EVIDENCE OF THE INSENSITIVITY OF THE -inc ALLELE TO THE FUNCTION OF THE HOMOTHALLIC GENES IN SACCHAROMYCES YEASTS

Isamu Takano ¹ and Kenji Arima ¹

¹ The Central Research Institute, Suntory Ltd., Wakayamadai, Shimamoto-cho, Mishima-gun, Osaka 618, Japan

The possible function of the -inc allele (an 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 -inc HO HM HMa and ho hm HMa gave rise mainly to nonmating clones (58 of 64 isolates) and a few clones (six of 64 isolates) showing 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/-inc diploid; the normal allele in /-inc cells was preferentially switched to an a allele. This observation further indicated that the HO/ho HM/hm HMa/HMa genotype is effective for the conversion of the to a and that the inconvertibility of the -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.