26-17 THE YEAST aLL 



Table 26-9 shows the number of days before fermentation be- 

 gan. The minus sign indicates fermentation did not occur. Of 79 

 colonies tested 4 finally fermented galactose after 12, 27, 21, and 

 17 days respectively. Twenty-two nonfermenters of meliJbiose ap- 

 peared while all cultures fermented sucrose on the second or third 

 day. Therefore, 557 which carries an unmistakable ME gene can 

 lose its ability to ferment in a fermentation tube in about one -fourth 

 of the single colony isolates if carried long enough in the absence 

 of melibiose. This is a very high rate of loss. However, conven- 

 tional diagnosis is still possible because selection in the presence 

 of the substrate favors the fermenters. 



Experiment DC The experiments (Spiegelman, Lindegren, and 

 Lindegren) which had been interpreted as indicating that the con- 

 tinued presence of a specific type of sugar could alter the pheno- 

 type of the progeny were repeated. Matings were made using hap- 

 lophases grown with glucose maintaining glucose in the tubes in 

 which matings occurred, keeping glucose on the g3rpsum slant and 

 dissecting the hybrid asci in glucose in an attempt to determine if 

 this would produce regular segregation of the glucose, melibiose 

 or maltose alleles. The first mating of 4 x 128 produced six 2 : 2 

 galactose asci but gave 2 irregular maltose asci (Table 26-10). 

 The mating L5C x 23 produced a large number of nonfermenters of 

 melibiose while the mating CIA x 15 produced 3 asci in which more 

 than 2 galactose fermenters were found. When galactose was main- 

 tained throughout the cycle (except for the fact that the haplophase 

 incapable of fermenting galactose was grown on glucose) 20 asci 

 were obtained from 4 x 128 in which 2 : 2 ratios of fermenter to 

 nonfermenter of galactose occurred. Three asci produced more 

 than 2 galactose fermenters, while 1 produced 3 nonfermenters of 

 galactose. Among 6 asci from a hybrid heterozygous for melibiose 

 fermentation there were none with more than 2 spores capable of 

 fermenting melibiose. The presence of maltose did not materially 

 change the kind of phenotype produced. Except for the fact that 

 fewer than the expected number of fermenters of melibiose 

 were obtained in glucose in the L5C x 23 mating, no significant evi- 

 dence of the effect of substrate was obtained. 



Conclusions: 



In the above series of experiments, the earlier claim that the 

 presence of a specific substrate could cause the maintenance of a 

 character for which no gene was present could not be substantiated. 

 Not only were inexplicable losses of fermentative ability occasion- 

 ally observed, but also abnormal ratios of fermenters to nonfermen- 

 ters in the segregants which were apparently independent of the sub- 

 strate on which growth and sporulation was achieved. Such ratios 

 were almost always in favor of fermenters, and at least in some 

 experiments could not be explained by self-diploidization or illegi- 

 timate copulation. 



