How do yeast change sex?

Abstract

The fission yeast Schizosaccharomyces pombe can switch between two mating types, M or P. This mitotic switch arises from the genetic transfer of either of the silent loci, mat2P or mat3M, to a transcriptionally active locus, mat1, which can contain either M or P alleles. Mating-type switching requires two consecutive asymmetric cell divisions, to produce one switched cell for every four granddaughters. During the first division, a chromosomal imprinting event marks one of the two sister chromatids such that only one of the daughters will be competent to switch during the next cell division. The sister of a newly switched cell will switch during the next generation – and one of the granddaughters (or grandsons) the generation after. The nature of the site- and strand-specific modification at the mat1 locus is still controversial, but it seems that the direction of the replication fork is crucial for switching1. The molecular mechanisms underlying mating-type switching have baffled researchers for several years.