Chapter 18 

 DEVELOPMENT OF FERTILE BREEDING STOCKS 



The genus Saccharomyces contains species which differ in tneir 

 ability to ferment various simple sugars, but there are none which 

 decompose complex carbohydrates and generally none whose in- 

 tact cells secrete proteolytic enzymes. Different species are dif- 

 ferentiated by their requirements for the B-vitamins and mutants 

 requiring amino acids, and nucleic acid components have been 

 found. There is, however, considerable interspecific sterility and 

 the project of developing fertile breeding stocks has involved an 

 enormous amount of labor. 



FERMENTATIVE CHARACTERS 



Many cultures of S. cerevisiae are incapable of producing an 

 abundance of viable ascospores. We tested a great variety of S. 

 cerevisiae cultures, but a strain which we obtained from Dr. E. M. 

 Mrak (93) and one which we isolated (FLD) are among the most 

 vigorous which we have discovered. Since many species of Sac- 

 charomyces are differentiated from S. cerevisiae by differences 

 in abili^^ to ferment different carbohydrates, we sought to bring 

 the genes controlling these differences into our stocks. Many spe- 

 cies are described which are incapable of fermenting galactose, 

 but in our first atten^)ts we were unable to obtain viable ascospores 

 from any of the deficient species. Our first success was in the 

 pedigree involving S. microellipsoideus (Chapter 19). It is a weak 

 fermenter of galactose. We obtained a culture from Dr. L.J. Wick- 

 erham and one ascospore dissected from it produced a culture 

 capable of fermenting galactose weakly. A hybrid between it and 

 a haplophase from S. cerevisiae (FLD), a strong fermenter of 

 galactose, produced only one viable ascospore. This haplophase 

 culture was not a weak fermenter, but was apparently incapable 

 of fermenting galactose even after standing 30 days in the fermen- 

 tation tube. An outcross of the non-fermenter to a haplophase fer- 

 menter from S. cerevisiae (Mrak 93) produced a pedigree in which 

 regular 2:2 segregations of both the a/a and the G/g alleles occur- 

 red in each ascus. The non-fermenters did not produce gas from 

 galactose although they were observed continuously until they fin- 

 ally dried up, a period of from 40 to 50 days. Fig. 18-1 shows the 

 steps by which the breeding stocks have been developed. 



Both S. cerevisiae and S. microellipsoideus are non-fermenters 

 of melibiose. S. carlsbergensis (Mrak 126) fermented this sugar 



18-1 



