Hemocytes are essential for wing maturation in Drosophila melanogaster

Newly eclosed flies have wings that are highly folded and compact. Within a n hour, each wing has expanded, the dorsal and ventral cuticular surfaces b onding to one another to form the mature wing. To initiate a dissection of this process, we present studies of two mutant phenotypes. First, the baton e mutant blocks wing expansion, a behavior that is shown to have a mutant f ocus anterior to the wing in the embryonic fate map. Second, ectopic expres sion of protein kinase A catalytic subunit (PKAc) using certain GAL4 enhanc er detector strains mimics the batone wing phenotype and also induces melan otic "tumors." Surprisingly, these GAL4 strains express GAL4 in cells, whic h seem to be hemocytes, found between the dorsal and ventral surfaces of ne wly opened wings. Ectopic expression of Ricin A in these cells reduces thei r 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 expressio n of PKAc in the presence of the batone mutant causes dominant lethality, s uggesting a functional relationship. We propose that batone is required for the release of a hormone necessary for wing expansion and tissue remodelin g by hemocytes in the wing.