Crossing over is rarely associated with mitotic intragenic recombination in Schizosaccharomyces pombe

Chromosomal rearrangements can result from crossing over during ectopic hom ologous recombination between dispersed repetitive DNA. We have previously shown that meiotic ectopic rccombination between artificially dispersed ade 6 heteroalleles in the fission yeast Schizosaccharomyces pombe frequently r esults ill chromosomal rearrangements. The same recombination substrates ha ve been studied in milotic recombination. Ectopic recombination rates in ha ploids were similar to1-4 x 10(-6) recombinants per cell generation, simila r to allelic recombination rates in diploids. In contrast, ectopic recombin ation rates in heterogous diploids 2.5-70 times lower than allelic recombin ation or ectopic recombination in haploids. These results were suggest that diploid-specific factors inhibit ectopic recombination. Very few crossover s occurred in ade6 mitotic recombination,either allelic or ectopic. Allelic intragenic rccombination was associated with 2% crossing over, and ectopic recombination between multiple different pairing partners showed 1-7% cros sing over. These results contrast sharply with the 35-65% crossovers associ ated with meiotic ade6 recombination,and suggest either differential contro l of resolution of recombination intermediates or alternative pathways of r ecombination in mitosis and meiosis.