17-4 THE YEAST CELL 



The nonfermenter DbZS^B was a haploid member of the galactose 

 fermenting "D" family of S. cerevisiae and practically all other 

 members of this family (Chap. 14 and fig. 14-2) were fermenters. In- 

 dividual cells of Db23B which became lermenters in galactose prob- 

 ably did not acquire an ability of which they were basically incapa- 

 ble. The gene controlling fermentation of galactose may have been 

 temporarily degraded and when it was repaired in the presence of 

 galactose the fermenters took over; the de novo acquisition of 

 new capacity is probably an extraordinarily unusual occurrence. 

 We have made thousands of tests of different genetical strains which 

 are incapable of producing mutants capable of fermenting galactose 

 even when they are held in galactose broth until it dries up (about 

 four weeks). 



PROGRESSIVE MUTATION 



Mutations discovered in the laboratory examination of flies and 

 fungi were at first considered comparable to the changes occurring 

 in progressive evolution. Stadler (1932) suggested that X-ray mu- 

 tations may not be comparable to those involved in progressive 

 evolution, but may in fact amount to destruction of the gene. The 

 acquisition of a fermentative character may depend upon an elabo- 

 rate series of changes in the organism. The haploid S. cerevisiae 

 culture Db23B which produced an increased number of fermenter 

 progeny from a weakly fermenting clone may never have been com- 

 pletely deficient in this capacity; from 2 to 15 per cent of the popu- 

 lation derived from a simple "non-fermenter" cell were capable 

 of fermentation, suggesting that most of the remainder ot the popu- 

 lation may have been capable of slow and irregular adaptation by 

 a mechanism similar to that which gave the diploid strain the abili- 

 ty to adapt with a high degree of regularity. The inability of the 

 haploid strain to ferment soon and regularly after exposure to sub- 

 strate may be due to a relative deficiency of some essential sub- 

 stance required for the formation of measurable amounts of the en- 

 zyme rather than total inactivation of the gene controlling the fer- 

 mentative process. Back-mutation or reactivation may appear to 

 occur in a random manner, but the process itself need not be fun- 

 damentally random. This view will be developed further in Chapter 

 28. Real progressive changes in yeasts may be very rare phenomena. 

 Yeasts have probably been adapted to grow as saprophytes for about 

 half a billion years. During this period they have developed forms 

 capable of fermenting most of the common sugars. The ability to 

 ferment any one of these, even including glucose, may be lost; once 

 it has been completely lost it appears impossible under ordinary 

 laboratory conditions to recover the ability. This is indicated by 

 the fact that our unadaptable galactose and melibiose strains never 

 produce a suffucient number of mutants to interfere with genetical 



