Juvenile Hormone Prevents Ecdysteroid-induced Expression of Broad Complex RNAs in the Epidermis of the Tobacco Hornworm, Manduca Sexta

A cDNA homolog of the Drosophila melanogaster Broad Complex (BRC) gene was isolated from the tobacco hornworm, Manduca sexta, which shows a predicted 88% amino acid identity with Drosophila BRC in the N-terminal BTB domain. Three zinc finger domains encoding homologs of the Drosophila Z2, Z3, and Z4 domains (93, 100, and 85% identity, respectively) were obtained by RT-PCR. In Manduca dorsal abdominal epidermis, BRC RNAs were not observed during the larval molt. Three BRC transcripts-6.0, 7.0, and 9.0 kb-first appeared at the end of the feeding stage of the fifth (final) instar when the epidermis is exposed to ecdysteroids in the absence of juvenile hormone (JH) and becomes committed to pupal differentiation. These RNAs were induced in day 2 fifth larval epidermis in vitro by 20-hydroxyecdysone (20E) in the absence of JH with dose-response and time courses similar to the induction of pupal commitment. This induction by 20E in vitro was prevented by the presence of JH I at levels seen in vivo during the larval molt. In the wing discs, the BRC RNAs appeared shortly after ecdysis to the fifth instar and coincided with the onset of metamorphic competence of these discs. Application of a JH analogue pyriproxifen during the fourth instar molt delayed and reduced the levels of BRC mRNAs seen in the wing discs in the early fifth instar, but did not completely prevent their appearance in this tissue that first differentiates at metamorphosis. The expression of the BRC transcription factors thus appears to be one of the first molecular indications of the genetic reprogramming of the epidermis necessary for insect metamorphosis. How JH prevents BRC expression in this epidermis may provide the key to understanding how this hormone controls metamorphosis.