The exogenous methyl farnesoate does not impact ecdysteroid signaling in the crustacean epidermis in vivo

Methyl farnesoate (MF) produced by the mandibular organ is a crustacean terpenoid hormone involved in the regulation of larval development, reproduction and male morphogenesis. But the receptor for MF has remained unresolved. In view of the fact that MF can bind to crustacean retinoid X receptor (RXR) and that the terpenoid mimic, pyriproxyfen, is capable of altering the expression of crustacean RXR gene, crustacean RXR has been proposed to be a candidate receptor for MF. It is well known that ecdysteroids signal through the ecdysteroid receptor (EcR), which heterodimerizes with the RXR in Crustacea. This study was aimed to investigate whether the exogenous MF impacts ecdysteroid signaling in vivo using N-acetyl-β-glucosaminidase (NAG) mRNA from epidermal tissue as a biomarker for ecdysteroid signaling. The NAG mRNA from the model crustacean Uca pugilator injected with 0, 0.2, 1, 5, and 20 ng/g wet weight of MF was quantified using quantitative real-time PCR (qRT-PCR). An assay of epidermal NAG activity in crabs injected with 0, 20, and 2000 ng/g wet weight of MF was also performed. The administration of the exogenous MF was found to have no effects on epidermal NAG gene transcription or NAG activity in U. pugilator. These results clearly show that MF is not capable of affecting epidermal ecdysteroid signaling in the fiddler crab, U. pugilator. Our data are not supportive of the notion that MF signals through the RXR in Crustacea.