Discussion 231 



oxidase. But we have an absolutely parallel effect between respiratory 

 adaptation and the induced "petite" mutation. 



Westergaard: What happened with your fourth system, that of 

 spontaneous mutation ? 



Slonimski: This gave a very curious result, but I did not want to talk 

 about it because selection is not excluded and the results are preliminary. 

 Cofactor E does not decrease the percentage of mutants ; on the contrary, 

 it increases it. Adenine does the same. So, from this point of view, it is 

 exactly the same as in the euflavine-induced mutation. However, the 

 situation is reversed because it is E which decreases the percentage of 

 mutants in the euflavine system, while it increases it in the spontaneous 

 mutation ; conversely, T, which increases the percentage of mutants in 

 the euflavine system decreases it in the spontaneous system. Spon- 

 taneous and induced mutants are both respiratory deficient, but recent 

 work by Ephrussi, Roman and Hottinguer showed that in many of the 

 highly mutable strains the "petites" are genetically different from the 

 acridine-induced ones. 



Westergaard: These are the somatic "petites"? 



Slonimski: I think that the "petites" of C982/19b are somatic (vegeta- 

 tive) but they may be suppressive — dominant instead of being recessive. 



Davis : How would you contrast the action of these substances in the 

 mutagenic system with the action of other ordinary mutagens? 



Slonimski: "Petite" mutation is genetically something rather unusual 

 and from this material one should not extrapolate hastily to any other 

 phenomenon. It shows, among other things, a unique relation between 

 the process of mutation and the process of adaptation, both processes 

 being, of course, quite distinct. This common reaction is specifically 

 inhibited by acridines and involves tetrose derivatives. How they act 

 is purely h\^3othetical at the present time. It seems to me possible that 

 a tetrose nucleotide or its polyiner is a part of the genetic material. 

 Another possibility is that it acts like a co-enzj^me in the synthesis of the 

 proper genetic material. ^Vllen we start getting down to the molecular 

 level it may be difficult to distinguish between the immediate product of 

 the action of a genetic determinant and parts of its structure. The point is 

 that tetrose derivatives seem to be specifically involved in both. 



Westergaard: What is the present status of the mitochondria in the 

 somatic "petite"? 



Slonimski: It contains mitochondria which are morphologically similar 

 to those from normal cells. "Petite" mitochondria don't contain cyto- 

 chrome oxidase, of course, therefore they will not be stained by Janus 

 green-B, but on fixed preparations they can be revealed by Altmann 

 staining. The present status of this problem has been reviewed by 

 Ephrussi, Slonimski and Yotsuyanagi (1955, loc. cit.). 



Pollock: Have you found any substance or even any conditions which 

 will affect the mutation rate to "petite" without producing a comparable 

 effect on adaptation? 



Slonimski: Mutation and adaptation can be dissociated by cellular 

 multiplication. The former requires proliferation, and mutant clones 

 derive almost exclusively from cells formed in the presence of the drug. 



