In the rosy mutant of Drosophila melanogaster, two types of autofluorescent cytoplasmic inclusions are found in the cells of the posterior region of the fatbody at the prepupal stage. Bright yellow autofluorescent granules accumulating within larger inclusions clearly demarcate this area of the fatbody which also contains cobalt blue fluorescent globular material. Such inclusions were not noted in the normal Ore-R strain at this stage nor in the series of mutant strains examined other than the rosy2 and maroon-like mutants. The pattern of biochemical deviation of the latter two mutants is known to be identical to that of the rosy mutant, and a portion of this mutant upset can be ascribed to the absence of xanthine dehydrogenase. These mutants lack the products of enzyme activity, uric acid and isoxanthopterin, and accumulate their precursors, hypoxanthine and 2-amino-4-hydroxypteridine. Chromatographic studies on the fatbody of rosy prepupae have shown that 2-amino-4-hydroxypteridine is limited to the posterior region; this correspondence in location as well as color of fluorescence indicates that the cobalt blue auto fluorescent globules in the fatbody contain 2-amino-4-hydroxypteridine. In the normal strain, isoxanthopterin was identified in the chromatograms of the posterior region of the fatbody, but it was not obtained from the anterior region of the fatbody. On the other hand, xanthine dehydrogenase activity could be demonstrated throughout the fatbody of the normal strain. The restriction of isoxanthopterin to a certain group of fat cells in the wild type strain and its absence from other fat cells can be explained by the differential distribution of its immediate precursor, 2-amino-4-hydroxypteridine, as displayed in the mutant rosy.

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