PROBLEMS OF MEASUREMENT OF MUTATION RATES 69 



whereas in man there are, perhaps, a sufficient number of homeostatic 

 mechanisms that we can readily detect only a small minority of the 

 changes in the genetic information. 



Demerec: A good answer to your question is to be found in an 

 observation made in pre-X-ray days by Timofeff-Ressovsky: that 

 in Drosophila funebris visible mutants are extremely rare. 



Neel: Dr. Demerec, I can't resist saying that the first paper which 

 I ever wrote was on the mutants of D. funebris (24) . It does indeed 

 seem to be the case, that despite a considerable amount of work, 

 D. funebris has yielded only a fraction of the clearcut, nice, clean 

 traits seen in D. melanogaster. Almost all the mutants recognized have 

 been extremely susceptible in their manifestations to temperature and 

 nutrition. What this means in terms of the structure of the genetic 

 material, who can say? 



Glass: Was there a comparison in respect to lethal rates, too? 



Neel: No. 



Auerbach: Wouldn't the difference be rather in the biochemistry of 

 the species than in the genetic material? 



Stern: Some of the types looked for were body or eye color 

 mutants which are frequent in melanogaster but were not found in 

 funebris. 



Auerbach: But this may be due to a difference in development be- 

 tween the species. I remember a striking case of this kind in Chlamy- 

 domonas. Although mutations to amino acid requirements were found 

 in Reinhardi, Gowans (15) was not able to induce them in eugametos. 

 Yet he obtained other types of requirement as mutations. It seemed 

 to me that this difference might have something to do with the 

 metabolism of these two species, perhaps something which makes 

 this kind of mutation lethal to eugametos. 



Lederberg: Cryptic autopolypoidy would be much more like it. 



Auerbach: But he got other mutations. 



Lederberg: But not the entire gene; part of it. 



Auerbach: But all the amino acid determinants would have to be 

 in one particular part of it. 



Lederberg: We are beginning to believe that now. 



Neel: But when you say it may be due to a difference in metabolism, 

 ultimately, I take it, this goes back to the genetic material. If there 

 are species which have a good deal of cryptic autopolypoidy, this cer- 

 tainly does interfere with comparing mutation rates from one to the 

 next, doesn't it? 



