26-27 THE YEAST CELL 



ly of g cells (previously called nonfermenters) capable of mutation 

 to G with sufficiently high frequency to insure long-term adaptation 

 to galactose. The difference between gg and g is therefore in the 

 exceptionally high mutation rate of g to G in the gg clones. 



EVIDENCE FOR MUTATION OF g TO G 



The times required for long-term adaptation of gg clones from 

 the same tetrad may vary from 4 to 25 days. These discrepancies 

 are not consistent with the view that the lag represents the period 

 during which the adaptive enzyme is accumulating in the entire pop- 

 ulation of cells, although discrepancies of this degree would be ex- 

 pected if mutation and selection occurred. In addition to the varia- 

 tions of time of gas formation for sibs, great variations occur in 

 the time of fermentation when nearly uniform amounts of inocula 

 are used and the same culture is used to inoculate a large number 

 of different tubes. This further supports the view that selection of 

 a mutation occurs in the positive tubes. 



Cells of the G mutant derived from a gg culture evolved gas 

 within 24 hours when inoculated into galactose broth. A mating was 

 made using one of the G mutants selected from a plate by a standard 

 fast fermenter of galactose. Nine 4-spored asci of this hybrid were 

 dissected and all the haploid progeny were fast fermenters of galac- 

 tose. The mating was made by inducing mass copulations of haplo- 

 phases heterozygous for mating type. The hybrid was heterozygous 

 for mating type, the fermentation of melibiose, alpha methyl gluco- 

 side, maltose, and the synthesis of thiamin, inositol, and pyridox- 

 ine, and was homozygous for adenine dependence. The latter factor 

 resulted in the production of only pink progeny. Proof that a hybrid 

 had been obtained derived from the fact that regular segregations 

 occurred in every ascus for the heterozygous alleles. 



It could be argued, however, that the G mutant (selected from 

 the plate) was a fast fermenter because it contained autonomous 

 cytoplasmic enzymes (plasmagenes) and that a uniform distribution 

 of these plasmagenes had occurred in the hybrid and among its 

 progeny. To test this view the derived G was mated to a standard 

 g. Ten asci yielded two G spores in each ascus. This hybrid was 

 heterozygous for six genes; each ascus was characterized for four 

 pairs of alleles: a/a, G/g, AD/ad, and MG/mg. AD indicates ade- 

 nine independence and MG indicates ability to ferment alpha methyl 

 glucoside. The data appear in Table 26-14. The fact that every 

 ascus except one yielded 2 pink and 2 white cultures proves that a 

 hybrid had been produced. Each pink culture is adenine dependent; 

 each white is adenine independent. That the exceptional ascus is 

 also a hybrid is shown by the segregation of other characters. In 

 this ascus conversion of both pink and mg genes has occurred. 



