Molecular characterization and functional analysis of the ultraspiracle (USP) in the oriental fruit moth Grapholita molesta (Lepidoptera: Olethreutidae)

Ecdysteroids play an important role in the growth, development, and reproduction of insects. The ecdysteroid signal is transduced through a heterodimeric complex of two nuclear receptors: the ecdysone receptor (EcR) and the ultraspiracle (USP). To investigate the role of USP during the development of Grapholita molesta, ultraspiracle (USP) cDNA was obtained from the transcriptome of G. molesta and verified by PCR. The deduced amino acid sequence of G. molesta USP (GmUSP) was highly homologous to those of other lepidopterans, especially the Manduca sexta USP-1 isoform. Relatively high expression levels of GmUSP were observed immediately prior to the last larval (L10) molt and during the early pupal stage (P1). Furthermore, high levels of GmUSP mRNA were found in the midgut during the last larval stage. Silencing of GmUSP could significantly down-regulate the transcriptional level of ecdysone-inducible genes and result in increased mortality during metamorphosis and a prolonged prepupal duration. In RNAi experiments, ecdysone-inducible genes were significantly down-regulated following GmUSP silencing, suggesting that they were under the control of GmUSP. In addition, the GmUSP knockdown resulted in increased mortality during metamorphosis and prolonged the prepupal period. Taken together, the present results indicate that GmUSP may directly or indirectly affect the development of G. molesta.