Enrichment of H3K9me2 on Unsynapsed Chromatin in Caenorhabditis elegans Does Not Target de Novo Sites

Many organisms alter the chromatin state of unsynapsed chromosomes during meiotic prophase, a phenomenon hypothesized to function in maintaining germline integrity. In Caenorhabditis elegans, histone H3 lysine 9 dimethylation (H3K9me2) is detected by immunolabeling as enriched on unsynapsed meiotic chromosomes. Loss of the SET domain protein, MET-2, greatly reduces H3K9me2 abundance and results in germline mortality. Here, we used him-8 mutations to disable X chromosome synapsis and performed a combination of molecular assays to map the sites of H3K9me2 accumulation, evaluate H3K9me2 abundance in germline vs. whole animals, and evaluate the impact of H3K9me2 loss on the germline transcriptome. Our data indicate that H3K9me2 is elevated broadly across the X chromosome and at defined X chromosomal sites in him-8 adults compared with controls. H3K9me2 levels are also elevated to a lesser degree at sites on synapsed chromosomes in him-8 adults compared with controls. These results suggest that MET-2 activity is elevated in him-8 mutants generally as well as targeted preferentially to the unsynapsed X. Abundance of H3K9me2 and other histone H3 modifications is low in germline chromatin compared with whole animals, which may facilitate genome reprogramming during gametogenesis. Loss of H3K9me2 has a subtle impact on the him-8 germline transcriptome, suggesting H3K9me2 may not be a major regulator of developmental gene expression in C. elegans. We hypothesize H3K9me2 may have a structural function critical for germline immortality, and a greater abundance of these marks may be required when a chromosome does not synapse.