Allele specific determinants of homothallism in Saccharomyces lactis.

N general, fertile yeasts are classified as being either heterothallic or homo’ thallic. Heterothallic strains exhibit one of two alternate mating types (usually designated as a and a! or + and -) with diploidization occurring between cells of opposite mating type only (e.g. a with a! and + with -) . In homothallic yeasts, on the other hand, restrictive mating systems are not observed and sister cells quite commonly fuse to form zygotes. The known genetic controls regulating homothallism in yeast are numerous and apparently complex in their action. For example, LEUPOLD (1958) working with the fission yeast Schizosaccharomyces pombe strain liquefaciens found that mating behaviour was controlled by a complex locus. Two heterothallic strains, h+ and h-, were genetically determined by closely linked mutant sites within this complex locus. Homothallic strains, designated H40 and Hgo, could be derived from these heterothallic cultures either through mutation or through recombination within the complex region. In the yeast Saccharomyces chevalieri the presence of a gene D in a single-spore culture committed the culture to homothallism (WINGE and ROBERTS 1949). The D gene was independent of the mating locus and was equally effective in the presence of either the a or alleles. Heterothallic cells of S. chevalieri, on the other hand, possessed the recessive d allele and exhibited either one of the alternate mating types a or a. OESER (1962) and HAWTHORNE (1963b) have reported that the D gene produces homothallism by directing mutation of a mating type allele to the alternate form in some of the cells within a culture. Following this mutation, the affected culture becomes homothallic since it consists of a mixture of a and a! sister cells capable of zygote formation. In Saccharomyces cerevisiae homothallism results either from a high mutation rate of one mating type allele to the other (AHMAD 1952) o r from the action of the complementary genes HM,HM, and HM,HM, (TAKAHASHI, AITO, and IKEDA 1958; TAKAHASHI 1958). To compare the nature of the D and H M genes, TAKAHASI and IKEDA ( 1959) prepared hybrid Saccharomyces diploids containing both the HM,HM, complement and the D gene of WINGE and ROBERTS (1949). The D gene and the H M genes segregated independently. This paper reports the identification of factors affecting homothallism in ’ This recearch supported by Public Health Service grants 1459 and 5R01 (AT-00328-14) Public Health Service postdoctoral fellowship 5-FZ-GM-13, 112-02 Present address Northern Utilization Reseaich and Development Di\ision, ‘IRS, U S Department of Agriculture, Peoria, Illinois