10-3 THE yEAST CELL 



erally distinguished by smaller size and diminished ability to pro- 

 duce 4-spored asci containing viable spores. Legitimate matings 

 between a and a haplophase cultures derived from a variety of strains 

 of S. cerevisiae usually produce large diploid cells which sporulate 

 to form 4-spored asci containing viable spores. The procedure is 

 as follows: One cc. of broth ^ is placed in a 150 x 18 mm. tube. The 

 broth is first inoculated with a large loop of cells from an agar slant 

 of the tester culture and shortly thereafter with a large loop from 

 the haploid culture to be tested. Copulations sometimes appear 

 within six hours. The tube is kept overnight in an incubator at 16*^C. 

 and then inspected under the microscope for the presence or ab- 

 sence of copulations. 



A large-scale experiment has demonstrated that two principal 

 alleles control mating-type specificity in S. cerevisiae. Fig. 10-2 

 shows the results of mating 58 different single ascospore cultures 

 derived from a variety of industrial bakers' yeasts. Ly, HD, M, 

 FLD, D and B represent standard legitimate diploid strains of com- 

 mercial baking yeasts; 800 is one of the baking strains of yeast ob- 

 tained from Dr. Wickerham of the Northern Regional Research Lab- 

 oratory, Peoria, Illinois. Two other cultures are hybrids, one of 

 800 X LK and the other (800 x LK) x Ly. Haplophases isolated from 

 the "(800 X L) x L" hybrid were generally quite fertile. Three be- 

 long to mating type a and seven belong to mating type a . Two were 

 sterile. Copulations invariably occurred when an a and an a culture 

 were mated, and haplophase lA produced illegitimate matings with 

 three other a type cultures. The Ly strain was also generally quite 

 fertile. Three a type haplophases and seven a type haplophases 

 were found in this culture. Only once did an a x a mating fail to 

 produce fusions. When the "(800 x L) x L" haplophases were mated 

 with the Ly haplophases, a high degree of fertility was demonstrated, 



