The origin of a centromere effect on mitotic recombination : A study in the fission yeast Schizosaccharomyces pombe.

Spontaneous meiotic and mitotic rates of recombination were measured for 50 intragenic intervals in 9 genes of Schizosaccharomyces pombe. A much smaller mitotic/meiotic recombination ratio is observed for genes unlinked to the centromere (average ratio: 0.005) than for genes close to the centromere (up to 0.17). The high ratio observed for the latter genes is due to a high rate of mitotic recombination rather than to a centromeric inhibition of meiotic recombination, since there was no meiotic inhibition in the genes considered. As already reported for one pair of genes in Saccharomyces cerevisiae (Hénaut et Luzzati, 1972), cells that have recombined at one locus have a meiotic rate of coincident recombination at an unlinked locus, even if the latter is not on the same chromosome. This coincidence is much lower when intragenic recombinants are selected in the centromere-linked gene lys1. In modification of an earlier hypothesis (Hurst and Fogel, 1964), we propose that: (a) pairing between homologous chromosomes is the major rate-limiting factor in mitotic recombination; (b) mitotic pairing of homologous chromosomes is frequent, possibly occuring in all vegetative cells, but is usually restricted to the centromere region; and (c) pairing along the whole genome is restricted to a small subpopulation among the mitotically dividing cells.