Insulin-like Peptides of the Western Flower Thrips Frankliniella occidentalis and Their Mediation of Immature Development

SIMPLE SUMMARY: The western flower thrips, Frankliniella occidentalis, is an invasive and polyphagous insect pest. Its regional and host diversity suggests variation in its developmental rate. In addition, a previous study indicated that insulin signaling is related to thrips development. However, no insulin-like peptide (ILP) was identified in F. occidentalis. This study identified two ILP genes (Fo-ILP1 and Fo-ILP2) and their expression profiles were investigated. Further, their expressions were dependent on feeding activity, and the starvation treatment greatly reduced the expression levels of both ILP genes. The RNA interference of the ILP gene expressions had adverse effects on the immature development of the thrips. However, different host plants causing variation in developmental rates also led to variation in the ILP gene expression levels. These results report the first ILP genes in thysanopteran insects and their functional relationship with the thrips’ development. ABSTRACT: Insulin-like peptides (ILPs) mediate various physiological processes in insects. Specifically, ILP expression is required for immature development in different insects. The western flower thrips, Frankliniella occidentalis, is polyphagous, but its occurrence and population density vary among different hosts. This study assesses the developmental variations in the thrips through quantitative analysis of their ILP expressions. Two types of ILPs (Fo-ILP1 and Fo-ILP2) were identified from the genome of F. occidentalis, and both ILPs were predicted to have the characteristics of signal peptides and B-C-A chains linked by cysteines. A phylogenetic analysis indicates that these two ILPs in the thrips are clustered with the ILP1 of Drosophila melanogaster, suggesting their physiological roles in growth. In addition, the two ILP genes were relatively highly expressed at all feeding stages, but their expression was reduced during the nonfeeding prepupal and pupal stages. Furthermore, RNA interference of each ILP expression led to significant developmental retardation. In validating the ILP expression in the thrips’ development, five different varieties of host hot peppers were assessed in a choice test, along with the immature development of F. occidentalis. The expression levels of the two ILP genes were highly correlated with variations in the immature developmental rates of different hot pepper varieties. These suggest that Fo-ILP1 and Fo-ILP2 mediate the immature development of F. occidentalis by sensing different nutritional values of hot peppers. This study is the first report on ILPs in thysanopteran insects.