Achieving final form: How ecdysone controls leg joint morphogenesis in Drosophila. C.K. Mirth , M.E. Akam. University Museum of Zoology, Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ.

   In the fly leg, the joints are patterned in the third instar, yet do not differentiate until mid-pupal periods. This temporal separation of patterning and differentiation is achieved by modulation of the steroid hormone ecdysone. In my studies, I describe the cell behaviours occurring during joint formation and show that ecdysone is required for these cell behaviours to occur in culture. Furthermore, above a threshold of 100ng/ml of ecdysone, different levels of hormone do not differentially affect joint formation. Rather the timing of when the ecdysone pulse occurs is important to joint differentiation. If the ecdysone pulse is delayed by twenty-four hours, no joints form. I have also identified several Gal4 lines that mark specific cell populations in the joint. Using these lines I can test 1) whether the regions destined to form joints retain joint markers, and hence joint identity, in the absence of ecdysone and 2) whether any of these joint markers change their expression pattern depending on the presence or absence of ecdysone. Markers that change in response to ecdysone could be insertions into genes whose proteins control joint cell behaviour. I find that joint cells retain their identify even in the absence of ecdysone. In addition, some of the markers tested appear to respond to ecdysone.