15-4 THE YEAST CELL 



Seven asci from an ad(P)MT x AD MT back cross hybrid pro- 

 duced 2 pink and 2 white per ascus. This further confirms the 

 regular segregation of the two genes. 



Adenine -dependence is the effect of the action of a single gene; 

 pink pigment is a correlated effect which depends on the synergis- 

 tic effect of other genes as well. Pink pigment is apparently pro- 

 duced following the interaction of a precursor of adenine and an 

 excess of methionine plus other substances. Pigment is usually 

 produced in organisms incapable of completing the synthesis of 

 adenine and capable of producing a considerable amount of methio- 

 nine. The variation in intensity of color in different pink organisms 

 indicates that many other factors affect color intensity. 



FALSE MUTATIONS 



Some of the adenine -dependent, methionine -dependent white 

 cultures were transferred to peptone agar to which an excess of 

 methionine had been added. Pink cultures appeared thus confirm- 

 ing the dependence of the pink character on the presence of methion- 

 nine. Added methionine did not induce the development of a pink 

 color in any of the adenine -independent white organisms. When the 

 cultures arising from homozygous ad mt stocks were grown on 

 agar, numerous small secondary pink papillae often appear sug- 

 gesting local accumulation of sufficient methionine to produce the 

 pink color. 



Variations in bacteria following environmental changes have 

 often been called *mutations"but the present experiments show that 

 variations may also be due to a deficiency either of external or in- 

 ternal origin which prevents the development of the characteristic 

 phenotypeon a deficient medium. False "mutations" from pink to 

 white may appear when growth occurs in the absence of sufficient 

 methionine to insure the production of the pink color; many pink 

 cultures have white borders which may arise when the supply of 

 methionine in the medium becomes exhausted. Transfer of these 

 false "mutants" to a medium containing sufficient methionine may 

 result in a false "reverse mutation" from white to pink without any 

 change occurring in the gene itself. 



DEPLETION MUTATIONS 



Tatum and Reaume discovered that white (ad MT) variant co- 

 lonies which retain their methionine synthesizing ability often arise 

 from pink cultures on vegetative propagation, - a fact which we have 

 confirmed, and which they will discuss in greater detail elsewhere. 

 Fig. 15-1 shows a typical plate of one of our pink segregants. Nu- 

 merous white variants of the original pink have appeared. These 

 white variants are ad MT like the white variants of Tatum and 



