46 Linkage Group V. 



VI. LINKAGE GROUP V. 



Group V includes five characters, thus equaling the number of 

 the workable characters in group III, the largest of the other auto- 

 somal groups (leaving steel out of account in the latter). It also 

 presents the same difficulties as does group III in the way of irregular 

 characters and characters that can not be used satisfactorily in 

 combination. 



DESCRIPTION, ORIGIN, AND COMPARISON OF CHARACTERS IN 



GROUP V. 

 Fused (fu). (Plate 4, Figure 4.) 



Description. — Fused is ordinarily used as a recessive, but may often be distin- 

 guished in heterozygous flies. In homozygous flies the third and fourth longitudinal 

 veins are fused or lie very close together from the base to the anterior cross-vein; 

 the -nings are held at an angle from the body; the ocelli and surrounding bristles on 

 top of the head are entirely gone; and usually some or all of the bristles on the 

 scutellum are lacking. In addition, the mutant is considerably weaker than the 

 normal and usually gives a deficient ratio in crosses. In heterozygous flies the ■s\ings 

 appear to be normal, but some or all the bristles on top of the head, and sometimes 

 the ocelli also, may be lacking. The effect of fused is exaggerated by net. Flies 

 heterozygous for fused and net, on the average, lack more bristles on the head than 

 those heterozygous for fused without net. The heads in this case are almost com- 

 pletely bald. 



Origin. — (V 778.) Fused, like many of the other non-sex-linked characters, was 

 first observed in a mass culture. 



Comparison. — Fused bears a slight resemblance to the sex-linked "fused" of D. 

 melanogaster, but not enough to indicate any probability of homology between the 

 two. 



Interrupted (i). (Plate 4, Figures 6 to 8.) 



Description. — Interrupted is a variable character grading into normal and fre- 

 quently not distinguishable. In extreme cases, the posterior cross-vein is broken or 

 almost gone (fig. 6, plate 4). The break may be near the middle, but is more often 

 near the fourth vein, or at the junction with this vein. It seems probable that inter- 

 rupted is exaggerated by approximated, for it seems to be distinguishable more often 

 in the presence of approximated than in its absence. It is difficult to make certain 

 of this without extensive tests, however, for environmental conditions appear to be 

 an important factor, and these may have been more favorable in the former case. 

 Large, well-fed flies tend to show the interrupted character more often than small, 

 under-fed ones. For instance, in one count of 130 flies (M 1) from stock every fly 

 was interrupted, while in others, especially old bottles, many appear normal. 



Origin. — (E 15.) From a pair mating in which both parents were heterozygous 

 for hunch and fused, 178 offspring were obtained, of which 28 lacked posterior cross- 

 veins. It subsequently developed that two mutant characters, interrupted and ap- 

 proximated, were involved here, and since their distinguishing features were not 

 known at the time, the records do not tell the exact number of each in this culture. 

 Since they are both recessives, however, it is apparent that each parent must have 

 been heterozygous for them. Subsequent tests showed that the fused stock carried 

 approximated (concealed by the fused character) ; hence the mutation responsible for 

 this character probably occurred eariUer than that for interrupted. 



Comparison. — Characters similar to interrupted are known in D. melanogaster and 

 in D. willistoni. We have tested two such characters in the former species, but 

 neither resembles interrupted sufficiently to be considered as a homologue. 



