Variation in crossover rates perturb crossover assurance without affecting meiotic chromosome segregation in S. cerevisiae. Saccharomyces cerevisiae S288c X YJM789

During meiosis cells undergo a single round of replication followed by Meiosis I and Meiosis II divisions to produce haploid gametes. The segregation of homologous chromosomes during the Meiosis I division requires crossovers between homologs. Meiotic crossing over is highly regulated to ensure an obligate crossover per homolog pair (crossover assurance). The effect of variation in crossover rates on crossover assurance is not well understood. In S. cerevisiae and mammals, Msh4 and Msh5 proteins stabilize Holliday junctions and its progenitors to facilitate crossing over. S. cerevisiae msh4/5 hypomorphs were identified recently that segregate homologous chromosome’s with up to a two-fold reduction in crossover levels at specific loci on chromosomes VII, VIII and XV. Here we use two S. cerevisiae msh4/5 hypomorphs (msh4-R676W and msh5-D680A) to study if crossover assurance is maintained in meiotic cells that experience variation in crossover rates. An S. cerevisiae S288c/YJM789 cross was used to generate high resolution genome wide recombination maps in the wild-type, msh4/5 hypomorphs and msh4Δ mutant. The msh4/5 hypomorphs made on average ~63 crossovers per meiosis compared to wild-type (~94 crossover/meiosis) and msh4 (~49 crossovers/meiosis) mutant. Crossover defects in msh4/5 mutants were similar across all chromosomes regardless of size. Non-crossovers in msh4/5 hypomorphs and msh4 were not significantly different from wild-type. About 40% of the four spore viable tetrads in msh4-R676W mutant showed the presence of at least one non-exchange chromosome pair on the small and medium sized chromosomes. Homolog pairs segregate normally in the msh4/5 hypomorphs despite the loss of crossover assurance. These observations highlight crossover independent mechanisms that can facilitate the segregation of a limited number of S. cerevisiae non-exchange chromosomes. Our results suggest variation in crossover rates can compromise crossover assurance without affecting homolog segregation.