Kinetic analysis of strand invasion intermediates during C. elegans meiosis reveals continuous sister- and homolog-directed repair.

Meiotic chromosome segregation requires reciprocal exchanges between the parental chromosomes (homologs). Exchanges form via tightly-regulated repair of double-strand DNA breaks (DSBs). However, since repair intermediates have been mostly quantified in fixed images, our understanding of the mechanisms that control repair progression remains limited. Here, we study meiotic repair kinetics in Caenorhabditis elegans by extinguishing new DSBs and following the disappearance of a crucial intermediate-strand invasion mediated by the conserved RecA-family recombinase RAD-51. Assuming exponential decay, RAD-51 foci have a half-life of 1 to 2 h, with >75% of foci disappearing within 4 h. Previous work suggested that sister-directed repair is specifically blocked throughout most of pachytene. In contrast, we find that RAD-51 foci half-lives are 1 to 2 h even in conditions where homolog engagement is prevented and only the sister is available as a template. This suggests that both sister- and homolog-engaged RAD-51 foci are continuously turned over during pachytene. We also use our kinetic information to revisit the total number of DSBs-the "substrate" for the formation of exchanges-and find an average of 20 to 38 DSBs per nucleus. Our work opens the door for analysis of the interplay between meiotic repair kinetics and the fidelity of genome inheritance.