Induced Mutational Changes in Yeast 131 



very interesting results. It was found that in the cells of the 

 heterozygous cultures, the activity of invertase was only one- 

 half of that in the homozygous cultures. Not only the original 

 S. glohosus cultures, but also those obtained from heterozygous 

 cells, but failing to ferment sucrose, have no active invertase 

 in the intact cell (Kossikov and Rayevskaya, 1957).* 



The experimental data on the inheritance of sucrose- 

 fermenting ability during spore formation and on the activity 

 of invertase in heterozygous and homozygous cells, give us 

 grounds to believe that, in this case, in the process of adapta- 

 tion to fermentation of sucrose the ability to produce the 

 active enzyme invertase appears in the cell. As yet, it is 

 difficult to say whether this capacity is related to the nuclear 

 or other structural elements of the cell. The detection of an 

 invertase activity in the homozygous diploid culture, twice 

 as high as that in the heterozygous diploid culture, leads 

 us to conclude that the production of invertase is related 

 to biochemical reactions and structures characteristic 

 of the haploid cell. Fig. 5 shows the scheme of inheritance, 

 by S. glohosus cells, of the ability to ferment sucrose during 

 sexual reproduction. 



Can one speak in this case of the heritability of acquired 

 characters? The sum-total of our data bearing on the stability 

 and heritability of new enzymic properties of yeast, developed 

 in the process of adaptation to fermentation of sugars (fourth 

 type of variation), leads us to answer this question in the 

 affirmative. However, one should bear in mind that not every 

 single character acquired by the organism is inherited. What 

 we are referring to is the heritability of those characters which 

 arise as a result of changes in the modus vivendi, this change 

 leading to stable variations in cell metabolism and to alter- 

 ations in cell function. 



* The activity of invertase was determined by the Berthran method. Cultures 

 failing to ferment sucrose had no active invertase if they were obtained from 

 cells cultivated for a long time in the presence of sucrose or from the spores of 

 heterozygous cultures. The activity of invertase adapted to fermentation of 

 sucrose in the heterozygous cultures was 17 and in the homozygous cultures 

 was 34 (expressed in mg. of glucose per 10 mg. of pressed yeast during 60 min.). 



