H3K27me3 natural variation selectively marks genes predicted to be important for differentiation in unicellular algae

In multicellular organisms H3K27me3 has been shown to be deposited by Polycomb Repressive Complex 2 (PRC2) to establish and maintain gene silencing, critical for cell fate and developmentally regulated processes. PRC2 complex is absent in both yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, which initially suggested that PRC2 arose with the emergence of multicellularity. However, its discovery in several unicellular species including microalgae questions its role in unicellular eukaryotes. Here, we show in the model diatom Phaeodactylum tricornutum (Pt), using mutants in the homologue of the catalytic subunit of PRC2, enhancer of zeste E(z), that Pt E(z) is responsible for di and tri-methylation of lysine 27 of histone H3. H3K27me3 depletion abolishes cell morphology in Pt providing evidence for a role of H3K27me3 in cell differentiation in unicellular species. Genome wide profiling of H3K27me3 in fusiform and triradiate cells further revealed genes that may specify cell identity. These results suggest a role for PRC2 and its associated histone mark in cell differentiation in unicellular species and highlights their ancestral function in a broader evolutionary context than is currently appreciated.