GENE CONVERSION 26-36 



fact that cultures of CIA and 609 did not copulate illegitimately 

 and produce ascospores. The possibility that a high rate of muta- 

 tion of the nonfermenter recessive genes to dominance would ex- 

 plain the irregularities in segregation was excluded by the fact 

 that the haploid clones to which CIA and 609 were mated had been 

 derived from pedigrees in which the stability of the recessives was 

 pronounced. 



SEGREGATIONS OF GALACTOSE AND MEUBIOSE 

 FERMENTER PHENOTYPES 



It has been pointed out above that the ambiguity in deciding 

 whether the extra -fer me nters in G G G g and G G G G (or ME ME 

 ME me and ME ME ME ME) tetrads originate from gene trans- 

 formations or from mutation can be resolved by minimizing the 

 chances of mutations in a haplophase segregant. The galactose and 

 melibiose pour plates made with inoculum obtained directly from 

 the microcolonies provided data on this point. Since the mutation 

 rates observed are usually of the order of 1 x 10-7 and the number 

 of cells in a microcolony at the time of pouring plates does not ex- 

 ceed 500,000, it may be assumed that the probability of recovering 

 fermenter mutants is rather remote at this stage of growth. 



The colonies in pour plates of both galactose and melibiose 

 agar are principally of three different sizes on the fourth or fifth 

 day after plating. These have been arbitrarily designated as 

 large-, medium- and minute-sized colonies. The large colonies 

 range in diameter from approximately 0.2 to 0.5 mm. in diameter. 

 The minute colonies are visible as specks in the agar. 



When a haploid clone is plated out, it may develop into uniform- 

 ly sized colonies falling in any one of these categories, or a com- 

 bination of two of these size classes, or it may show no growth at 

 all. In Tables 26-18 and 26-19 when one class alone is indicated 

 for a segregant, it is implied that the colonies in that particular 

 pour -plate are uniformly sized. 



The medium- and large-sized colonies are obviously fermen- 

 ters and consistently corroborate the phenotype of the segregant 

 obtained from the corresponding glucose slant. However, the 

 minute colonies may or may not be fermenters. For instance, in 

 cases where minute and large colonies occur in the same plate, 

 single minute colonies inoculated with a micropipette into sugar 

 broth achieve fermentation and confirm the characteristic of the 

 clone from the corresponding slant. In other cases, however, 

 minute colonies occurring uniformly in a plate were generally found 



found to be nonfermenters. 



