Hormonal control of a metamorphosis-specific transcriptional factor Broad-Complex in silkworm

Abstract

Insect metamorphosis is induced by the steroid 20-hydroxyecdysone (20E) in the absence of sesquiterpenoid juvenile hormone (JH). In Drosophila melanogaster, the Broad-Complex (BR-C) transcriptional factor plays critical roles during metamorphosis. We isolated cDNAs encoding BR-C in the silkworm Bombyx mori and examined their mRNA expression. cDNAs for three BR-C isoforms with zinc finger pairs (Z1, Z2 and Z4) and four isoforms lacking them were cloned. Their mRNAs were expressed in multiple tissues at the larval–pupal metamorphosis. In the anterior silk gland, BR-C mRNAs were expressed at the end of the last larval instar but not expressed during the penultimate instar. 20E administration induced BR-C mRNA expression and JH suppressed 20E-induced BR-C expression in this tissue both in vivo and in vitro. Thus, BR-C mRNAs are inducible by 20E only in the absence of JH, a finding that explains their metamorphosis-specific expression.