Payne: Variations and Selection 



41 



Summary 



1. Selection was effective in increasing the number of 

 bristles up to a maximum of 15 in the female and 14 in the 

 male and until the mean bristle number was 9.123. 



2. The increase in bristle number was not continuous, but 

 there were three distinct rises, the first from the first to the 

 eleventh generations, the second from the fourteenth to the 

 eighteenth generations, and the third from the twenty-fifth 

 to the twenty-ninth generations. 



3. A back selection line started from the eleventh genera- 

 tion of the selection line and carried for twenty-five genera- 

 tions did not return to the normal. 



4. The females have a higher bristie number than do the 

 males. This difference in bristle number is not due to a dif- 

 ference in size but to the presence of a sex-linked factor for 

 extra bristles. 



5. The linkage tests demonstrate a factor for extra 

 bristles in the X-chromosome, somewhere near the zero end. 



6. When the X-chromosome is eliminated by crossing to 

 ''bar" and the line inbred, extra bristles are still present, 

 demonstrating the presence of one or more factors for extra 

 bristles outside the X-chromosome. Since the bristle number 

 cannot be raised to what it was in the selection line, this 

 experiment also indicates the presence of a factor, in the 

 X-chromosome. 



7. In F2 of the cross of the selection line vsith black, pink, 

 bent, the pink flies have very few extra bristles. This indi- 

 cates the presence of a factor for bristle number in the third 

 chromosome. The same result is obtained by crossing to any 

 other third chromosome mutant. 



8. When the males of the cross of the selection line 

 female with a sepia spineless kidney sooty rough male, are 

 mated back to sepia spineless kidney sooty rough females, 

 the sepia spineless kidney sooty rough males have no extra 

 bristles. These males get their X-chromosomes and both 

 third chromosomes from the sepia spineless kidney sooty 

 rough line, indicating further that there is a factor for extra 

 bristles in the third chromosome. 



9. There are at least two factors for extra bristles on 

 the scutellum. There may be more if the three rises in the 



