CRISPR/Cas9-induced DNA breaks trigger crossover, chromosomal loss, and chromothripsis-like rearrangements

Abstract

Abstract DNA double-stranded breaks (DSBs) generated by the Cas9 nuclease are commonly repaired via nonhomologous end-joining (NHEJ) or homologous recombination (HR). However, little is known about unrepaired DSBs and the type of damage they trigger in plants. We designed an assay that detects loss of heterozygosity (LOH) in somatic cells, enabling the study of a broad range of DSB-induced genomic events. The system relies on a mapped phenotypic marker which produces a light purple color (betalain pigment) in all plant tissues. Plants with sectors lacking the Betalain marker upon DSB induction between the marker and the centromere were tested for LOH events. Using this assay, we detected a tomato (Solanum lycopersicum) flower with a twin yellow and dark purple sector, corresponding to a germinally transmitted somatic crossover event. We also identified instances of small deletions of genomic regions spanning the T-DNA and whole chromosome loss. In addition, we show that major chromosomal rearrangements including loss of large fragments, inversions, and translocations were clearly associated with the CRISPR-induced DSB. Detailed characterization of complex rearrangements by whole-genome sequencing and molecular and cytological analyses supports a model in which a breakage–fusion–bridge cycle followed by chromothripsis-like rearrangements had been induced. Our LOH assay provides a tool for precise breeding via targeted crossover detection. It also uncovers CRISPR-mediated chromothripsis-like events in plants.