THE GENETIC BASIS OF X-RAY INDUCED SOMATIC DAMAGE 



It should be noted that Drosophila is ideally suited for this work because the 

 appropriate stocks can be readily synthesized and because exposure of the 

 larval stages, having the various developmental and physiological processes 

 associated with growth and differentiation crowded into a relatively short 

 period of time, tends to accelerate the appearance of induced damage. In a 

 sense then, death resulting shortly before or after hatching from the pupa 

 case which takes place 4 to 5 days following irradiation, represents a shorten- 

 ing of the life-span comparable to that observed in longer-lived higher forms. 



MATERIALS AND METHODS 



For all the experiments to be reported hybrid third instar Drosophila melano- 

 gaster larvae from crosses of two unrelated stocks were used. This avoided 

 the use of individuals already homozygous for deleterious genes. The treated 

 individuals were exposed to 1280 r (135 kV; 20 mA; 1 mm Al filtration; 

 160 r/min) when their outer surfaces were fairly dry. Unirradiated but 

 similarly handled larvae served as controls. By the use of suitably marked 

 stocks which facilitated the separation of the sexes it was possible to segregate 

 large numbers of male and female larvae. The males or females to be com- 

 pared resembled each other genotypically and phenotypically but differed 

 in chromosomal morphology. The following is the general type of scheme 

 which was used in order to obtain easily recognizable individuals of both 

 sexes. The particular scheme shown below was employed to obtain ring- 

 shaped chromosome- and rod-shaped chromosome-bearing individuals. 



cJ(? 



$ $ (virgins with 

 attached 

 X-chromosomes) 



(brown mouth-parts) 

 or 



sc«.Y(y+) 



"7^ 



66 



(black mouth-parts) 



(brown mouth-parts) 



sc».Y(y + ) 



66 



(black mouth-parts) 



?? 



X 



or 



c2 



(brown mouth-partsj 



-^ 



66 



sc^ .\\y' ) 

 (black mouth-parts) 



(The inheritance of the markers, y is. y + , which produces a difference in the colour of the larval 

 mouth-parts allows for the rapid recognition of the sexes of immature individuals.) 



269 



