Transcriptomic Analyses of Secreted Proteins From the Salivary Glands of Wheat Midge Larvae

Both the wheat midge (Sitodiplosis mosellana) (Géhin) (Diptera: Cecidomyiidae) and the Hessian fly (Mayetiola destructor) (Say) (Diptera: Cecidomyiidae) belong to a group of insects called gall midges (Diptera: Cecidomyiidae), and both are destructive pests of wheat. From Hessian fly larvae, a large number of genes have been identified to encode secreted salivary gland proteins (SSGPs), which are presumably critical for the insect to feed on and manipulate host plants. For comparison, we conducted an analysis on transcripts encoding SSGPs from the first instar larvae of the wheat midge. In total, 3,500 cDNA clones were sequenced, from which 1,301 high-quality sequences were obtained. Approximately 25% of the cDNAs with high-quality sequences encoded SSGPs. The SSGPs were grouped into 97 groups based on sequence homology. Among the SSGP-encoding transcripts, 206 encoded unique proteins with no sequence similarity to any known protein and 29 encoded proteins similar to known proteins including proteases, serpines, thioesterases, ankyrins, and ferritins. Most (~80%) SSGP-encoding genes appear under strong selection for mutations that generate amino acid changes within the coding region. Identification and characterization of SSGPs in wheat midge larvae provide a foundation for future work to reveal molecular mechanisms behind wheat midge–wheat interactions and the role of these putative effector proteins in insect virulence. Availability of the SSGP transcripts will also facilitate comparative analyses of insect effectors from related species.