Cloning of an ecdysone receptor homolog from Manduca sexta and the developmental profile of its mRNA in Wings

Using the Drosophila melanogaster ecdysone receptor (DmEcR) BI cDNA clone, we isolated three genomic clones for EcR from the tobacco hornworm, Manduca sexta. Subsequent isolation and sequencing of several cDNAs yielded a homolog of the BI isoform with 50, 95 and 70% amino acid identities with DmEcR in the N-terminal A/B, the DNA binding and the ligand binding domains respectively. Unlike Drosophila, an intron occurs between the exons encoding the two zinc fingers of Manduca EcR (MsEcR). A 6.0 kb mRNA encoding MsEcR was found in both larval wing discs and prothoracic glands and in pupal wings. During the final larval instar, the mRNA was maximal in the wing discs at one day after wandering (W1), whereas in the prothoracic gland EcR mRNA increased rapidly to high levels on day 2 and remained high thereafter. During the onset of adult development, two peaks of EcR mRNA were observed in wings from day 3 to 5 and on day 8 after pupal ecdysis. These two peaks correlated with the time of increasing titers of ecdysone (E) and 20-hydroxyecdysone (20E), respectively. The EcR mRNA peaks always preceded the large ecdysteroid peak, suggesting that the transcription of the EcR gene is induced by a low concentration of ecdysteroid in vivo.