Insect development and metamorphosis are controlled by two major hormones; 20-hydroxyecdysone (20E) and juvenile hormone (JH). 20E signaling pathway is well recognized while JH signaling is still ambiguous. For a better understanding of JH biosynthesis and signaling we worked on two parallel projects; reverse genetic and forward genetic studies.
In the reverse genetic study, we have tested the potential functional redundancy between Methoprene-tolerant (Met) and germ cell-expressed (gce), two paralog bHLH-PAS transcription factors in Drosophila that were suggested to be JH receptors. Met null mutants are viable, resistant to JH and low fecundity. No gce mutant was available at the begening of this project. We generated a gce null allele and found that it phenocopies Met mutants. Met-gce double mutants are lethal at prepupal stage, which is similar to the JH-deficient flies. Krüppel homolog1 (Kr-h1) and broad (br) are two known JH signaling componets. Further investigations revealed that Met-gce double mutant diminishes Kr-h1 expression, induces precocious br expression, and causes premature and enhanced caspase-dependent programmed cell death. Therefore, we conclude that Met and Gce are functionally redundant in transducing JH signals.
Expression of br is induced by 20E, but its induction can be suppressed by JH. In the forward genetic study, we designed and conducted a novel genetic screen to isolate mutations that can de-repress br expression at early larval stages. From 4,400 lethal lines, 55 mutations were isolated based on the precocious br expression in 2nd instar larvae. Genes associated with these 55 mutations include apterous, InR, NMAR1, Fpps and Kr-h1, which are known to be involved in JH biosynthesis or signaling. Other genes encode proteins with various molecular functions, including enzymes, signal transduction molecules, and transcriptional factors. Among them, there are three Wnt signaling components, Axin (Axn), supernumerary limbs (slmb), and naked cuticle (nkd) and two TGF-β signaling components, thick vein (tkv) and mothers against Dpp (mad). We further demonstrated that Wnt signaling mediates JH signaling by regulating Met and gce expression, and that TGF-β signaling controls JH biosynthesis by upregulating transcription of JH acid methyltransferase (jhamt), a key regulatory enzyme of JH biosynthesis
