Studies of Eye Pigments of Drosophila. I. Methods of Extraction and Quantitative Estimation of the Pigment Components.

ICROSCOPIC observation, as well as solubility tests and studies of the M effects of gene substitutions, have shown that the normal, wild type eye color of Drosophila melafiogaster is due to the presence of two pigments, red and brown. These two pigments are the end products of two largely independent chains of reactions. Recent work on the genetic control of eye color in Drosophila was almost entirely confined to the reaction chain leading to the formation of brown pigment. The hormone-like diffusible substances derived from tryptophane and representing intermediate links of this chain were the most intensively studied phase of this process. The study of the reaction chain resulting in the formation of red pigment was on the contrary very much neglected and so was the study of the end products of both reaction chainsnamely, the red and brown pigments themselves. Both technical and theoretical factors were responsible for this uneven advance. Technically, the diffusible nature of the hormone-like substances makes them particularly accessible for a very direct experimental attack. The intermediate products involved in the formation of red pigment offer no such technical advantages. The origin of the red pigment, in so far as we know, is entirely intracellular and local. The study of the pigments themselves has encountered technical obstacles, chiefly in finding suitable solvents, which have greatly retarded the understanding of their chemical nature. Only lately a most promising study by BECKER (1939,1942) somewhat clarified the chemical nature of the pigments, but this lead was broken by BECKER’S death on the Russian front. Two theoretical considerations may be added to the above technical ones. In the early phases of the work under discussion it was plausible to assume a rather direct relationship between the diffusible substances and the genes controlling their production. It was a reasonable hope that, starting from the tangible diffusible substances, it would be easier to trace back to the genes the whole chain of intermediate reactions, connecting the genes with the characters they control. This possibility had a special appeal to certain investigators, while to others the diffusible substances were of particular interest as means for the discovery of spatial and temporal correlations in development. Whatever the reason in each particular case may have been, the net result is that the chain of reactions leading to the formation of brown pigment is now rather well defined, while that leading to the formation of red is almost entirely un-