Biochemical Genetics (/) 



273 



defective in different ways. In this event, a 

 second explanation can be based upon the 

 possibihty that the biochemical effects of the 

 two different mutants are produced independ- 

 ently of each other. This can be visualized in 

 the following way. Suppose that brown pig- 

 ment has as two of its precursors a v+ substance 

 produced by the v+ gene and a c/i+ substance 

 produced by the cn+ gene. If these sub- 

 stances are produced independently of each 

 other, then a v/v cn+jcn'^ larva should produce 

 only cn+ substance, and a v+/v+ cn/cn larva 

 only v+ substance. Reciprocal eye anlage 

 transplants between these two larvae should 

 produce wild-type eye color in the implants, 

 if both precursors are diffusible, since which- 

 ever of the two precursors is missing in the 

 implant will be furnished by its host. 



Thirdly and finally, the two precursors of 

 brown pigment may be different but related 

 to, or dependent upon, each other in their 

 production. The amount of relation or de- 

 pendency between the two precursors might 

 be of several kinds, and of almost any degree 

 — ranging from almost complete independ- 

 ence to complete dependency. Dependency 

 would be complete and unambiguous if the 

 formation of one precursor had to precede 

 the formation of the other precursor. Sup- 

 pose, to make this example specific, that the 

 hypothesized cn+ substance is synthesized 

 from the hypothesized v+ substance. This 

 means that v+ substance is a precursor of cn+ 

 substance. In this particular case, what 



should be the result of implanting a v anlage 

 in a en host? The host (v+/v+ cn/cn) manu- 

 factures v+ substance (which the implant, 

 v/v cn^jcn'^ cannot make) ; the implant can 

 convert this into cn+ substance which, in turn, 

 can be converted to brown pigment. This 

 pigment together with red would make the 

 implant eye wild-type. What is the result 

 expected from the reciprocal transplant of 

 a en anlage in a v larva? Since the implanted 

 en disc (v+/v+ cn/cn) lacks the ability to make 

 cn+ substance, and does not receive this from 

 the v host {v/vcn+/cn+), no brown pigment 

 should be formed and the transplant should 

 form a bright red eye color. 



When the reciprocal transplantations actu- 

 ally were made (D into C, and C into D in 

 Figure 31-1) the specific results last hypothe- 

 sized were obtained; that is, a en disc in a v 

 host remains cinnabar, while a v disc in a en 

 host becomes wild-type. Such results also 

 rule out the first two types of explanations 

 presented. What we are presumably dealing 

 with is a chain of chemical reactions (Figure 

 3 1-2 A) involving a minimum of four chemical 

 substances: a precursor (1) which through the 

 action of gene v+ is converted to v+ substance 

 (2) which is converted by gene cn+ into cn+ 

 substance (3), which is in turn converted to 

 brown pigment (4). Note that v blocks the 

 chemical pathway to brown pigment at an 

 earlier stage than does en. The transplanta- 

 tion results further support the concept that 

 the chemical intermediates, formed prior to 



FIGURE 31-2. C/iain ofcfiemical reactions involved in the formation of brown eye 

 pigment in Drosophila. 



A. Precursor -^ v^ Substance -^^ cn"^ Substance 4 Brown 



" Pigment 



Trypto- 

 phan 



Kynurenine 



3-Hydroxy- 

 kynurenine 



Brown 

 Pigment 



