gaul: induced mutants in seed-propagated species 



221 



mutagenic treatment of cross-pollinating populations will result 

 in additional and possibly new genetic variability. 



A conclusive experiment of recurrent irradiation with success- 

 ful selection for small mutations was reported in Drosophila mel- 

 anogaster (11, 97). This may serve here as a model for plant popu- 

 lations. The subject of selection was a quantitative character, the 

 sternopleural bristle number of the flies. In this experiment, two 

 strains were treated with X-rays at every second generation. Two 

 further strains, serving as controls, were nonirradiated but sub- 

 jected to the same selection procedure as the irradiated lines. As 

 illustrated in Figure 2, the two irradiated strains (Ap and Bp) 

 responded strikingly to the selection. The hair number could be 



i s 



CYCLES of SELECTION 



Figure 2. — Progress of artificial selection for high number of sterno- 

 pleural hairs of Drosophila melanogaster in two X-ray-treated lines 

 (Ap and Bp) and in two nontreated lines (Cp and Dp). From Buzzati- 

 Traverso and Scossiroli (11). 



