308 Gene Action 



possessing all the other wild-type alleles. If the brown pigment 

 is absent or reduced in amount, the eye is redder than in the 

 wild type. In the cardinal {cdcd) fly, there is less brown pig- 

 ment than in the wild type and consequently the eye color is 

 closer to the red color. In the vermilion (vv) and cinnabar 

 {cncn) types, there is no brown pigment and the eye is bright 

 red. Definite hormones have been discovered which are neces- 

 sary for the formation of the brown pigment. In the vermilion 

 flies, the r^^ hormone is absent, and therefore the brown pig- 

 ment is not formed. In the wild type, this hormone is present. 

 The v^ hormone is formed in the Malpighian tubules, in fat 

 bodies, and in the eye tissue. In the cinnabar fly, the v+ hor- 

 mone is present, but another hormone, the cn+, is absent. This 

 hormone is also necessary for the formation of brown pigment, 

 and is formed in the wild-type fly in the Malpighian tubules 

 and in the eye but not in the fat bodies. The cn+ hormone is 

 formed from the v^ hormone. 



Chemical studies have carried even further our knowledge of 

 pigment formation. The ultimate basis of the brown pigment 

 is tryptophane. In the wild-type fly, the v+ gene produces an 

 enzyme which oxidizes tryptophane to another substance, ky- 

 nurenin, which seems to be the v+ hormone. The chemical rela- 

 tionship of these two substances is: 



H H H H 



-C— C— COOH 

 H NH2 



C=C— C— COOH 



COOH NH2 



NH2 



+ 



Tryptophane Kynurenin or a hormone 



Of course, if the v+ gene is not present, f + hormone is not 

 formed and none of the subsequent steps in the formation of the 

 brown pigment can occur. Actually, even in the vermilion fly, 

 a small amount of kynurenin is formed. Most of it is converted 

 into the inactive kynurenic acid, but a very small amount is con- 

 verted into brown pigment. 



In the wild-type fly, some of this kynurenin is probably trans- 

 formed into the inactive kynurenic acid, but much of it is trans- 

 formed into cn+ hormone by an enzyme secreted by the cn + 



