22-37 THE YEAST CELL 



THE FUNCTIONS OF RECESSIVE AND DOMINANT 

 ALLELES IN ADAPTATION 



The analysis of the "slow" fermentation of galactose (Chapter 

 11) shows that the process of "slow" fermentation occurs in two 

 phases. During the first phase of primary adaptation there is 

 an abundant increase of cells with galactose the source of carbon, 

 although none of the cells produced during this period is capable of 

 rapid fermentation of galactose and no gas appears in the tube. 

 However, when the cells are removed from the fermentation tube, 

 placed in a Smith tube over mercury and provided with substrate, 

 they produce gas. Therefore, they are actually capable of ferment- 

 ing galactose, but at a much slower rate than the dominant genotype. 

 The slow growth in galactose is the result of utilization of galactose 

 by the cell through the agency of the recessive gene by the conven- 

 tional fermentative apparatus; the recessive gene is not incapable 

 of fermentation but merely ferments the sugar at a lower rate. The 

 period during which the recessive cells are multiplying in the me- 

 dium comprises the primary adaptation. The failure of the cells 

 during the primary adaptation to produce visible gas in the fermen- 

 tation tube results from the diffusion of the gas into the atmosphere 

 more rapidly than it is evolved. That gas is actually produced is 

 shown by the evolution of gas over mercury in the Smith tubes. Ad- 

 aptation of the recessive by treatment with substrate has been shown 

 to increase its ability to produce gas in the Smith tube. The fre- 

 quency of mutation from recessive to dominant is in the order of 

 1 X 10 ' and when the population attains this magnitude a sufficient 

 number of dominant cells is present to take over and the secondary 

 adaptation begins. During secondary adaptation the dominant gene 

 ferments galactose at such a rapid rate that the gas collects in the 

 fermentation tube before it can diffuse into the atmosphere. 



These facts appear to bear a direct relation to the phenomenon 

 of adaptation to a pantothenate -free medium by the pantothenate - 

 dependent cultures. Fig. 22-1 shows that the growth rate in a me- 

 dium containing a full supply of pantothenate follows a simple sig- 

 moid curve, but that the growth in lower concentrations is a more 

 complex phenomenon resembling the diauxie growth curves de- 

 scribed by Monod. The phase of the primary adaptation involves 

 long lag followed by a slow rate of growth; for example, in the first 

 17 curve the lag persists for about 170 hours and the slow growth 

 following primary adaptation continues until about the 380th hour. 

 From the 170th to the 380th hour the recessive alleles are presum- 

 ably synthesizing the pantothenate required for growth. About this 

 time the mutation occurs initiating the rapid growth characteristic 

 of the secondary adaptation. When a larger amount of pantothenate 

 is present all these steps occur more rapidly because of the re- 

 duction of the synthesizing "load". 



