production, loss of nucleolar structure via Pol I depletion led to severe meiotic chromosome abnormalities, widespread changes in chromatin accessibility, and a dampening of the typical distal-proximal H3K4me3 gradient required for oogenesis, resulting in fewer but significantly larger oocytes. Despite their promoters becoming more accessible, oogenesis genes did not show large changes in steady-state mRNA, consistent with transcriptional repression prior to fertilization. Instead, Pol I depletion prematurely remodeled oogenic chromatin, through a misdirection of H3K4me3 deposition towards promoters normally primed for zygotic genome activation. These findings reveal an epigenetic gating function for nucleolar integrity in oocyte maturation: Pol I preserves three-dimensional chromatin organization and maintains proper spatiotemporal regulation of histone modifications, independent of ribosome production. Given the evolutionary conservation of nucleolar dynamics and histone modifications during gametogenesis, our work suggests that nucleolar stress, whether from environmental factors, aging, or genetic disorders, could broadly compromise fertility by disrupting oogenic chromatin priming.