Conversion of Wine Strains of Saccharomyces cerevisiae to Heterothallism

A general method to convert homothallic strains of the yeast Saccharomyces cerevisiae to heterothallism is described which is applicable to genetically well-behaved diploids, as well as to strains that sporulate poorly or produce few viable and mating-competent spores. The heterothallic (ho) allele was introduced into three widely used wine strains through spore x cell hybridization. The resultant hybrids were sporulated, and heterothallic segregants were isolated for use in successive backcrosses. Heterothallic progeny of opposite mating type and monosomic for chromosome III produced by sixth-backcross hybrids or their progeny were mated together to reconstruct heterothallic derivatives of the wine strain parents. A helpful prerequisite to the introduction of ho was genetic purification of the parental strains based on repeated cycles of sporulation, ascus dissection, and clonal selection. A positive selection to isolate laboratory-wine strain hybrids requiring no prior genetic alteration of the industrial strains, coupled with a partial selection to reduce the number of spore progeny needed to be screened to isolate heterothallic segregants of the proper genotype made the procedure valuable for genetically intractable strains. Trial grape juice fermentations indicated that introduction of ho had no deleterious effect on fermentation behavior.