16-10 THE YEAST CELL 



vigorous fermentation after 24 to 48 hours incubation at 30 degrees 

 C. This rapid adaptation could be due to either a specific factor in 

 the natural medium or simply to the inclusion of different substrates 

 which permit multiplication of the unadapted cells thus providing a 

 large population for the selection of mutants. 



CROSSES AND BACKCROSSES WITH STRAIN NO. 97 



Several crosses of the unadapted strain No. 97 with glucose - 

 utilizing yeasts were made by Mrs. Lindegren. Only a few asco- 

 spores were obtained in each successful case. The ascospores 

 were isolated on a lactate medium like that described above but 

 matings were made in standard peptone glucose broth and the natu- 

 ral presporulation medium was used to induce sporulation. No clear 

 segregation of the progeny has been found. In cases where all four 

 spores from an ascus were viable, two often grew rapidly while two 

 grew slowly, but when tested later for glucose utilization all would 

 be positive. Viable progeny from back-crosses of these cultures 

 with strain No. 97 were similarly positive. It is clear, however, 

 from the evidence obtained later that rapid reversion to glucose 

 utilization occurs on standard glucose broth and that such rever- 

 sions would take place on the complex medium required for sexual 

 fusion (Chapter 3). The progeny not only may have been quickly 

 adapted but may have been derived In the first place only from glu- 

 cose-adapted parent cells. 



THE MECHANISM OF ADAPTATION 



These results fit the view that adaptation occurs by the recov- 

 ery of a recently lost ability; it seems likely that not only the mul- 

 tiplication of the mutants but the mother cells themselves are in- 

 fluenced by the presence of the substrate, and that the mutant does 

 not arise merely by chance. The reversion to glucose utilization 

 is probably not a single step change occurring uniformly in indi- 

 vidual cultures to restore the mutant to the original wild type. This 

 suggests that the strain incapable of utilizing glucose did not arise 

 by specific single step change in the metabolic apparatus of the 

 wild type. 



