Chapter 17 

 BACK-MUTATION AND PROGRESSIVE MUTATION 



The standard Mendelian mutant ordinarily used in genetical 

 analysis usually involves the loss of some synthetic ability; it is 

 deficient as compared to normal. Back-mutation to normal, there- 

 fore, involves the reacquisition of a capacity which has been tem- 

 porarily lost. A progressive mutation involves the acquisition of 

 an entirely new ability. The present chapter describes back-mu- 

 tation of a non-fermenter of galactose to a fermenter of this su- 

 gar. It is proposed that back-mutation probably involves the re- 

 pair of a gene which is only partially deficient or the reactivation 

 of a non-allelic dormant gene which can be brought into functional 

 activity in the presence of certain essential components (Chapter 

 22). Although it seems clear that progressive mutations must be 

 the basis of evolutionary advances, the frequency of which they oc- 

 cur may be very low. Apparent progressive mutation so frequently 

 encountered among microorganisms in the laboratory are probably 

 back-mutations or reactivations rather than progressive mutations. 



ADAPTIVE FERMENTATION OF GALACTOSE 

 WITHOUT FORMATION OF NEW CELLS 



S. cerevlsiae is an active fermenter of galactose. One of our 

 diploid cultures of S. cerevisiae (Lk, a derivative of the L family, 

 fig. 14-1, ferments galactose vigorously as measured by the evolu- 

 tion of gas in nitrogen. In preparing for Warburg analyses the cells 

 grown on glucose were washed in M/15 KH2PO4 and resuspended 

 in this buffer containing 4 per cent purified galactose. Under such 

 conditions, cell counts remain constant over long periods of time. 

 Spiegelman, Lindegren, and Hedgecock (1944) showed that adapta- 

 tion to galactose fermentation occurs quickly and almost explosive- 

 ly after 180 minutes in the Warburg vessel (fig. 17-1). Total counts 

 of the population showed no change in the number of individuals pre- 

 sent and are plotted on the same graph. This is adaptation without 

 the formation of new cells. The three hour lag period is reprodu- 

 cible imder standard contitions and is characteristic for this strain. 



ADAPTATION BY « BACK-MUTATION" 

 (FROM NON-FERMENTATION TO FERMENTATION OF GALACTOSE) 

 FOLLOWED BY SELECTION 



Practically all haplophase stocks of S. cerevisiae ferment galac- 

 tose. However, one haplophase culture of S. cerevisiae from the 



17-1 



