432 



IfADIATION BIOLOGY 



nuliatioii may proccccl lliroufih tho action of chomical intcrmcdiutos. 

 Moit'ovoi-, it may ho possible to deduce important characteristics of the 

 radiosensitive cell constituents from a knowledge of the reactions in which 

 radiomimetic chemicals will specifically participate. Discussions of 

 chemical mutagenesis dealing mainly with Ncurospora have appeared 

 (Tatum, 1950; Jensen, rt al.. 1*)51). Table 11-1 is representative but is by 

 no means an exhaustive list of chemicals for which claims of mutagenic 

 activity have l)een put foith. 



Table 11-1. Somk ("ukmicals PuoDtTriNO Hadiatioxi.ikk Effects in Fuxgi 



Clicmiral 



Mustard gas 



Mustard gas 



Nitrogen mustard 



Nitrogen mustard 



Nitrogen mustard 



Nitrogen mustard 



20-Methvlcholanthrene- 



Fungus 



Neurospora crassa 

 Aspergillus nidulans 

 Neurospora crassa 



Coprinus fimetarhis 

 PenicilUum notatum 

 Saccharomyces cerevisiae 



Reference 



Horowitz et al., 1946 

 Hockenhull, 1948 

 McElroy et al., 1947; Miller 

 and McElroy, 1947, 1948 

 Fries, 1948 



Stahmann and Stauffer, 1947 

 Reaume and Tatum, 1949 



It should be noted, particularly for chemical mutagens, that the muta- 

 tions observed may not always be caused directly by the agents used or in 

 the manner anticipated. Ulie unmasking of genetic variability, which is 

 concealed in polyploid or particularly in heterokaryotic complexes, is a 

 basis for induced variation in fungi which, if unrecognized, may lead to 

 erroneous conclusions about the actions of inducing agents. It is also 

 important to examine the experimental conditions to evaluate the possi- 

 bility of selection of preexisting spontaneous mutants as opposed to direct 

 causation (Lederberg, 1948), an evaluation which has not always been 

 critically attempted. Spontaneous genetic changes are well known both 



