Isolation of the ecdysone receptor and retinoid X receptor from the araneomorph funnel-web spider Agelena silvatica (Araneae: Agelenidae)

Similar to other arthropods, molting is essential for spider development and previous studies suggest molting is also regulated by ecdysteroids. In insects, ecdysteroids bind to a heterodimer of the ecdysteroid receptor (EcR) and ultraspiracle (USP) an ortholog of the vertebrate retinoid X receptor (RXR). However, in some insects, crustaceans, ticks and scorpions EcR binds with RXR instead of USP. The EcR/RXR complex shows high binding affinity with both ecdysteroids and the ecdysone response elements (EcRE) of target genes to regulate gene transcription. However, to date EcR and RXR have not been identified from spiders. Therefore, we identified AsEcR and AsRXR from the araneomorph funnel-web spider Agelena silvatica. The isolated AsEcR was 533 amino acids and AsRXR 466 amino acids. Both AsEcR and AsRXR sequences contained the A/B domain, a DNA binding domain (DBD), a hinge area (D domain), and a ligand binding domain (LBD) common to nuclear receptors. AsEcR and AsRXR amino acid sequence homology of each domain was compared with EcRs, RXRs and USPs of other animals. Both AsEcR and AsRXR showed high identities with the DBD of other organisms. The LBD showed high identities with arachnids: a scorpion and ticks but low identities with other insects and crustaceans. For RXR, LBD showed higher identities with other invertebrate RXRs and human RXRá than USPs. As an internal control AsActin 5C was also identified and the expression of AsEcR and AsRXR were analyzed. Both genes were constantly expressed in 3rd instars nymph. These results indicate AsEcR and AsRXR function as transcription factors but the response to ligands maybe different. Further studies are required to clarify the regulation of molting in spiders. These studies will contribute to understanding the evolution of regulatory mechanisms in arthropods.