Chapter 26 



IRREGULAR SEGREGATIONS AS SUPPORTING EVIDENCE FOR 

 GENE CONVERSION 



The tetrad analyses in Chapter 25 are conclusive evidence of 

 the transformation of alleles because they involve well-marked 

 chromosomes and exclude the possibility of illegitimacy or multiple 

 factors producing the non-Mendelian ratios. Our first evidence 

 supporting the idea of conversion was derived from the analysis of 

 hybrids between galactose -melibiose-fermenters with "non-ferment- 

 ing** haplophases. It has been pointed out in Chapter 11 that the so- 

 called "non-fermenters" are capable of producing enzyme but do not 

 produce detectable amounts of gas imder the conditions of the test. 

 They are called •*non-fermenters'* with this qualification. Many 

 more data of this t3rpe are now available and are presented in this 

 chapter. In the tetrad analysis of many yeast hybrids produced by 

 matings of fermenter with '*nonfermenter** stocks, each ascus con- 

 tains 2 fermenter and 2 **nonfermenter*' spores. In other pedigrees 

 involving apparently similar matings 1, 2, 3, or 4 fermenters are 

 obtained from each ascus. The pedigree shown in Table 26-1 was 

 developed by mating the haploid culture CIA (Table 18-1), a single 

 ascospore culture derived from S. carlsbergensis (culture 126 from 

 Dr. Mrak's collection), capable of fermenting both galactose and 

 melibiose (aGME),with "nonfermenter" cultures Nos. 35 and 85 (a g 

 me) (Table 19-2). Both of the latter cultures did not produce gas 

 from either galactose or melibiose; heterozygous diploids were 

 produced. The a G Me x a g me matings revealed that these alleles 

 segregated independently of each other. The a/a alleles behaved 

 regularly in nearly every ascus and the exceptions concerned prin- 

 cipally the fermentative characters. 



Nine complete 4-spored asci from the hybrid 35 x CIA (a g 

 me X a G ME), were analyzed. One GGGG and one GGGg ascus and 

 seven of the expected GGgg asci were obtained. This mating pro- 

 duced 10 complete 4-spored asci in which ME/^e were diagnosed. 

 Eight were the expected ME ME me me; one was ME ME ME ME 

 and one was ME ME ME me. A second similar mating (85 x CIA) 

 produced three GGgg with 1 GGGG and two ME ME me me with two 

 ME ME ME me. 



Matings of an individual (184) from one of the irregular asci re- 

 vealed non-Mendelian transmission of both G and ME (184 x 166 and 

 184 X 187). A mating of 16 by 164, and 4 by 159 each produced two 

 regularly segregating asci. 



26-1 



