Instability of the Mating Type Alleles in Saccharomyces

Copulation in Saccharomyces cerevisiae is controlled by a pair of a/a alleles. Ascospores are haploid for either the a or the a gene and produce clones, the members of which copulate pairwise when the cultures are mixed. The a by a matings result in abundant copulations, forming legitimate diploid zygotes capable of undergoing reduction to produce viable 4-spored asci. The a by a and the a by a matings fail to result in copulations. The four cultures from the single haploid ascospores which were the basis of the experiments and illustrations in our earlier papers (Lindegren and Lindegren, '43 b, c) have been carried in culture in the laboratory for over a year and have been observed to produce a great variety of morphological mutants. That they still retain their haploid character is shown by the shape and size of the cells. Recently the earlier experiments were duplicated by mating the cultures in all possible pairs, following exactly the same procedure as before. These matings failed to produce either copulations or diploid cells. No asci were obtained by transferring the mated cultures to our presporulation medium and gypsum slants (Lindegren and Lindegren, '44). The cultures are now completely neutralized as to sex and have been transformed into typical members of the genus Torula or Torulopsis. They are incapable of producing diploid sporulating cultures either by legitimate or illegitimate copulation. This is undoubtedly due to mutation either of the principal a/a alleles or of modifying genes which inhibit copulation. The genus Torula or Torulopsis was invalidated by ?atava ('34) and Winge and Laustsen ('39) suggested that the genus Zygosaccharomyces should be dropped. We have presented evidence on both these points ('43b), and the names of the genera are used in the present discussion without implication of generic status.