The ecdysone agonist tebufenozide (RH-5992) blocks the progression into the ecdysteroid-induced regulatory cascade and arrests silkmoth oogenesis at mid-vitellogenesis

Abstract

The non-ecdysteroidal ecdysone agonist tebufenozide (RH-5992) induces early and early–late gene expression in lepidopteran target tissues in a fashion similar to that of the naturally occurring molting hormone, 20-hydroxy-ecdysone (20E), but the RH-5992 induced response becomes subsequently arrested and the expression of genes that require a decline in hormone titre is suppressed. Here, we demonstrate that injection of RH-5992 into developmentally arrested pupal abdomens of the silkmoth, Bombyx mori, results in the initiation of vitellogenesis in ovarian follicles but the ensuing development towards the production of an eggshell (chorion) is not sustained. The developmental arrest occurs during mid-vitellogenesis (around stage “−20”), prior to the initiation of a cascade of changes in the expression of regulatory factors that include the nuclear receptors BmEcR, BmHR3A, BmFTZ-F1 and BmE75C, the adaptor protein BmSH3, the membrane protein P30 and the transcription factor BmGATAβ. The developmental arrest cannot be rescued by culturing the follicles in vitro, either in the absence or presence of 20E. Interestingly, both the decline of BmHR3A and the up-regulation of BmFTZ-F1 that normally occur during middle to late vitellogenesis are prevented by RH-5992, suggesting that changes in the normal expression of genes involved in the first part of the ecdysteroid-controlled cascade are responsible for the arrest. RH-5992 arrested follicles provide a powerful system that can be used for the identification of regulatory factors that promote the transitions from mid to late vitellogenesis and choriogenesis through arrest–rescue experimentation.