DEVELOPMENTAL PROFILES AND ECDYSTEROID REGULATION OF THE MESSENGER-RNAS FOR 2 ECDYSONE RECEPTOR ISOFORMS IN THE EPIDERMIS AND WINGS OF THE TOBACCO HORNWORM, MANDUCA-SEXTA

Ecdysteroids acting through multiple isoforms of the ecdysone receptor (EcR) initiate molting and metamorphosis of insects. Two isoforms of EcR, A (this paper) and BI (Fujiwara et al., Insect Biochem. Mol. Biol . 25, 845-856, 1995), were isolated from the tobacco hornworm, Manduca sexta, and shown to be similar to the corresponding Drosophila EcR is oforms. The developmental profiles of both EcR-A and EcR-B1 (determine d by both analysis of isoform-specific mRNAs and use of monoclonal ant ibodies that detect either EcR-B1 or all forms), however, were differe nt in Manduca epidermis, which produces sequentially the larval, the p upal, and the adult cuticles. EcR-B1 predominated through the larval, pupal, and early developing adult stages with an upregulation early in each molt. By contrast, EcR-A was present only at the onset of new cu ticle synthesis during the larval malt, but in the pupal and adult mol ts was upregulated slightly later than EcR-B1 during the commitment pe riod and was present during the predifferentiative phase. Both isoform s appeared in the larval wing discs after pupal commitment and persist ed through pupal differentiation. The mRNAs for both isoforms were dir ectly induced in larval epidermis in vitro by 20-hydroxyecdysone, but EcR-B1 mRNA accumulated more rapidly, peaking at 3 hr. In the presence of a protein synthesis inhibitor, the accumulation of EcR-B1 mRNA was slower and its subsequent decline was prevented, but the accumulation of EcR-A mRNA was unaffected. Thus, in this polymorphic epidermis bot h isoforms appear in every molt, with EcR-B1 present during the commit ment and predifferentiative phases and then at the onset of cuticle sy nthesis EcR-A prevails. Additionally, EcR-A is apparently associated w ith the switching and predifferentiative events necessary for a new sy nthetic program. (C) 1996 Academic Press, Inc.