クサグモ(Agelena silvatica)幼体における核内受容体遺伝子の同定と解析

Growth of spiders occurs only during a molt, so studies on the regulation of molting are essential for use of spiders as a bioresource or control of harmful spiders. Molting of arthropods progresses mainly through sequential regulation by ecdysteroids (E) and gene regulatory factors. In insects, E regulates molting through two nuclear receptors, the E receptor (EcR) and retinoid X receptor (RXR). Early genes directly regulated by E/EcR/RXR complex also function as transcriptional factors to regulate other genes. Characterization of these transcriptional factors is necessary to understand the regulation of molting in spiders. Therefore, the EcR, RXR and E75 genes were identified and their expression patterns analyzed in the spider Agelena silvatica. The complete open reading frames of two nuclear receptor AsEcRA isoforms, AsRXR and a partial sequence of AsEcRB were identified from the spider A. silvatica. These nuclear receptors show high identities with other species especially in the DNA binding domains (DBD) and ligand binding domains (LBD). AsEcR and AsRXR showed the highest homologies with EcRs and RXRs of the hard tick and scorpion, which can bind to the Drosophila EcRE. These results indicate that AsEcR and AsRXR can bind to the EcRE in the presence of E to regulate gene expression. Two types of the ecdysone early gene AsE75 were also isolated and analyzed in this study. AsE75A contained all of the conserved nuclear receptor domains but AsE75D lacked the DBD. The DBD of AsE75A was identical to other arthropods, indicating AsE75A can bind with DNA to regulate gene expression. The expression of AsEcR and AsRXR mRNA occurred throughout the third instar nymphal stage. On the other hand, AsE75A expression was low or non-existent at first, then suddenly increased from D7 to D10. The E titers of A. silvatica have not been determined yet but another spider Pisaura mirabilis showed an E peak just before molting, and apolysis and synthesis of new cuticle also occurred during this period. The length of the molting cycles differ between P. mirabilis and A. silvatica, but the regulation of molting is likely similar in both spiders, so the increase in AsE75A in A. silvatica is likely a result of E binding to the AsEcR/AsRXR heterodimers that are constantly expressed to upregulate AsE75A expression. Further studies are needed to clarify the E titers and functions of these receptors in A. silvatica for better understanding of the regulation of spider molting.