EMBRYONIC ORIGIN OF HEMOCYTES AND THEIR RELATIONSHIP TO CELL-DEATH INDROSOPHILA

We have studied the embryonic development of Drosophila hemocytes and their conversion into macrophages. Hemocytes derive exclusively from t he mesoderm of the head and disperse along several invariant migratory paths throughout the embryo. The origin of hemocytes from the head me soderm is further supported by the finding that in Bicaudal D, a mutat ion that lacks all head structures, and in twist snail double mutants, where no mesoderm develops, hemocytes do not form. All embryonic hemo cytes behave like a homogenous population with respect to their potent ial for phagocytosis. Thus, in the wild type, about 80-90% of hemocyte s become macrophages during late development. In mutations with an inc reased amount of cell death (knirps; stardust; fork head), this figure approaches 100%. In contrast, in these mutations, the absolute number of hemocytes does not differ from that in wild type, indicating that cell death does not 'induce' the formation of hemocytes. Finally, we s how that, in the Drosophila embryo, apoptosis can occur independently of macrophages, since mutations lacking macrophages (Bicaudal D; twist snail double mutants; torso(4021)) show abundant cell death.