Ultrabithorax and the control of cell morphology in Drosophila halteres

The Drosophila haltere is a much reduced and specialised hind wing, which f unctions as a balance organ. Ultrabithorax (Ubx) is the sole Hox gene respo nsible for the differential development of the fore-wing and haltere in Dro sophila. Previous work on the downstream effects of Ubx has focused on the control of pattern formation. Here we provide the first detailed descriptio n of cell differentiation in the haltere epidermis, and of the developmenta l processes that distinguish wing and haltere cells. By the end of pupal de velopment, haltere cells are 8-fold smaller in apical surface area than win g cells; they differ in cell outline, and in the size and number of cuticul ar hairs secreted by each cell. Wing cells secrete only a thin cuticle, and undergo apoptosis within 2 hours of eclosion, Haltere cells continue to se crete cuticle after eclosion, Differences in the shape of wing and haltere cells reflect differences in the architecture of the actin cytoskeleton tha t become apparent between 24 and 48 hours after puparium formation. We show that Ubx protein is not needed later than 6 hours after puparium formation to specify these differences, though it is required at later stages for th e correct development of campaniform sensilla on the haltere, We conclude t hat, during normal development, Ubx protein expressed before pupation contr ols a cascade of downstream effects that control changes in cell morphology 23-48 hours later. Ectopic expression of Ubx in the pupal wing, up to 30 h ours after puparium formation, can still elicit many aspects of haltere cel l morphology. The response of wing cells to Ubx at this time is sensitive t o both the duration and level of Ubx exposure.