Yet in Sturtevant's gynandromorphs in which only a small part of the body 

 including eye tissue was vermilion, the appearance of that tissue was usually 

 not vermilion but wild type — as though some substance had diffused from 

 wild-type tissue to the eye and caused it to become normally pigmented. 



It was on the basis of this observation that Ephrussi and I transplanted 

 vermilion eyes into wild type larvae. The result was as expected — the trans- 

 planted eyes were indeed wild type. 



At that time there were some 26 separate eye-color genes known in Droso- 

 phila. We obtained stocks of all of them and made a series of transplants 

 of mutant eyes into wild-type hosts. We found only one other clear-cut non- 

 autonomous eye character. This was cinnabar, a bright red eye color, like ver- 

 milion but differentiated by a second chromosome recessive gene. We had a 

 third less clear case, claret, but this was never entirely satisfactory from an 

 experimental point of view because it was difficult to distinguish claret from 

 wild-type eyes in transplants. 



The vermilion and cinnabar characters are alike in appearance; both lack 

 the brown pigment of the wild-type fly but retain the bright red component. 

 Were the diffusible substances that caused them to develop brown pigment 

 when grown in wild-type hosts the same or different? If the same, reciprocal 

 transplants between the two mutants should give mutant transplanted eyes 

 in both cases. If two separate and independent substances were involved, such 

 reciprocal transplants should give wild-type transplanted eyes in both in- 

 stances. 



We made the experiment and were much puzzled that neither of these 

 results was obtained. A cinnabar eye in a vermilion host remained cinnabar, 

 but a vermilion eye in a cinnabar host became wild type. 



To explain this result we formulated the hypothesis that there must be two 

 diffusible substances involved, one formed from the other according to the 

 scheme: -* Precursor -*■ v + substance -*■ cn + substance -* Pigment . . . where 

 v + substance is a diffusible material capable of making a vermilion eye become 

 wild type and cn + substance is capable of doing the same to a cinnabar eye (9). 



The vermilion (v) mutant gene blocks the first reaction and the cinnabar 

 (en) mutant gene interrupts the second. A vermilion eye in a cinnabar host 

 makes pigment because it can, in its own tissues, convert the v + substance into 

 cn + substance and pigment. In it, the second reaction is not blocked. 



This scheme involves the following concepts: 



a. A sequence of two gene-regulated chemical reactions, one gene identified 

 with each. 



s-79 



