Abstract
The methyl farnesoate receptor (MfR) orchestrates aspects of reproduction and development such as male sex determination in branchiopod crustaceans. Phenotypic endpoints regulated by the receptor have been well-documented, but molecular interactions involved in receptor activation remain elusive. We hypothesized that the MfR subunits, methoprene-tolerant transcription factor (Met) and steroid receptor coactivator (SRC), would be expressed coincident with the timing of sex programming of developing oocytes by methyl farnesoate in daphnids. We also hypothesized that methyl farnesoate activates MfR assembly. Met mRNA was expressed rhythmically during the reproductive cycle, with peak mRNA accumulation just prior period of oocytes programming of sex. Further, we revealed evidence that Met proteins self-associate in the absence of methyl farnesoate, and that the presence of methyl farnesoate stimulates dissociation of Met multimers with subsequent association with SRC. Results demonstrated that the Met subunit is highly dynamic in controlling the action of methyl farnesoate through temporal variation in its expression and availability for receptor assembly.
Highlights
Methyl farnesoate is an acyclic sesquiterpenoid hormone and the unepoxidated form of juvenile hormone III found in insects
We demonstrated that methyl farnesoate activated the daphnid methoprene-tolerant transcription factor (Met) in the presence of the steroid receptor coactivator (SRC) ortholog derived from Aedes aegypti[15]
Subsequent experiments performed with Met and SRC cloned from Daphnia magna and Daphina pulex corroborated our earlier results and confirmed the protein complex as a methyl farnesoate responsive receptor (MfR)[16]
Summary
Methyl farnesoate is an acyclic sesquiterpenoid hormone and the unepoxidated form of juvenile hormone III found in insects. Some environmental chemicals, known as insect growth regulating insecticides (IGRs), can stimulate male sex determination in some crustacean species[6,7]. Several IGRs activated the daphnid MfR in gene transcription reporter assays[15,16], with similar relative potency as observed with the stimulation of male sex determination in vivo. These experiments supported Met and SRC as being the receptor protein complex mediating male sex determination. Shaded regions denote the periods of embryo susceptibility to male sex determination by methyl farnesoate
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