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We isolated heterothallic strains from a homothallic strain of S. exiguus by mutagenization with UV or ethylmethanesulfonate (EMS). A gene, not linked to the mating-type locus, was found to control homothallism in the yeast, as in S. cerevisiae. alpha Pheromone of S. exiguus (alpha se pheromone) induced formation of large pear-shaped cells (shmooing) in alpha strains of S. exiguus, S. cerevisiae, and S. kluyveri, and sexual agglutinability of an inducible alpha strain of S. cerevisiae. alpha se Pheromone is a peptidyl substance a little different from alpha pheromone of S. cerevisiae. alpha Pheromone of S. exiguus acts only on alpha cells of S. exiguus. Contrary to the above results, neither sexual agglutination nor zygote formation occurred among these three Saccharomyces yeasts.

The alpha mating pheromones synthesized in three Saccharomyces yeasts (S. cerevisiae, S. kluyveri, and S. exiguus) displayed interspecific actions on the a cells of all three species despite the fact that the amino acid sequences of all three alpha pheromones are different. Mating between species, however, did not occur. The interspecific alpha pheromone--a cell reaction was not necessarily more effective than the interspecific one.

A DNA fragment homologous to the homothallism (HO) gene of Saccharomyces cerevisiae was isolated from Saccharomyces paradoxus and was found to contain an open reading frame that was 90.9% identical to the coding sequence of the S. cerevisiae HO gene. The putative HO gene was shown to induce diploidization in a heterothallic haploid strain from S. cerevisiae. Phylogenetic analysis revealed that the coding and 5'-upstream regulatory regions from five Saccharomyces sensu stricto HO genes have coevolved, and that S. paradoxus is phylogenetically closer to S. cerevisiae than to S. bayanus. Finally, heterothallic haploid strains were isolated from the original homothallic type strain of S. paradoxus by disrupting the S. paradoxus HO gene with the S. cerevisiae URA3 gene.