A high-resolution map of non-crossover events reveals impacts of genetic diversity on mammalian meiotic recombination

During meiotic recombination in most mammals, hundreds of programmed DNA Double-Strand Breaks (DSBs) occur across all chromosomes in each cell at sites bound by the protein PRDM9. Faithful DSB repair using the homologous chromosome is essential for fertility, yielding either non-crossovers, which are frequent but difficult to detect, or crossovers. In certain hybrid mice, high sequence divergence causes PRDM9 to bind each homologue at different sites, 'asymmetrically', and these mice exhibit meiotic failure and infertility, by unknown mechanisms. To investigate the impact of local sequence divergence on recombination, we intercrossed two mouse subspecies over five generations and deep-sequenced 119 offspring, whose high heterozygosity allowed detection of thousands of crossover and non-crossover events with unprecedented power and spatial resolution. Both crossovers and non-crossovers are strongly depleted at individual asymmetric sites, revealing that PRDM9 not only positions DSBs but also promotes their homologous repair by binding to the unbroken homologue at each site. Unexpectedly, we found that non-crossovers containing multiple mismatches repair by a different mechanism than single-mismatch sites, which undergo GC-biased gene conversion. These results demonstrate that local genetic diversity profoundly alters meiotic repair pathway decisions via at least two distinct mechanisms, impacting genome evolution and Prdm9-related hybrid infertility.