The nuclear hormone receptor BgE75 links molting and developmental progression in the direct-developing insect Blattella germanica

Ecdysteroid hormones regulate key developmental processes throughout the life cycle of insects. 20-Hydroxyecdysone (20E) acts upon binding to a heterodimeric receptor formed by the nuclear receptors EcR and USP. The receptor, once 20E bounds to it, elicits cascades of gene expression that mediate and amplify the hormonal signal. The molecular characterization of the 20E-mediated hierarchy of transcription factors has been analyzed in detail in holometabolous insects, especially in Drosophila melanogaster, but rarely in more basal hemimetabolous species. Using the hemimetabolous species Blattella germanica (German cockroach) as model, we have cloned and characterized five isoforms of B. germanica E75, a member of the nuclear receptor family participating in the 20E-triggered genetic hierarchy. The five isoforms present characteristic expression patterns during embryo and nymphal development, and experiments in vitro with fat body tissue have shown that the five isoforms display specific 20E responsiveness. RNAi experiments in vivo during the penultimate and last nymphal instars of B. germanica revealed that BgE75 is required for successfully complete nymphal–nymphal and nymphal–adult transitions. Detailed analysis of knockdown specimens during the last nymphal instar showed that BgE75 is required for the rise of circulating ecdysteroids that occurs towards the end of the instar. The main cause of ecdysteroid deficiency in BgE75 knockdowns is the premature stage-specific degeneration of the prothoracic gland. As a consequence, BgE75 knockdown nymphs do not molt, live for up to 90 days and start the adult developmental program properly, in spite of remaining as nymphs from a morphological point of view. Finally, RNAi of specific isoforms during the last nymphal instar of B. germanica has showed that they are functionally redundant. Furthermore, it also revealed the occurrence of a complex regulatory relationship among BgE75 isoforms, which is responsible of their sequential expression.