The lines that acquired recessive lethal mutations were maintained for at least three generations. Worms were rinsed off with deionized water and concentrated. Genomic DNA was purified using Puregene Core Kit A (QIAGEN, Valencia, CA). DNA sequencing was performed at the Novogene Bioinformatics Institute (Beijing, China). Sequence reads were mapped to the C. elegans reference genome version WS230 (http://www.wormbase.org) using the short-read aligner BWA (Li and Durbin 2010 (link)), which resulted in an average sequencing depth for each sample ranging from 22× to 57× with a median of 34×. Single-nucleotide variations (SNVs) and small insertions/deletions were identified and filtered with the help of the SAMtools toolbox (Li et al. 2009 (link)). Candidate variants at genomic locations for which the parental N2 strain had an agreement rate with the reference genome < 95% were eliminated from further consideration. Each variant was annotated with a custom Perl script and gene information downloaded from WormBase version WS230. Copy numbers were estimated from the alignments with a procedure analogous to that of Itani et al. (2016 (link)) using 5-kb wide overlapping sliding windows with the alignments from the parental strain used as the reference.