26-37 THE YEAST CELL 



Evidence that the differences in colony sizes result from differ- 

 ing growth rates was obtained from the following experiment: A 

 suspension of a single minute fermenter colony (clone M 533) from a 

 galactose plate was plated out in galactose agar and was found to 

 yield only minute colonies. A galactose pour -plate of a suspension 

 of a single large fermenter colony of the same clone produced both 

 large and minute colonies. Therefore, the minute colonies are slow- 

 er growing clones descended from the large colonied clones. This 

 explains the occurrence of fermenter colonies of two size classes 

 in the same plate. However, the fact that when only minute colonies 

 occur on a plate, they are generally nonfermenters indicates that 

 the genotype is intrinsic in the spore from which the clone descend- 

 ed. 



The uniformity in size of pour -plate colonies in a large number 

 of tetrads, supported by the fact that both large and small colonies 

 were similar in fermentative ability is strong evidence that muta- 

 tions with regard to fermentative ability occurred rarely, if at all, 

 in the microcolony. 



In determining the fermentative abilities of the minute colonies, 

 large numbers of them were inoculated into the same broth tube to 

 establish their phenotype with greater certainty. This is especial- 

 ly important when the corresponding glucose slant clone is a non- 

 fermenter. The use of a large number of minute colonies insures 

 that corroboration of the nonfermenter phenotype from glucose was 

 not coincidental. 



On the other hand, the extra -fermenters in a tetrad would have 

 been diagnosed as such if only a single mutation (g to G; me to ME) 

 occurred on a slant; or, if there were a single fermenter colony 

 amidst the more numerous nonfermenter colonies and this excep- 

 tional one were inoculated into sugar broth together with its sister, 

 nonfermenter colonies. 



To test for variation among the individual colonies on a plate 

 in the case of tetrads with extra -fermenters, the following experi- 

 ment was performed: Two representative tetrads having extra - 

 fermenters were selected (M532 - M535; M544 - M547) and the 

 clones known to be galactose and melibiose fermenters were plated 

 out in glucose nutrient agar. Ten individual colonies picked at 

 random from each plate were inoculated each into a separate galac- 

 tose (or melibiose) Durham fermentation tube. All tubes contained 

 gas within 24 hours. 



This experiment indicates that individual colony variation with 

 respect to galactose and melibiose fermentations did not occur in 

 the pour -plates; it also confirms that after an originally recessive, 

 nonfermenter clone has acquired the fermentative ability (and thus 

 become the extra -fermenter in the tetrad) this new capacity is not 

 affected by an intervening period of growth on glucose. 



