MANNER OF PRODUCTION OF MUTATIONS 565 



16. INFLUENCE OF TEMPERATURE, INFRARED, 

 AND OTHER PHYSICAL FACTORS ON HIGH-ENERGY MUTAGENESIS 



Early experiments of Stadler (1928c) on dry barley seeds showed 

 no significant influence of temperature on the induced-mutation rate. 

 Experiments of Muller (1930) on sex-linked lethals induced in Drosophila 

 spermatozoa had indicated a somewhat higher frequency when the irradi- 

 ation was carried out at a cold (5°C) than at a warm (37°C) temperature, 

 and a similar more marked efTect in the same direction (1930, unpub- 

 lished data) was obtained for chromosome aberrations. However, it was 

 thought that these differences might have been caused by the faulty tech- 

 niciue of having used a copper container, to the scattered secondary radi- 

 ation of which the flies at the cold temperature might have been more 

 exposed because of their lying immobilized directly upon it when at this 

 temperature. Subsequent tests of the same type on Drosophila lethals 

 by Medvedev (1935b, 1938) and on translocations by Papalashwili 

 (1935) again gave higher frequencies for the cold series, for both types of 

 changes, but still were not clearly free of technical sources of error. For 

 example, in Medvedev's work one series had been warmed with a glass 

 coil containing air while the other had been cooled with a coil containing 

 water, which gives more scattered radiation. Certain later tests on 

 Drosophila lethals by Timofeeff-Ressovsky (Timofeeff-Ressovsky, 1937a; 

 Timofeeff-Ressovsky and Zimmer, 1939) and on Drosophila translocations 

 by Makhijani (see Muller, 1940, 1941a, b; Makhijani, 1945) gave results 

 that appeared negative. However, definitely positive results showing a 

 considerably higher frequency of translocations in the cold series were 

 obtained by Mickey (1939) and by Kanellis (1946). Moreover, E. D. 

 King (1947), Baker and Sgourakis (1949, 1950), and Novitski (1949) have 

 obtained the same type of effect for sex-linked lethals. Thus there can 

 no longer be any doubt about the enhancing effect of cold on the induc- 

 tion of both chromosome changes and point mutations in Drosophila 

 spermatozoa. 



That the influence on sex-linked lethals was mainly exerted on gene 

 mutations is shown by the finding in the work of Baker and Sgourakis 

 that the effect is about as strong with a dose of 1000 r as at higher doses, 

 even though far fewer of the lethals can have been associated with chro- 

 mosome changes at 1000 r. Their work is also more analytical than that 

 of the other investigators in that it shows the promoting effect of cold on 

 lethal frequency to be produced only when oxygen is present. This indi- 

 cates that in their experiments the cold effect results mainly from the fact 

 that more oxygen is dissolved in the tissues at the lower temperature. 

 This effect is probably intensified by the lower rate of withdrawal of 

 oxygen by metabolic processes at lower temperatures. In view of the 

 findings in plant material, already cited, of an increased production of 



