Evidence of the Insensitivity of the alpha-inc Allele to the Function of the Homothallic Genes in Saccharomyces Yeasts.

The possible function of the alpha-inc allele (an alpha mating-type allele that is insensitive to the function of the homothallic gene system) was investigated by means of protoplast fusion. The fusion of protoplasts prepared from haploid strains of alpha-inc HO HMalpha HMa and alpha ho hmalpha HMa gave rise mainly to nonmating clones (58 of 64 isolates) and a few clones (six of 64 isolates) showing alpha mating type. Thirty of the 58 nonmating clones showed the diploid cell size and 28 clones had a larger cell size. Tetrad analysis of the nonmating clones with diploid cell size indicated that they were a/alpha-inc diploid; the normal alpha allele in alpha/alpha-inc cells was preferentially switched to an a allele. This observation further indicated that the HO/ho HMalpha/hmalpha HMa/HMa genotype is effective for the conversion of the alpha to a and that the inconvertibility of the alpha-inc allele is due to the insensitivity of the mating-type allele to the functional combination of the homothallic genes. It was suspected that fusion products larger than diploid cells might have been caused by multiple fusion of protoplasts.