26-5 THE YEAST CELL 



pected (GGgg) type, but the numerous exceptions indicate that the 

 Mendelian mechanism is not operating precisely. The ME alleles 

 in 168 and 170 when backcrossed to me behave like standard dom- 

 inants producing only ME ME me me asci. However, 167 x me 

 produced one ME ME ME ME ascus and 169 x me yielded 3 ME me 

 me me asci. In the backcross to the fermenter only cultures 167, 

 168, and 169 were studied. 168 and 169 behaved rather regularly 

 as dominants with the G allele, but 167 produced several unexpect- 

 ed me progeny. The intra-ascus crosses which were matings of 

 fermenters by fermenters also yielded a considerable number of 

 nonfermenter progeny. An intra-ascus hybrid produced by mating 

 168 X 169 yielded only galactose fermenters and a single nonfer- 

 menter of melibiose among 18 progeny. (This mating was made 

 possible by the fact that 169 acted as an a culture after many 

 transfers.) The fact that the intra-ascus matings produced non- 

 fermenter progeny indicates that the phenotypical fermenters in 

 the ascus are not fully potent. The data suggest that spores 168 

 and 169 carry stroi^ dominants for both G and ME. The illegiti- 

 mate mating of 170 x 170 threw more "nonfermenters" of galactose 

 and melibiose than fermenters. 



THE ORI(3N OF NONFERMENTERS OF MALTOSE 



Numerous attempts were made to bring a maltose nonfermenter 

 into the pedigree by selecting haploids from a number of natural 

 nonfermenters of maltose. However, many of them failed to pro- 

 duce viable spores and none of those with viable spores was cap- 

 able of producing hybrids. S. microellipsoideus from which the 

 members of this pedigree are descended is ordinarily classified as 

 a nonfermenter of both maltose and lactose, but no data are avail- 

 able now concerning the cultures which we used. 



The progeny were diagnosed for ability to ferment maltose after 

 the original cultures had been lost. Whatever the maltose ferment- 

 ing ability of the parents, it is clear that the asci do not follow any 

 regular behavior. With regard to maltose the first mating yielded 

 1 ma ma ma MA, 2 ma ma MA MA, 2 ma MA MA MA, and 3 MA 

 MA MA MA segregations. Regular segregations occurred in some 

 asci, notably 237-240, and 249-252. Other asci from the same hy- 

 brid were MA MA MA MA, or MA MA MA ma. However, this re- 

 sembled the circumstances surrounding melibiose and galactose 

 fermentation and did not negate the possibility that a reliable non- 

 fermenting allele of maltose had been uncovered. Further analysis 

 showed that ma x ma matings generally produce only ma ma ma ma 

 asci, indicating that the nonfermenter is not capable by itself of 

 achieving fermentative ability The precision with which the MA/ma 

 alleles segregate in the mating of 520 x 525, Table 26-3, proves 

 that fermentation of maltose is under genie control. 



