Induced Mutational Changes in Yeast 111 



sucrose when seeded into the medium containing sucrose and 

 autolysed yeast extract, propagate more or less intensively, 

 especially within the first few days after seeding. In an 

 experiment where 32-35 million cells were introduced into the 

 test-tubes, within 6 days the number of cells had increased to 

 150-200 million. Later on, the reproduction of cells slowed 

 down considerably, and only when sucrose-fermenting cells 

 appeared in the test-tubes did the numbers of cells increase 

 rapidly. 



Testing of experimental cultures for their ability to ferment 

 maltose: The sediment of yeast cells from each test-tube was 

 transferred to new test-tubes, with gas-traps, containing a 

 maltose medium. One of these cultures fermented maltose 

 very well; this was culture 72/349, which had previously 

 adapted to fermentation of sucrose on a sucrose medium. 

 Subsequent reseedings showed that the ability to ferment 

 sucrose and maltose, developed by this culture, was very 

 strongly heritable. 



The appearance of yeast cells capable of fermenting maltose 

 in a sucrose medium was of special interest for the elucidation 

 of the question of the nature of such changes. In the first 

 place, it was necessary to find out how often these changes 

 occur. In a series of experiments, 55 cultures were developed 

 which were adapted to fermentation of sucrose, when culti- 

 vated on a sucrose medium. Out of these 55 cultures, 42 were 

 developed from single spores of S. globosus, and 13 from single 

 spores of the second generation of hybrids from S. ellipsoideus 

 X S, globosus. These hybrids, prior to their adaptation to 

 fermentation of sucrose, did not even ferment sucrose, let 

 alone maltose. Many of the cultures mentioned in these 

 experiments became adapted to fermentation of sucrose (in 

 2-3 seedings or more). 



All 55 cultures were tested for their ability to ferment 

 maltose. It was found that as well as culture 72/349, culture 

 73/349 also fermented both sucrose and maltose. Therefore, 

 out of 42 cultures of S, globosus, 2 cultures became adapted 

 on the sucrose medium to fermentation, not only of sucrose, 



