WGS of C. elegans and A. thaliana mutation accumulation lines identify TONSOKU/TONSL/tnls-1 as a suppressor of TDs

Tandem duplications (TDs) are a common class of genomic rearrangements with both adaptive and pathogenic consequences. Although prevalent in genomically unstable cancer genomes, TDs are rarely observed in healthy tissues, suggesting the existence of robust protective mechanisms. However, the molecular underpinnings of TD suppression remain poorly understood. Here, we identify the histone chaperone TONSL/TONSOKU (tnsl-1 in C. elegans) as a critical suppressor of TD formation. Loss of tnsl-1 leads to TD accumulation in two distinct size classes, with medians of ~25 kb and ~300 kb. These TDs originate in developmentally distinct cell types: small TDs arise in rapidly dividing embryonic cells, whereas large TDs originate in slower-dividing germline progenitors. Both TD classes require polymerase theta-mediated end joining (TMEJ), implicating this double-strand break repair pathway in TD formation. Genetic disruption of break-induced replication (BIR) via Pif1 helicase loss reduces TD size, revealing a mechanistic link between BIR and TD expansion. Strikingly, TONSL-deficient Arabidopsis thaliana display an identical TD signature, underscoring the evolutionary conservation of this genome surveillance function. Together, our findings establish TONSL as a cross-kingdom guardian of genome integrity through suppression of TD formation.