A Deletion in Yeast and Its Bearing on the Structure of the Mating Type Locus.

is growing evidence for the composite nature of the loci controlling T z E g types in yeasts as well as in the higher fungi (Coprinus lagopus: DAY 1963; Collybia uelutipes: TAKEMARU 1961 ; Pleurotus ostreatus: TERAKAWA 1960; and Schizophyllum commune: RAPER, BAXTER, and ELLINGBOE 1960). In the yeast Schizosaccharomyces pombe, LEUPOLD ( 1958) has demonstrated recombination (0.4 percent) between the + and alleles of the heterothallic strains to give a third phenotype for homothallism. The tetrads with a homothallic segregant also contain the reciprocal recombinant which although of the f phenotype is distinguished from the original 4parent by its failure to give homothallic recombinants when backcrossed to the parent. The results are consistent with the interpretation that there are separate cistrons determining the + and activities, and when neither is defective, the strain is homothallic. In Saccharolmyces cereuisiae, an investigation of mating type mutations has led to the following evidence for a similar complexity of the mating type locus.