Polycomb repressive complex 2 regulates sexual development in Neurospora crassa
In most branches of the fungal kingdom  sexual development marks a dramatic shift from simple multicellular hyphae to complex fruiting bodies composed of multiple tissues and cell types. It is essential to tightly regulate development to prevent unnecessary energy investment into building these structures. Polycomb Repressive Complex 2 (PRC2) is a highly conserved regulator of development in multicellular organisms. In plants  animals  and some fungi  PRC2 represses genes by depositing histone H3 lysine 27 tri-methylation (H3K27me3) in promoters and gene bodies of conditionally activated genes. In Neurospora crassa  H3K27me3-associated genes are poorly conserved and stably repressed in most RNA-seq datasets. Through analysis of publicly available RNA-seq experiments  we found that PRC2-methylated genes are broadly and uniquely activated during sexual development. PRC2-methylated genes comprise a distinct subset of developmentally induced genes (DIGs) characterized by an exceptionally high degree of cell-type specificity. Loss of PRC2 activity results in the precocious formation of perithecia-like structures (dubbed false perithecia)  even in the absence of a compatible mating-type partner. These structures show a unique gene expression profile with activation of both PRC2-methylated and unmethylated DIGs  showing evidence of a transcriptional reprogramming event. Together  these data suggest that PRC2 is part of a developmental checkpoint that restricts fruiting body development to the appropriate conditions. Overall design: isogenic siblings were isolated from a backcross between FGSC11182 (set-7) and wild-type N. crassa. Individual wild-type and set-7 progeny were inoculated on plates containing SCM agar overlain with cellophane and grown at room temperature in constant darkness for 12 days. Tissue was harvested from the cellophane surface and then snap frozen and ground in liquid nitrogen. RNA was isolated using TRIzol reagent  using methods optimized for solid fungal tissue.