Genome sequencing ofC. elegansbalancer strains reveals previously unappreciated complex genomic rearrangements

Genetic balancers inCaenorhabditis elegansare complex variants that allow lethal or sterile mutations to be stably maintained in a heterozygous state by suppressing cross over events. Balancers constitute an invaluable tool in theC. elegansscientific community and have been widely used for decades. The first/traditional balancers were created by applying (X-rays, UV, or gamma radiation) onC. elegansstrains generating random genomic rearrangements. Their structures have been mostly explored with low-resolution genetic techniques (e.g. fluorescence in situ hybridization or PCR), before genomic mapping and molecular characterization through sequencing became feasible. As a result, the precise nature of most chromosomal rearrangements remains unknown, whereas, more recently, balancers have been engineered using the CRISPR-Cas9 technique for which the structure of the chromosomal rearrangement has been predesigned. Using short-read whole-genome sequencing (srWGS) and tailored bioinformatic analyses, we previously interpreted the structure of four chromosomal balancers randomly created by mutagenesis processes. Here, we have extended our analyses to 5 CRISPR-Cas9 balancers and 17 additional traditional balancing rearrangements. We detected and experimentally validated their breakpoints and have interpreted the balancer structures. Many of the balancers were found to be more intricate than previously described, being composed of complex genomic rearrangements (CGRs) such as chromoanagenesis-like events. Furthermore, srWGS revealed additional structural variants and CGRs not known to be part of the balancer genomes. Altogether, our study provides a comprehensive resource of complex genomic variations inC. elegansand highlights the power of srWGS to study the complexity of genomes by applying tailored analyses.