auerbach: effects of chemicals 143 



MacKey: I greatly appreciate the cautiousness shown by Dr. Auer- 

 bach in discussing results from different organisms in relation to muta- 

 genic treatment. 1 believe there is great danger in trying to explain 

 different radiobiological phenomena observed in different research 

 objects according to one pattern. For example, the radiosensitivity in 

 relation to polyploidy goes a completely different way in a ploidy series 

 of wheat or any higher plant and yeast. From diploid level onwards, 

 radioresistance gradually increases with genome number in the first 

 case but decreases in the latter. The results cannot be explained only 

 by substitution of cells in the multicellular organism. It seems likely that 

 a factor not yet discussed here intervenes, viz., the mutual interaction of 

 cells in a multicellular tissue. Work by Dr. A. M. Clark with Habro- 

 bacon favors such an interpretation. He found haploid embryos more 

 resistant to X-rays during cleavage. No significant difference in radio- 

 sensitivity existed at blastema and early larval stages, and at late larval, 

 prepupal, and pupal stages, the diploids showed the higher resistance. 



Kramer: There is perhaps some precedent for this approach in the 

 use of such chelating agents as ethylene diamine tetra acetic acid on 

 Chlamydomonas and Drosophila. Some preliminary work by Nuffer, I 

 believe, has indicated less success with corn, but this may be due to 

 inadequate methods of treatment. Surely this approach has value both 

 in plant breeding and in learning more about recombination. 



Auerbach: The importance of induced recombination for plant breed- 

 ing deserves, I think, more consideration than it has received so far. 

 It seems that the efficiencies of mutagens in affecting crossing over, on 

 the one hand, and producing chromosome rearrangements, on the other, 

 are only loosely correlated. It might, therefore, be promising to under- 

 take a screening program — first, cytologically, then genetically — in a suit- 

 able organism like maize for chemicals that enhance recombination 

 with little concomitant sterility or other undesirable effects resulting 

 from rearrangements. 



Vallentyne: I would like to comment briefly on the first of your three 

 initial hopes, viz., the possibility of learning something about the chem- 

 ical nature of the hereditary unit from a knowledge of the chemistry of 

 mutagenic substances. As you were talking about the action of formalde- 

 hyde and nitrous acid, it occurred to me that there is a reaction that 

 should be considered in relation to the natural process of mutation. 

 This is the reaction between keto and amino compounds first studied 



