Induced Mutational Changes in Yeast 129 



agree that there exists only one type of mechanism of develop- 

 ment of these variations, and that we should consider them 

 either as being unstable (bearing on quantitative shifts in the 

 enzymic organization of cells) or stable ones of the mutation 

 type. In each and every case, the character of variation will 

 be determined by the nature of the organism undergoing 

 change and by the peculiarities of the factor which exerts its 

 influence in those organisms. As regards the variation in 

 fermentative properties of yeasts, one can speak of four types 

 of adapted variations based on existing experimental data : 



The first type of variation involves the fermentative system 

 of the cell. An example of this type of variation is the capacity 

 acquired by the yeast cell to split sucrose by means of maltase. 

 This is not a case of qualitative change in the fermentative 

 system, but of alterations in the permeability of the cell 

 (Oparin, Helman and Zhukova, 1955). As our investigations 

 have shown, a most important factor here is the substrate on 

 which the cell acquires the ability to produce the active enzyme 

 maltase. If this ability is acquired on a medium containing 

 maltose then the splitting of sucrose usually becomes difficult. 

 Cultures obtained in this way from S. glohosus start to ferment 

 sucrose usually after a lapse of 10 days, but ferment maltose in 

 1-2 days. The same cultures oiS. glohosus, if they acquire the 

 ability to produce the active enzyme maltase on a medium 

 containing sucrose, will ferment sucrose within the first 2-3 

 days. In both cases hydrolysis of sucrose is effected by 

 maltase. The time of cultivation of such cells on the sucrose 

 medium is also of great significance. S. heterogenicus, S. pro- 

 stoserdovi and S. chodati were tested for ability to ferment 

 maltose and sucrose (but not raffinose), and it was demon- 

 strated (Kossikov, Helman and Rayevskaya, 1956) that all 

 of these hydrolyse sucrose by means of maltase, since maltase 

 but not invertase was detected in the cells after fermentation 

 of sucrose. Fermentation of sucrose in these species began at 

 varying times (from 2 to 5 days). 



The second type of variation was detected in the adaptation 

 of S. paradoxus to fermentation of maltose. In this case, the 



