Mating-type gene switching in a homothallic fission yeast

Abstract

When single cells of a homothallic strain of the fission yeast ( Schizosaccharomyces pombe 968h 90 ) are plated upon sporulation agar, a couple of cell divisions yield four preconjugal cells ordered in a line. Within a line, conjugation occurs either between sibs (the pair at either end of the line) or between cousins (the central pair of cells) or not at all. Miyata & Miyata (1981) have shown that sib matings are favored over cousin matings, the ratio of sib:cousin:sterile being 96:23:10. To have mating within these 4-cell clones means that the mating types of some of the cells have switched. In a further analysis of their data, we come to a series of deductions, one of which is that switching of mating-type genes in the fission yeast must be asymmetrical. We propose a random model and a deterministic model based upon asymmetrical switching. Either model could generate the ratios provided, but the models are sufficiently different that experimental tests should be able to discriminate between the two.