Mating-type Switching and Mitotic Crossing-over at the Mating-type Locus in Fission Yeast

In homothallic strains of the fission yeast Schizosac-charomyces pombe, a remarkable system of reversible genetic hanges is operating during mating-type switch-ing. Silent information is activated by copy transposi-tion to a closely linked expressible site on the same chromosome. Seemingly, as a side reaction of this inter-nal recombination mechanism, mitotic crossing-over is unusually frequent at that site between the two homo-logs in a diploid configuration. S. pombe is essentially a haploid species. Both conju-gation and meiosis occur under the same physiological conditions, i.e., after starvation for a source of nitro-gen. Normally, therefore, zygotes immediately proceed through sporulation, and only rarely can a young wild-type zygote be diverted to diploid growth and division