EGO-1, a putative RNA-dependent RNA polymerase, is required for heterochromatin assembly on unpaired DNA during C-elegans meiosis

During meiosis in C. elegans, unpaired chromosomes and chromosomal regions accumulate high levels of histone H3 lysine 9 dimethylation (H3K9me2), a modification associated with facultative heterochromatin assembly and the resulting transcriptional silencing [1, 2]. Meiotic silencing of unpaired DNA may be a widely conserved genome defense mechanism [3-5]. The mechanisms of meiotic silencing remain unclear, although both transcriptional and posttranscriptional processes are implicated [3-5]. Cellular RNA-dependent RNA polymerases (RdRPs) function in development and RNA-mediated silencing in many species [3,6, 7] and in heterochromatin assembly in S. pombe [3, 8]. There are four C. elegans RdRPs, including two with known germline functions. EGO-1 is required for fertility and robust germline RNAi [9-11]. RRF-3 acts genetically to repress RNAi and is required for normal meiosis and spermatogenesis at elevated temperatures [12] (S. L'Hernault, personal communication). Among C. elegans RdRPs, we find that only EGO-1 is required for H3K9me2 enrichment on unpaired chromosomal regions during meiosis. This H3K9me2 enrichment does not require Dicer or Drosha nuclease or any of several other proteins required for RNAi. ego-1 interacts genetically with him-17, another regulator of chromatin and meiosis [13], to promote germline development. We conclude that EGO-1 is an essential component of meiotic silencing in C. elegans.