Nematode samples were cross-linked with 2% formaldehyde for 30 min at room temperature. Next, formaldehyde was then quenched by addition of 1M Tris (pH 7.5). After sonication, cell extracts containing DNA fragments with an expected range between 200 and 800 bp were immunoprecipitated using anti-H3K4me3 (Abcam ab8580). ChIP-Sequencing library preparation was performed according to Illuminas protocol Preparing Samples for ChIP Sequencing of DNA.