272 



CHAPTER 31 



You may recall (p. 53) that the dull-red 

 eye color of wild-type Drosophila is due to 

 the presence of both brown and red pigments. 

 There are a large number of allelic and 

 nonallelic point mutants that change eye 

 color. Homozygotes for the mutant brown, 

 bw, which are otherwise genetically wild-type, 

 have brown eyes because they cannot make 

 the red pigment. On the other hand, mutant 

 individuals pure for vermilion, v, or for cinna- 

 bar, en, cannot make the brown pigment, 

 and so, even though otherwise genetically 

 wild-type, have bright red eye color. Of 

 course, flies pure for bw, and either v or en, 

 have white eyes, since they cannot make 

 either the red or the brown pigments. 



A series of investigations was initiated ^ to 

 determine the effect that transplantation of 

 prospective eye tissue has upon the type of 

 eye color it later shows. The larva of Dro- 

 2 By G. W. Beadle and B. Ephrussi. 



FIGURE 31-1. Results of transplanting eye anlage 



between Drosophila larvae. 



sophila contains prospective compound eye tis- 

 sue in the form of anhf^en, or imaginal discs. 

 It became feasible to transplant an eye anlage 

 from one larva into the body cavity of another 

 and have development continue to comple- 

 tion. When the adult emerged from the pupal 

 case, its own eye color and, after being dug 

 out, the eye color of the implant, could be 

 scored. It was found that if an implant was 

 genetically the same as the host, all eyes 

 developed the expected color (Figure 31-1, E). 

 Moreover, the eye anlage of most mutants, 

 when transplanted into wild-type larvae, de- 

 veloped the mutant eye color, even though 

 the host eyes were wild-type. In these cases, 

 therefore, the implant developed autono- 

 mously, and the host provided nothing which 

 modified the development of the mutant 

 phenotype in this respect. 



There were two exceptions, however. If 

 either v or en discs (from homozygotes for 

 V or en) were transplanted into a wild-type 

 host (B into A in the Figure), they developed 

 into eyes with the dull-red color of the wild- 

 type. In these cases, then, the wild-type host 

 supplied something which the transplanted 

 anlage needed to develop brown pigment. 

 It was shown that what the wild-type host 

 contributed was a diffusible substance. This 

 substance itself was not brown pigment, but 

 could be utilized metabolically by the implant 

 to synthesize brown pigment. 



It is a reasonable presumption, therefore, 

 that en and v are defective in the produc- 

 tion of some substance which is present in 

 wild-type, and which is essential for the 

 subsequent formation of brown pigment. 



Three kinds of explanations are possible for 

 the two exceptional transplantation results. 

 First, en and v may be defective in the same 

 way and fail to produce the same chemical 

 precursor of brown pigment. If this is so, 

 then reciprocal transplants of eye anlage 

 between en and v larvae should produce only 

 the mutant (bright red) eye color. But, it is 

 also possible that the en and v mutants are 



