MANNER OF PRODUCTION OF MUTATIONS 547 



genie potency of the products of ultraviolet by dinitrophenol, anoxia, and 

 cold, respectively. Since in the work with Drosophila there was an 

 interval of about 7 minutes between the end of irradiation and the 

 beginning of the temperature treatment, the temperature-susceptible pre- 

 mutational stage in this organism must have lasted at least as long as 

 this. 



Another condition which has been found to cause distinct enhancement 

 (again using the term only descriptively) of the ultraviolet-induced- 

 mutation frequency is pretreatment with the mutagen nitrogen mustard, 

 in concentrations which alone are only slightly mutagenic, as shown by 

 Swanson and Goodgal (1947; also Swanson, 1952) for Aspergillus and by 

 Swanson, McElroy, and Miller (1949) for Neurospora. With Neurospora 

 it was found that with a mustard pretreatment the frequency of muta- 

 tions of a morphological type was not only enhanced at doses in the rising 

 portion of the ultraviolet dosage curve but was also able to attain a corre- 

 spondingly higher peak, whereas mutations to biochemical inadequacies 

 were so enhanced that no peak level appeared to be attained. This 

 difference may be interpreted on the supposition that the complete 

 medium used protected the biochemical mutants better than the morpho- 

 logical ones frorli adverse selection. An examination of the data shows 

 that, in the Neurospora work, the mustard, in the amount used, caused 

 an approximate doubling of the spontaneous-mutation rate (i.e., of that 

 obtained without ultraviolet) and somewhat less than a doubling of that 

 obtained with ultraviolet. It is therefore conceivable that the mustard 

 in both cases acted in essentially the same way, by hindering the destruc- 

 tion or increasing the availability or potency of mutagenic materials which 

 were being formed spontaneously or by ultraviolet, as the case might be. 

 If this is the case, it might further be inferred that the mutagenic mate- 

 rials here are of a similar kind, no matter whether formed spontaneously 

 or by ultraviolet. 



Similarly, Jensen et al. (1952) report that, when Neurospora spores are 

 treated with the mutagen diazomethane and are at the same time irradi- 

 ated with ultraviolet, there is a marked synergism in the effects. Here 

 too the data given, when examined with reference to this point, would 

 suggest that the mutation frequency produced by the combination treat- 

 ment is approximately that to be expected from a dose which was the 

 product rather than the sum of either dose alone (if the probable con- 

 vexity of the dosage curve is taken into account at the same time). 

 With the conditional mutagen formaldehyde also, these investigators 

 report an enhancement of mutagenesis when ultraviolet (or hydrogen 

 peroxide) is applied, but in this case the interaction is of a different kind 

 and can apparently take place outside the cell (see pp. 548-552). 



The more general conclusion to be derived from all these results, 

 namely, that there is in ultraviolet mutagenesis a premutational stage or 



