26-33 THE YEAST CELL 



EVIDENCE IN WELL-MARKED HYBRIDS THAT 

 CONVERSIONS OF ALLELES OCCUR IN 

 THE HETEROZYGOTE* 



Mundkur and Lindegren's (1949) demonstration that the pheno- 

 menon of long-term adaptation to galactose was explicable as the 

 result of mutation of non-fermenter clones (g) to the fermenter 

 allele (G) has raised the question whether the non-Mendelian seg- 

 regations of galactose and melibiose phenotypes encountered in 

 the above pedigrees were indeed instances of conversion in the 

 heterozygote during meiosis, or merely mutations occurring after 

 the spore had germinated. The extra -fermenters in non-Mendelian 

 tetrads described in this chapter ferment as rapidly as the bona- 

 fide fermenter segregants and this indicates strongly that they are 

 different from those which ferment "slowly". In the latter case the 

 number of days elapsihg before a "slow" clone ferments is very 

 variable, suggesting chance mutation. However, an irregular tet- 

 rad is diagnosed as such only if more than the expected number of 

 segregants ferment at the saae time, usually within 18 hours; 

 or if greater than the expected number of segregants do not fer- 

 ment over an extended period of time. 



Tetrad analysis involves the transfer of single-spore glucose- 

 grown microcolonies to glucose agar slants. When the four haplo- 

 phase clones have attained mature growth on the slants they are 

 tested for their mating type specificity and their abilities to fer- 

 ment various sugars and synthesize vitamins, amino acids and 

 nucleic acid components. This initial growth on glucose may not 

 involve selection of phenotypes, and mutants of recessives to 

 dominant fermenter phenotypes should have equal opportunities for 

 growth. When a hybrid ascus heterozygous for G/g and ME/me is 

 dissected, one expects two ascospores to be nonfermenters. How- 

 ever, if mutation occurred on glucose after these two spores ger- 

 minated, a segregant culture could comprise mixtures of original 

 g and mutant G clones. If this mixed population were used as ino- 

 culum in testing for galactose (or melibiose) fermentation the mu- 

 tant cells would be selected and the culture would ferment. A 

 highly mutable (g to G; me to ME) haplophase segregant is thus 

 liable to be diagnosed as an extra -fermenter although the asco- 

 spore giving rise to the mutant clones was genotypically g or me 

 when isolated from the ascus. 



Similarly, loss mutations (G to g; ME to me) are liable to 

 yield fewer than the expected number (two) of fermenter phenotypes 

 when mature glucose -grown clones are tested for fermentation. 



This section contains material extracted from 

 Mr. Balaji D. Mundkur's unpublished doctoral dissertation 



