Abstract
Meiotic DNA double-strand breaks (DSBs) initiate crossover (CO) recombination, which is necessary for accurate chromosome segregation, but DSBs may also repair as non-crossovers (NCOs). Multiple recombination pathways with specific intermediates are expected to lead to COs and NCOs. We revisited the mechanisms of meiotic DSB repair and the regulation of CO formation, by conducting a genome-wide analysis of strand-transfer intermediates associated with recombination events. We performed this analysis in a SK1 × S288C Saccharomyces cerevisiae hybrid lacking the mismatch repair (MMR) protein Msh2, to allow efficient detection of heteroduplex DNAs (hDNAs). First, we observed that the anti-recombinogenic activity of MMR is responsible for a 20% drop in CO number, suggesting that in MMR–proficient cells some DSBs are repaired using the sister chromatid as a template when polymorphisms are present. Second, we observed that a large fraction of NCOs were associated with trans–hDNA tracts constrained to a single chromatid. This unexpected finding is compatible with dissolution of double Holliday junctions (dHJs) during repair, and it suggests the existence of a novel control point for CO formation at the level of the dHJ intermediate, in addition to the previously described control point before the dHJ formation step. Finally, we observed that COs are associated with complex hDNA patterns, confirming that the canonical double-strand break repair model is not sufficient to explain the formation of most COs. We propose that multiple factors contribute to the complexity of recombination intermediates. These factors include repair of nicks and double-stranded gaps, template switches between non-sister and sister chromatids, and HJ branch migration. Finally, the good correlation between the strand transfer properties observed in the absence of and in the presence of Msh2 suggests that the intermediates detected in the absence of Msh2 reflect normal intermediates.
Highlights
Meiotic crossovers (COs) are reciprocal exchanges of chromosome arms between homologous chromosomes
A subset of double-strand breaks (DSBs) that do not give COs is repaired without reciprocal exchange of chromosome arms and gives noncrossover products (NCOs) that can only be identified by gene conversions associated with the recombination process
This study provides for the first time a genome-wide view of heteroduplex DNAs (hDNAs) associated with COs and NCOs
Summary
Meiotic crossovers (COs) are reciprocal exchanges of chromosome arms between homologous chromosomes (homologs). They generate genetic diversity and establish physical links between homologs. A subset of DSBs that do not give COs is repaired without reciprocal exchange of chromosome arms and gives noncrossover products (NCOs) that can only be identified by gene conversions associated with the recombination process. After DSB formation and Spo removal from the 59 ends of the breaks [5], 39 single-stranded tails are generated and initiate recombination with homologous sequences [6] to produce COs and NCOs. Genetic and physical analyses performed in Saccharomyces cerevisiae suggest that the decision to form either a CO or a NCO is made before or during the transition between DSB formation and strand invasion of the
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