590 RADIATION BIOLOGY 



distilled water) was used, there being always about the same amoiuit of 

 increase in the mutation rate over the noncentrifuged controls. 



20. QUESTIONS CONCERNING SELECTIVITY OF RADIATION MUTAGENESIS 



Despite the general similarity between spontaneous and radiation- 

 induced mutations in much material, as well as between mutations pro- 

 duced by different types of radiation or by radiation applied under differ- 

 ent conditions, it has not yet been possible, in any material, to obtain 

 quantitative evidence that the locus-by-locus frequencies of mutations, 

 relative to one another, or the relative frequencies of alleles at the same 

 locus — to include both points, the mutational "spectra" — are actually 

 the same in these cases. In fact, if both chromosome changes and point 

 mutations are included in the reckoning, the spectra are certainly not the 

 same since these two general categories are known not always to run 

 parallel. Since in some material both are capable of giving hereditary 

 changes having visible expressions, but expressions which differ to some 

 extent, this at least would cause differences in the spectra of visible muta- 

 tions obtained under different conditions. A more interesting question, 

 however, is whether or not the point mutations themselves, when arising 

 from different primary causes, show differences in their relative 

 frec^uencies. 



Gustafsson, in his work on mutations produced by exposing barley 

 seeds to X rays, brought forward evidence in 1940 and thereafter 

 (Gustafsson, 1946, 1947; D'Amato and Gustafsson, 1948) that a certain 

 rare mutant, alboxantha, which had arisen several times after irradiation 

 of wet seeds, had never arisen after irradiation of dry seeds. According 

 to Haldane, as cited by Gustafsson, the difference was statistically sig- 

 nificant. On the other hand, tigrina, also rare, seemed to arise somewhat 

 oftener when the irradiated seeds were dry. D'Amato and Gustafsson 

 (1948) and Gustafsson and Nybom (1948) further showed that, when 

 colchicine-treated seeds were irradiated, there was a distinct increase in 

 the relative frequency of xantha, alboviridis, and a number of other 

 rare types, an effect not evident under other special conditions of 

 irradiation except possibly when potassium cyanide had been applied. 

 Although it might be speculated that these types were connected with 

 chromosome changes that were more likely to be produced in chromo- 

 somes of the condensed type favored by colchicine, there is no proof of 

 this. Gustafsson did show, however, that some of the kinds of visible 

 mutants observed by him, notably xantha and viridis, were present 

 (heterozygously) only in those first-generation plants (i.e., those grown 

 from irradiated seeds) which showed some infertility and hence may 

 have carried chromosome changes, whereas the most frequent kind of 

 mutant, albina, was evidently a point mutation, uncorrected with 



