Genetic analysis of intergeneric hybrids obtained by protoplast fusion in yeasts.

Prototrophic hybrids have been obtained by the fusion of various auxotrophic haploid strains of Saccharomyces cerevisiae and Schwanniomyces castellii. The fusion hybrids showed starch fermentability which derived from one of the fusion parents, S. castellii. Surprisingly, these fusion hybrids were found to exhibit excellent sporulation and spore germination. The progenies of these fusion hybrids showed a few aberrant segregations, but mostly normal segregation for auxotrophic genetic markers. They also showed many tetrads with an apparently digenic segregation (2:2, 3:1 and 4:0) for starch fermentation. On the other hand, mating types of segregants of the fusion hybrids were determined by the prototrophic recovery method. Consequently, tetrad types for mating type were mostly 2a:1 alpha: 1 non-mater and several asci showed tetrad types of 2a: 2 non-mater and 2a:2 alpha. The 60 prototrophic fusion hybrids and its segregants did not secrete alpha-amylase on the starch agar plate. However, all of the data suggested that fusion hybrid could carry two dominant genes (STAB and STAC) to ferment starch, and that the two genes STAB and STA2 may be identical or allelic as may be the genes STAC and STA3.