Hemocytes are essential for wing maturation in Drosophila melanogaster

Newly eclosed flies have wings that are highly folded and compact. Within an hour, each wing has expanded, the dorsal and ventral cuticular surfaces bonding to one another to form the mature wing. To initiate a dissection of this process, we present studies of two mutant phenotypes. First, the batone mutant blocks wing expansion, a behavior that is shown to have a mutant focus anterior to the wing in the embryonic fate map. Second, ectopic expression of protein kinase A catalytic subunit (PKAc) using certain GAL4 enhancer detector strains mimics the batone wing phenotype and also induces melanotic "tumors." Surprisingly, these GAL4 strains express GAL4 in cells, which seem to be hemocytes, found between the dorsal and ventral surfaces of newly opened wings. Ectopic expression of Ricin A in these cells reduces their number and prevents bonding of the wing surfaces without preventing wing expansion. We propose that hemocytes are present in the wing to phagocytose apoptotic epithelial cells and to synthesize an extracellular matrix that bonds the two wing surfaces together. Hemocytes are known to form melanotic tumors either as part of an innate immune response or under other abnormal conditions, including evidently ectopic PKAc expression. Ectopic expression of PKAc in the presence of the batone mutant causes dominant lethality, suggesting a functional relationship. We propose that batone is required for the release of a hormone necessary for wing expansion and tissue remodeling by hemocytes in the wing.