An a mating type mutant of the yeast Saccharomyces cerevisiae which expressed high sexual activities during vegetative growth was isolated and characterized. Its constitutive sexual agglutinability was higher than the sexual agglutinability of its parental strain induced by α pheromone. It produced a pheromone and α-pheromone-inactivating substances in larger amounts than its parental strain. It also produced large pear-shaped cells (shmooed cells) without α pheromone, was more sensitive to α pheromone, and grew vegetatively more slowly than its parental strain. When the mutant was crossed to a wild type α strain isogenic with the parental strain, a mating type segregants with high constitutive sexual agglutinability showed self-shmooing. However, in a mating type segregants self-shmooing was not observed regardless of the degree of their sexual agglutinability. The cross between a and α segregants, both of which carried the mutation, had higher frequency of zygote formation than the crosses between a and α cells one of which or both of which were of wild type.
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Suzuki et al. (1986) studied this question.