Submitted to: Insect Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2008
Publication Date: 2/18/2009
Citation: Li, H., Sun, L., Muir, W., Xie, J., Schemerhorn, B.J., Pittindrigh, B. 2009. Transcriptional Signatures in Response to Wheat Germ Agglutinin and Starvation in Drosophila melanogaster Larval Midgut. Insect Molecular Biology. p. 21-31. Interpretive Summary: Plant lectins are carbohydrate-binding proteins. Wheat germ agglutinin (WGA), present in wheat and wheat-based foods, is a lectin that binds carbohydrates containing N-acetyl-D-glucosamine (GlcNAc) such as chitin. Lectins, including WGA, possess cytotoxic, anti-insect and anti-nematode properties. To date only a few studies on the precise mode of insecticidal action of WGA have been conducted. WGA is thought to bind insect gut tissues while resisting enzymatic degradation. Chitin is a key component of important structure within the gut of most phytophagous insects and may be a target for dietary WGA. Using transmission electron microscopy and microarrays we investigated the structural/quantitative changes in third instar larvae of fruit flies fed on a WGA rich diet and during starvation. Microarray analysis revealed that several chitin-binding gene transcripts respond to dietary WGA. Transcription factor-binding motif analysis indicated that one motif is involved in the formation of cytoskeletons in WGA-fed insects. Our observations collectively support the hypothesis that dietary WGA: (i) impacts larval midgut cell structures in insects; and, (ii) causes differential expression of genes associated with starvation as well as with carbohydrate and lipid metabolism. This work will benefit nearly all aspects of agriculture, from research, to breeders, to farmers themselves. This work opens up new dimensions in possible targets for innate, non-pesticide-based strategies to control insect infestations in agriculturally important crops.
Technical Abstract: One function of plant lectins such as wheat germ agglutinin (WGA) is to serve as defenses against herbivorous insects. The midgut is one critical site affected by dietary lectins. We observed marked cellular, structural, and gene expression changes in the midguts of Drosophila melanogaster third-instar larvae that were fed WGA. Some of these changes were similar to those observed in the midguts of starved insects. Dietary WGA caused shortening, branching, swelling, distortion and in some cases disintegration of the midgut microvilli (Mv). Starvation was accompanied primarily by shortening of the Mv. Microarray analyses revealed that dietary WGA evoked differential expression of 61 transcripts; seven of these were also differentially expressed in starved insects. The differentially-regulated gene clusters in WGA-fed larvae were associated with (i) cytoskeletal organization, (ii) digestive enzymes, (iii) detoxification reactions, and (iv) energy metabolism. Four possible transcription factor binding motifs (TFBMs) were associated with the differentially-expressed genes. One of these exhibited substantial similarity to MyoD, a TFBM associated with cellular structures in mammals. These results are consistent with the hypothesis that WGA caused a starvation-like effect and structural changes of midgut cells of Drosophila third-instar larvae.