Interconversion of Yeast Mating Types I. Direct Observations of the Action of the Homothallism (HO) Gene.

The HO gene promotes interconversion between a and alpha mating types. As a consequence, homothallic diploid cells are formed by mating between siblings descended from a single alpha HO or a HO spore. In order to determine the frequency and pattern of the mating-type switch, we have used a simple technique by which the mating phenotype can be assayed without losing the cell to the mating process itself. Specifically, we have performed pedigree analysis on descendants of single homothallic spores, testing these cells for sensitivity to alpha-factor.The switch from alpha to a and vice versa is detectable after a minimum of two cell divisions. 50% of the clones tested showed switching by the four-cell stage. Of the four cells descended from a single cell, only the oldest cell and its immediate daughter are observed to change mating type. This pattern suggests that one event in the switching process has occurred in the first cell division cycle. Restriction of the switched mating-type to two particular cells may reflect the action of the homothallism system followed by nonrandom segregation of DNA strands in mitosis.The mating behavior of cells which have sustained a change in mating type due to the HO gene is indistinguishable from that of heterothallic strains.