Mating Type and Sporulation in Yeast I. Mutations Which Alter Mating-Type Control Over Sporulation

In Saccharomyces cerevisiae, meiosis and spore formation as well as mating are controlled by mating-type genes. Diploids heterozygous for mating type (aalpha) can sporulate but cannot mate; homozygous aa and alpha-alpha diploids can mate, but cannot sporulate. From an alpha-alpha diploid parental strain, we have isolated mutants which have gained the ability to sporulate. Those mutants which continue to mate as alpha-alpha cells have been designated CSP (control of sporulation). Upon sporulation, CSP mutants yield asci containing 4alpha spores. The mutant gene which allows alpha-alpha cells to sporulate is unlinked to the mating-type locus and also acts to permit sporulation in aa diploid cells. Segregation data from crosses between mutant alpha-alpha and wild-type aa diploids and vice versa indicate (for all but one mutant) that the mutation which allows constitutive sporulation (CSP) is dominant over the wild-type allele. Some of the CSP mutants are temperature-sensitive, sporulating at 32 degrees, but not at 23 degrees. In addition to CSP mutants, our mutagenesis and screening procedure led to the isolation of mutants which sporulate by virtue of a change in the mating-type locus itself, resulting in loss of ability to mate.