Factors bearing on the non-malignancy of tumors in Drosophila.

Several investigators have reported the occur rence of tumors in Drosophila (Burdette in D. melanogaster [4]; Makino in D. virUis [9]; Milani in D. simulans [10]; and BrnciÃ3and Koref in sev eral other species, [3]). In Drosophila melanogaster more than 24 different tumor strains are known, characterized by growths that first make their ap pearance during the larval stages, soon becoming encapsulated in a sheath of melanin. During pupa tion the tumor tissues undergo histolysis along with the rest of the larval tissues, but the highly inert melanin persists as one or more black masses in the abdomen or thorax and seldom interferes with the activities of the fly. Some of the tumors are the result of multiple genetic factors, while others are due mainly to the action of one gene (4). In the majority of the tumor strains the penetrance is well below 100 per cent, and environmen tal factors tend to influence the occurrence of the melanotic growths considerably (4, 14). These growths are benign in the sense that they do not cause death. Several workers, notably Wilson (15) and Russell (13), have suggested that the behavior of the melanotic tumors may be the result of the holometabolous nature of the insect; that is, the tumor regresses at the time of metamorphosis in the same manner as those normal tissues which are replaced by adult structures. It is, therefore, conceivable that, even if tissue corresponding to a vertebrate malignant tumor were present, it would have had its growth stopped along with the growth of most other tissues. As the tumors grow only dur ing the larval stages, prolonging the period during which the fly is a larva might throw further light on the nature of Drosophila tumors. It might also be mentioned that the pre-imaginal individuals of some lethal-bearing strains of Drosophila manifest melanistic inclusions during their moribund stages, but as yet no conclusive