412 RADIATION BIOLOGY 



fecundity, it is necessary to use a much lower dose than that commonly 

 employed in Drosophila. Yet even at this low dose the frequency of 

 minute deletions,- hke that of gross structural changes, is probably a good 

 deal highe.r than for the same dose when applied to Drosophila. These 

 del'etions then would tend, by their numbers, to obscure from view the 

 occurrence of gene mutations. Even when all these allowances are made, 

 however, it still seems probable that gene mutations in maize, if produced 

 at all by ionizing radiation, arise at a considerably lower frequency, for a 

 given dose, than is the case in Drosophila. Nevertheless, in view of 

 facts given in Chap. 8, there is theoretical ground for inferring that some 

 gene mutations must be produced by this means even in maize. 



' 17. AGENTS OTHER THAN RADIATION 

 WHICH SEPARATELY AFFECT MUTATION FREQUENCY 



In order that the mutagenic influence of radiation may be viewed in 

 better perspective, consideration mil be given in this section to the other 

 agents which, acting without significant amounts of radiation, affect the 

 occurrence of mutations. In this connection, it will be taken for granted 

 that natural (earth and cosmic) radiation is ordinarily far too meager to 

 play an appreciable role in the mutation process, and that therefore, in 

 the absence of artificially applied radiation, these other influences are not 

 working through any interaction with radiation effects. A review of the 

 evidence for this will be postponed until the more detailed treatment of 

 radiation mutagenesis presented in Chap. 8. For that chapter also will 

 be reserved a discussion of the influence of other factors when they do 

 work in conjunction with radiation. 



The first condition of any kind found to affect mutation frequency was 

 temperature (Muller and Altenburg, 1919; MuUer, 1928c). Drosophila 

 raised and bred at 27°C showed a frequency of origination of mutations 

 (sex-linked and other lethals) per generation two to three times as high as 

 did those at 17°C, despite the fact that in some of these experiments each 

 generation passed at the lower temperature lasted two to three times as 

 long. Thus, as worked out more exactly by Timofeef-Ressovsky (1935b) , 

 the temperature coefficient, Qio, of the mutation frequency, was 5 to 6, a 

 figure markedly higher than the Qio of 2 to 3 which is characteristic of the 

 processes of development and of metabolism, and of the reactions ordi- 

 narily dealt with by chemists. 



This result could simply mean, as pointed out by Delbruck (1935), on 

 entering this field from physics, that the chemical change involved in 

 mutation has an especially high energy threshold, but takes place when- 

 ever this threshold is passed. If it is assumed that the frequency of 

 mutation at a given temperature depends entirely on the frequency with 

 which this threshold amount of kinetic energy is attained at that tem- 



