Responses to Ecdysone Express D ifferent Ecdysone Receptor lsoforms

In D. melanogaster a pulse of the steroid hormone ecdysone triggers the larval-to-adult metamorphosis, a complex process in which this hormone induces imaginal t issues to generate adult structures and larval tissues to degenerate. We show that the EcR gene encodes three ecdysone receptor isoforms (EcR-A, EcR-81 , and EcR-B2) that have common DNAand hormone-binding domains but different N-terminal regions. We have used isoform-specific monoclonal antibodies to show that at the onset of metamorphosis different ecdysone target t issues express different isoform combinations in a manner consistent with the proposition that the different metamorphic responses of these tissues require different combinations of the EcR isoforms. We have also determined temporal developmental profiles of the EcR isoforms and their mRNAs in whole animals, showing that different isoforms predominate at different developmental stages that are marked by a pulse of ecdysone.