Discussion 291 



Davis: Do you know whether this kind of study has been apphed to 

 allehc mutants blocked in different sites within the same locus? 



Westergaard: It has nothing to do with the locus. It is only relevant to 

 the mutant strain which you use. We can get other alleles at the same 

 locus with quite a different reverse-mutation pattern. This pattern 

 probably depends upon how the gene was originally damaged. 



Pontecorvo: Is this the first example of real specificity in mutagenic 

 action ? 



Westergaard: You find the same thing, for instance, if you use dimethyl 

 sulphate and diethyl sulphate and if you use diepoxybutane. This is 

 where it was first found. Giles finds the same pattern with X-rays and 

 ultraviolet light. It all depends upon the particular allele and the pattern 

 for each allele has to be worked out. 



Demerec: What is the available evidence about the potentialities of 

 antibiotics as chemical mutagens ? 



Westergaard: The only evidence is that all the antibiotics which have 

 been tested in plants seem to induce chromosome breakage and re- 

 arrangement. This has been shown for penicillin by Levan, and by 

 Wilson and others in the United States for streptomycin and chloro- 

 mycin. I think that all inducers of chromosome breakage should be 

 considered mutagens, because everything which will induce chromosome 

 breakage may also cause rearrangement. I don't agree with Auerbach 

 in this discrimination between chromosome breakage and mutation. 



Alexander: Would you call maleic hydrazide a mutagen? 



Westergaard: Definitely so. This again is relatively specific on the 

 cytological level, because it seems to attack only the heterochromatin 

 preferentially and so does S-oxj^caffeine. 



Alexander: Have you been able to test in your system whether one of 

 the mutagens acted by changing the gene so that it now transmits a 

 new property, as opposed to destroying the gene so that it no longer has 

 any biological function ? Is it possible to distinguish between these two 

 different types of processes in your system ? 



Westergaard: We have so far induced only back mutations. Demerec 

 has induced both forward and back mutations, but we have not done it 

 on a quantitative scale. We hope to induce forward mutations with 

 urethane, which will not induce back mutations in these two particular 

 loci. We are now trying to get the forward and back mutation patterns 

 worked out, inducing the forward mutations by different mutagens. 



Alexander: Surely it is within the realms of possibility that something 

 which is a powerful chromosome breaker may only act in the waj^ of 

 destroying the gene, but never introducing a positively changed gene ; 

 i.e. a function has been lost without a new function taking its place. 



Westergaard: I would still call them mutations. 



Alexander: There are some mutagens which can produce back muta- 

 tions which would indicate that they can actually change the gene 

 material in such a way that it transmits a new and different code. 



Westergaard: This is what I believe, but I am not sure that everybody 

 would agree with me. 



Pontecorvo: Can back mutations be produced with X-rays? 



