Remarkably high rate of meiotic recombination in the fission yeast Schizosaccharomyces pombe

In most eukaryotes, the number of meiotic crossovers (COs) is limited to 1-3 per chromosome, which are prevented from occurring close to one another by CO interference. The fission yeast Schizosaccharomyces pombe, an exception to this general rule, lacks CO interference and seems to have the highest CO number per chromosome. However, global CO frequency was indirectly estimated in this species, raising doubts about this exceptional recombination level. Here, we used an innovative strategy to directly determine COs genome-wide in S. pombe. We confirm the absence of crossover interference and reveal the presence of co-variation in CO number across chromosomes within tetrads, suggesting that a limiting pro-CO factor varies stochastically between meiocytes. CO number per chromosome varies linearly with chromosome size, with the three chromosomes having, on average, 15.9, 12.5, and 7.0 COs, respectively. This is significantly lower than previous estimates but reinforces S. pombes exceptional status as the eukaryote with the highest CO number per chromosome described to date and among the species with the highest rate of COs per unit of DNA.