566 RADIATION BIOLOGY 



chromosome changes by high-energy radiation when oxygen is present, 

 probably through an effect on breakage, it seems very Hkely that the 

 temperature influence on chromosome aberrations in Drosoyhila is also, 

 mainly at least, an effect on the breakage process exerted by the greater 

 amount of oxygen present at lower temperatures. However, it is sug- 

 gested by the findings reported by Haas et al. (1952) on Drosophila trans- 

 locations, findings which are rather divergent from those of Baker and 

 Sgourakis, that temperature may also have an effect of its own on the 

 result. 



In this connection, the findings of D. Lewis (1949) on the influence of 

 temperature on the frequency of X-ray-induced mutations of the self- 

 sterility gene, S, in Oenothera onagrensis are of much interest. Lewis 

 reports a marked increase of frequency of these mutations (which are 

 almost certainly gene mutations) with rise in temperature, when the 

 irradiation and temperature are applied during the early prophase of the 

 first meiotic division. However, as the metaphase is approached, the 

 induced-mutation frequency rises to about the same extent (two to three 

 times) even without raising the temperature, and a rise in temperature 

 at the period (metaphase) of the maximum induced rate fails to raise the 

 frecjuency further. 



It may be recalled that several investigators (Sax and Enzmann, 1939; 

 Faberge, 1940b; Rick, 1940; Sax and Brumfield, 1943; Catcheside, Lea, 

 and Thoday, 1946) had obtained more abundant chromosome aberrations 

 in Tradescantia when lower temperatures were used during and/or for an 

 hour after irradiation (although, according to Faberge, 1947, the effect is 

 only produced by temperatures applied during irradiation), and they had 

 interpreted this result as being caused by a promoting effect of higher 

 temperature on the joining (and more especially on the restitutional join- 

 ing) of broken ends. There must undoubtedly be more opportunity for 

 exchange unions when irradiation is carried out at a low temperature, 

 in cases in which joining can occur to an appreciable extent during expo- 

 sure, provided that the exposure occupies an appreciable portion of the 

 period during which unions occur; for the delay of union caused by the 

 low temperature will make more breaks available simultaneously for 

 exchange union. It has, however, become evident that such effects do 

 not constitute the whole story. For one thing, Darlington and La Cour 

 (1945) brought out the point that in some types of work, temperature, by 

 influencing the rate of development of successive phases of the mitotic 

 cycle, will occasion differences in the number of aberrations scored at any 

 fixed time after exposure, even in cases where the number actually pro- 

 duced is the same. Illustrating a different point was the fact that, even 

 in the early experiments of Faberge (1940b), the temperature effect was 

 found to be about as strong for fractionated as for continuous doses. 

 This might have been interpreted to mean that the frequency of break- 



