410 RADIATION BIOLOGY 



16. DIFFERENCES BETWEEN THE PRODUCTION OF MUTATIONS 



IN DIFFERENT SPECIES 



As will be discussed in Chap. 8, some physicoohemical conditions accom- 

 panying irradiation have a pronounced influence on the frequency of 

 production by radiation of both gene mutations and structural changes. 

 In view of this and of the foregoing results showing the influence of 

 cellular stage, it is to be expected that some genetic differences also would 

 influence these frequencies. Dubovsky (1935) reported finding statis- 

 tically significant, although not very large, differences in the frequency of 

 X-ray-induced lethals on irradiation of spermatozoa of different stocks of 

 Drosophila. However, these results do not seem secure, when account is 

 taken of various possible sources of error, such as the possible presence in 

 some stocks of detrimental genes, which by a synergistic effect cause 

 other detrimental genes, induced by the irradiation, to be classified as 

 lethals. Similarly, much more striking differences in the induced muta- 

 tion frequency observed in different stocks of bacteria may have had their 

 origin in differences in the number of chromosome sets present ("ploidy ") 

 which affected the detection of the mutants. On the other hand Kossikov 

 (1937) found sensibly the same X-ray-induced frequency of sex-linked 

 lethals in Drosophila simulans as in D. melanogaster although here the 

 genetic difference, being of species rank, must have been far greater than 

 in any of the above cases. Timofeeff-Ressovsky (1931a) found only a 

 slight difference in this respect between the much more widely separated 

 species D. melanogaster and D. funebris. It therefore seems Hkely that 

 large differences in induced frequency are comparatively rarely caused by 

 such genetic differences as commonly exist between individuals of the 

 same population. 



There is much more reason for supposing marked differences to exist in 

 the induced mutation frequency between organisms of widely differing 

 systematic groups. The experimental establishment of this point meets 

 with very serious difficulties, however. This is in part because compari- 

 sons of the over-all frequencies of any given phenotypic class of mutations 

 are of very uncertain meaning. The differences in developmental and 

 physiological processes between two organisms of very diverse kinds are 

 so great that a given phenotypic category for one type of organism cannot 

 be taken as being in genetic respects equivalent to any apparently cor- 

 responding category for the other type. Again, it has not as yet been 

 possible, in organisms other than Drosophila, to obtain even an approxi- 

 mate estimate of the total number of genes, still less of the number of 

 genes underlying any given "over-all" phenotypic category (such as 

 lethals), and without such knowledge the significance of any comparative 

 results on over-all mutation rates must remain unclear. 



Nevertheless, comparisons of the work on widely different organisms 



