Molecular Characterization and Expression of the Ecdysone Receptor and Ultraspiracle Genes in the Wheat Blossom Midge, Sitodiplosis mosellana

20-hydroxyecdysone (20E) is essential for insect development and diapause. Ecdysone receptor (EcR) and ultraspiracle (USP) proteins are crucial regulators of 20E signaling. To explore their potential roles in the development of <i>Sitodiplosis mosellana</i>, a major wheat pest that undergoes obligatory diapause as a larva, one <i>SmEcR</i> and two <i>SmUSPs</i> (<i>SmUSP-A</i> and <i>SmUSP-B</i>) from this species were isolated and characterized. The deduced SmEcR and SmUSP-A/B proteins contained a conserved DNA-binding domain with two zinc finger motifs that bind to specific DNA sequences. Expression of <i>SmEcR</i> and the <i>SmUSPs</i> was developmentally controlled, as was 20E induction. Their transcription levels increased as the larvae entered pre-diapause, followed by downregulation during diapause and upregulation during the shift to post-diapause quiescence, which is highly consistent with ecdysteroid titers in this species. Topical application of 20E to diapausing larvae also elicited a dose-dependent expression of the three genes. Expression of <i>SmEcR</i> and <i>SmUSPs</i> decreased markedly during the pre-pupal stage and was higher in adult females compared to males. These findings suggested that 20E-induced expression of <i>SmEcR</i> and <i>SmUSPs</i> has key roles in diapause initiation and maintenance, post-diapause quiescence, and adult reproduction, while the larval-pupal transformation may be associated with a decrease in their expression levels.