674 



RADIATION BIOLOGY 



in frequency of rearrangements or in total number of induced breaks 

 when flies were exposed to temperatures of 8, 18, or 28°C before, during, 

 or after exposure to X rays. More recent studies of sex-linked lethals by 

 King (1947) and of translocations by Kanellis (1946) and Baker (1949) 

 indicate that irradiation at a temperature in the 0-to-4°C range gives a 

 higher frequency of aberrations than irradiation at 20 to 32°C. As an 

 illustration, Fig. 9-15a and b shows the frequencies of exchanges among 

 the second, third, fourth, fifth, and Y chromosomes of D. virilis deter- 

 mined at two temperature levels by Baker. The data indicate that not 

 only the frequency of rearrangements, but also the slopes of the dose- 



1000 



4000 



1000 



2000 

 DOSAGE, 



3000 4000 



2000 3000 

 ( a ) DOSAGE, r ((,) 



Fig. 9-15. Dose-frequency relations for viable types of chromosome rearrangements 

 induced by irradiation of spermatozoa of Drosophila virilis. (a) Number of exchanges 

 per tested sperm, (b) Minimum number of breaks per tested sperm. The method 

 of determining the shapes of the theoretical curves presented in these figures is 

 described in detail in the original article. {From Baker, 1949.) 



frequency curves are influenced by the temperature at the time of treat- 

 ment. Either more breaks are produced when irradiation is carried on at 

 a low temperature (as Baker concluded), or the properties of breakage 

 ends of chromosomes are modified so as to increase the number of viable 

 types of exchange (as Kanellis concluded). Evidence that such modi- 

 fication can occur has accumulated in recent years, and will be considered 

 in the next section. 



3-le. Qualitative Differences among Breaks. There is considerable evi- 

 dence that the capacity of breakage ends to participate in recombination 

 varies greatly in different organisms and types of cells. In some cases, 

 as in the irradiated eggs of Ascaris, the ends of chromosomes bordering on 

 induced breaks appear to have no capacity for uniting with one another, 

 for chromosomal rearrangements do not occur (Bauer and LeCalvez, 

 1944; cf. the similar studies of Hughes-Schrader and Ris, 1941, and Ris, 

 1942, on Hemiptera). That differences may exist in the behavior of 

 breakage ends in different tissues of the same organism was clearly shown 



