Frequency of mating-type switching in homothallic fission yeast

Abstract

THE genes determining mating type in the fission yeast Schizosaccharomyces pombe are of particular interest in the general study of the genetic control of differentiation. The genetic segments concerned, mat1^M and mat2^P, specifying mating type (−) and (+), respectively¹, are present, closely linked, in homothallic strains², but on any one chromatid (that is in the cis position) they are only expressed in a mutually exclusive manner³. The segments of the mating-type locus, mat1^M and mat2^P, govern the transition from vegetative growth to sexual reproduction. They act in a complementary fashion—one functional M segment and one functional P segment are always required to initiate both conjugation (usually among haploid cells) and meiosis (leading to sporulation in either zygotes or diploid cells), the prominent events in differentiation in S. pombe. A mating-type mutant defective in meiosis but not in conjugation (mat1^M mat2^Pm) when crossed with wild type, produces sporulating and non-sporulating zygotes (Fig. 1). These two classes indicate that in individual cells of either parent strain only one or the other mating-type segment is active³. Here I describe a single-cell evaluation of this cross, and show (also by single-cell techniques) that the frequency of mating-type switching in haploid and diploid strains is approximately random. A favoured mechanism for this type of switching is the inversion of a promoter element between the structural genes, the direction of the promoter determining which mating-type segment is switched on.