Molecular characterization and functional analysis of a putative sex-peptide receptor in the tobacco cutworm Spodoptera litura (Fabricius, 1775) (Lepidoptera: Noctuidae)

The sex-peptide receptor (SPR) acts in Drosophila females to detect male sex-peptide (SP) and trigger changes in female behaviour. Further study on SPR and its ligand family from different taxa will not only provide insight into ligand-receptor coevolution but also facilitate finding new approaches for pest control. Our previous studies on the tobacco cutworm Spodoptera litura (Fabricius, 1775) showed that mating usually results in marked changes in female behaviour and determined that factors from male accessory gland (MAG) secretions play a key role in such switches in females after mating. In the present study, we report the cloning of a transcript encoding a G-protein-coupled receptor from S.litura. This 3kb transcript contains an ORF encoding a protein of 421 amino acids that shared significant amino acid identity to the SPR identified in Drosophila. Real-time PCR showed that the expression levels of the receptor in adults correlate well with the process of sexual maturation and mating rates; mating resulted in distinct sexual dimorphic differences in gene expression. Our results suggest that this gene is involved in mediating reproductive behaviour in adult moths. To test the hypothesis, we further studied the possible function of this receptor (SplSPR) on mediating female post-mating behaviour by using RNA interference (RNAi) and treatment with MAG secretions. RNAi reduced the expression levels of SplSPR by 65-88% and significantly prevented the effect of MAG secretions. Injection of MAG secretions causes normal virgin females to behave as though they had mated (they are sexually unreceptive and start to lay eggs), whereas SPR RNAi females fail to respond to MAG factors and continue to show virgin behaviours (lay very few eggs and copulate readily). These results support the hypothesis that the SplSPR functions in mediating female post-mating behaviour and the existence of an SP-like ligand in MAG secretions in S.litura.