Abstract
1. Cytoplasmic pigment granules are found in some of the fat cells of the recessive mutant, rc, of Drosophila melanogaster. These scattered red fat cells are located chiefly in the thorax and head, but a few occur in the abdomen and appendages of the adult. Using genetic methods it had been shown previously that these pigment accumulations are related to the synthesis of the brown eye pigment of this insect (Jones and Lewis, 1957).2. The pigmentation in the rc fat cells is suppressed when the rc gene is combined with the recessive factor, tuw. This combination, however, in no way alters the expression of the characteristic pattern of tuW as revealed by the presence of melanotic tumors in the caudal fat masses of the homozygous tuwrc flies. After a period of larval starvation, the tuwrc flies develop both the red-pigmented fat cells and melanotic tumors. The time of appearance of the rc pigment has been shifted under these nutritional and genetic conditions. The cytoplasmic pigment granules appear in the cells originating from the anteriormost section of the larval fatbody which is closely associated with the anterior Malpighian tubules. During the reorganization accompanying metamorphosis from the larval to the adult stage, these cells are redistributed mostly to the thoracic and cephalic regions while a few are found in the abdomen and appendages. An explanation is thus provided for the cytodifferentiation of pigmented and nonpigmented fat cells found side by side in the adult fly.3. The nature of the pigment granules has been examined in in vitro preparations at each of these periods of development, and of particular interest is the internal threadlike structure of these cytoplasmic inclusions during the early stages of pigment formation.
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