DHR3 is required for the prepupal-pupal transition and differentiation of adult structures during Drosophila metamorphosis

Pulses of the steroid hormone ecdysone activate genetic regulatory hierarch ies that coordinate the developmental changes associated with Drosophila me tamorphosis. A high-titer ecdysone pulse at the end of larval development t riggers puparium formation and induces expression of the DHR3 orphan nuclea r receptor. Here we use both a heat-inducible DHR3 rescue construct and clo nal analysis to define DHR3 functions during metamorphosis. Clonal analysis reveals requirements for DHR3 in the development of adult bristles, wings, and cuticle, and no apparent function in eye or leg development. DHR3 muta nts rescued to the third larval instar also reveal essential functions duri ng the onset of metamorphosis, leading to lethality during prepupal and ear ly pupal stages. The phenotypes associated with these lethal phases are con sistent with the effects of DHR3 mutations on ecdysone-regulated gene expre ssion. Although DHR3 has been shown to be sufficient for early gene repress ion at puparium formation, it is not necessary for this response, indicatin g that other negative regulators may contribute to this pathway. In contras t, DHR3 is required for maximal expression of the midprepupal regulatory ge nes, EcR, E74B and beta FTZ-1. Reductions in EcR and beta FTZ-F1 expression , in turn, lead to submaximal early gene induction in response to the prepu pal ecdysone pulse and corresponding defects in adult head eversion and sal ivary gland cell death. These studies demonstrate that DHR3 is an essential regulator of the beta FTZ-F1 midprepupal competence factor, providing a fu nctional Link between the late larval and prepupal responses to ecdysone. I nduction of DHR3 in early prepupae ensures that responses to the prepupal e cdysone pulse will be distinct from responses to the late larval pulse and thus that the animal progresses in an appropriate manner through the early stages of metamorphosis. (C) 1999 Academic Press.