Chapter 15 

 DEPLETION MUTATIONS 



The abundant variation which occurs when one plates a haplo- 

 phase culture has been described in Chapter 13. This phenomenon 

 seems to be in direct opposition to the reliability which we have 

 demonstrated in our mass mating technique. The answer to this 

 apparent contradiction is that many of these variants, although 

 stable on vegetative reproduction, cannot be transmitted through 

 the sexual cycle. In other words, when such variants are mated 

 to a normal individual they produce not half normal and half vari- 

 ant progeny, as one would expect, but only normal progeny. We 

 have called this type of variation a depletion mutation, implying 

 that something in the genome is lost which can be restored in the 

 zygote. The present chapter describes the analysis of the first 

 depletion mutation which we discovered. 



RELATION OF PINK COLOR TO ADENINE 

 AND METHIONINE SYNTHESIS 



Culture No. 4, an a mating type galactose-, maltose -fermenting 

 haplophase segregant of Saccharomyces cerevisiae (Mrak 93) was 

 subjected to mustard gas treatment by Tatum and Reaume (in ms.) 

 and produced an adenine -dependent mutant with pink colonies. The 

 symbol ad (P) indicates the adenine -dependent variant producing 

 pink colonies; ad indicates the same allele carried by a white pheno- 

 type. The symbol AD indicates the dominant allele; no secondary 

 symbol is necessary for these are always white. The adenine- de- 

 pendent pink (ad (P) ) culture was shown to be a gene mutation by 

 hybridizing it with an adenine -independent white (AD) haplophase 

 of S. cerevisiae. 



MATING HYBRID SEGREGANTS 



haploidxhaploid diploid haploid haploid haploid haploid 



gene ad(P) x AD ad(P)/AD ad(P) ad(P) AD AD 



color pink x white white pink pink white white 



Forty-six asci were dissected from the white hybrid and in 42 

 asci, two white and two pink cultures arose from each ascus, pro- 

 ving that gene mutation was involved (Table 15-1). In these 42 asci 

 the pink cultures were adenine -dependent and the white cultures 

 were adenine -independent. 



15-1 



