Mating types in Saccharomyces: their convertibility and homothallism.

GENETIC systems controlling homothallism uersus heterothallism in Saccharomyces yeasts have been reported by several authors ( WINGE and ROBERTS 1949; TAKAHASHI 1958; HERMAN and ROMAN 1966; TAKANO and OSHIMA 1967). Allelism tests indicated that all the homothallism gene systems in S. cereuisiae and its related species are the same (TAKANO and OSHIMA 1970a). The controlling system in these strains consists of two unlinked genes, HO, and HM. The HO, gene acts as a specific mutator for the 01 mating-type allele and changes 01 to a shortly after spore germination (TAKANO and OSHIMA 1970b). A cell converted from 01 to a can make a zygote with its neighboring 01 cell. An a allele converted from 01, designated a’, is as stable as normal a, and responds like a normal a to HO,. Thus the 01 HO, hm clone is homothallic, and after meiosis it segregates 2 homothallic : 2a in every ascus (TAKANO and OSHIMA l967,1970a, 1970b). The HM gene has no effect alone, but when combined with HO,, it converts a to homothallism (TAKANO and OSHIMA 1967). This paper presents data demonstrating that the combination of two complementary genes, HO, and HM, brings about the conversion of an a allele to a. An 01 mating-type allele, 01’) derived from the a or d allele is stable and cannot be distinguished from normal O( either in its ability to form zygotes with a cells or in its response to HO,.