Effect of tryptophan metabolites on fluorescent granules in the Malpighian tubules of eye color mutants of Drosophila melanogaster.

Fluorescent granules that are stained with Nile blue sulfate are present in larval Malpighian tubules in the wild type strain of Drosophila melanogaster, Oregon-R. These granules emit a weak blue fluorescence and most of them are about 2 microns or more in diameter. The ommochrome precursor 3-hydroxy-kynurenine (3-HK) is actively transferred into the tubules of Oregon-R. Changes in the fluorescent granules in the Malpighian tubules on administration of ommochrome precursors were investigated in eye color mutants of Drosophila. The fluorescent granules in the tubules of the nonautonomous mutants v;bw and cn bw emit a strong blue fluorescence and most of them are about 1 micron or less in diameter. When v;bw and cn bw larvae were cultured on medium supplemented with kynurenine or 3-HK, respectively, the fluorescence intensity of their granules decreased, and their size increased. These additions resulted in almost equal accumulation of 3-HK to that in Oregon-R. On the other hand, no 3-HK accumulated in the tubules of larvae of the autonomous mutants bw;st, ltd bw and w, which lack the fluorescent granules. These findings indicate that the fluorescent granules are an important intracellular site for uptake or storage of ommochrome precursors in larval Malpighian tubules of Drosophila.