30 



Indiana University Studies 



It is interesting tho that the number of flies with the reduced 

 bristle number is greater than in stock cultures. I hope to 

 be able to repeat such a test. 



In the sixth generation of the line selected for extra 

 bristles (parent number 113) appeared a male with only one 

 bristle on the scutellum. Males with two and three bristles 

 on the scutellum had appeared before in other strains, and 

 when paired with normal females the minus condition showed 

 no indication of being inherited. However, the male with 

 only one bristle was mated to a normal female. In all 

 flies (328) were normal. Five pairs of the F^'s were mated 

 and gave in Fo 1,634 males and 1,594 females. All the 

 females were normal. Of the males, 834 were normal, but 

 804 or approximately one-half of them showed a reduced 

 bristle number. There was no definite number of bristles 

 among these ''reduced" flies. Two hundred fifty-five of them 

 had no bristles, 296 one bristle, 219 two bristles, and 34 three 

 bristles. From this and further tests it was evident that this 

 reduced condition was a mutation and behaved as a sex-linked 

 Mendelian character. The character was transferred to the 

 female line by mating ''reduced" males to Fo females. The 

 line was then made pure by inbreeding reduced males and 

 females. In this reduced stock, selection has been practiced 

 in two directions, one to produce a strain with no bristles and 

 the other to raise the strain to normal again. These two 

 lines have been subjected to selection for more than 21 genera- 

 tions and the experiment is still in progress. In the minus 

 line the number of bristles has been gradually reduced until 

 in the seventeenth generation 99.52 per cent of the flies had 

 no bristles, and the flies which did have bristles seldom had 

 more than one. In the plus line the number of bristles has 

 been gradually increased until in the twenty-first generation 

 47.77 per cent of the flies had the normal number, four. Most 

 of the other flies had two or three bristles and in the later 

 generations of selection a fly with no bristles rarely occurred. 

 The normal flies, however, are not normal genetically, for 

 when mated to wild stock they throw the characteristic sex- 

 linked ratio. Occasionally a fly with five bristles appears in 

 this line. The two lines have thus been gradually pulled apart 

 until they are no longer the same, and sufficient tests have 

 been made to show that they are not the same genetically, 

 but just what this difference is I am not prepared as yet to 



