A De Novo Transcriptomics Approach Reveals Genes Involved in Thrips Tabaci Resistance to Spinosad

The onion thrip, Thrips tabaci (Thysanoptera: Thripidae) is a major polyphagous pest that attacks a wide range of economically important crops, especially Allium species. The thrip's damage can result in yield loss of up to 60% in onions (Allium cepa). In the past few decades, thrip resistance to insecticides with various modes of actions have been documented. These include resistance to spinosad, a major active compound used against thrips, which was reported from Israel. Little is known about the molecular mechanisms underlying spinosad resistance in T. tabaci. We attempted to characterize the mechanisms involved in resistance to spinosad using quantitative transcriptomics. Susceptible (LC50 = 0.6 ppm) and resistant (LC50 = 23,258 ppm) thrip populations were collected from Israel. An additional resistant population (LC50 = 117 ppm) was selected in the laboratory from the susceptible population. De novo transcriptome analysis on the resistant and susceptible population was conducted to identify differently expressed genes (DGEs) that might be involved in the resistance against spinosad. In this analysis, 25,552 unigenes were sequenced, assembled, and functionally annotated, and more than 1500 DGEs were identified. The expression levels of candidate genes, which included cytochrome P450 and vittelogenin, were validated using quantitative RT-PCR. The cytochrome P450 expression gradually increased with the increase of the resistance. Higher expression levels of vitellogenin in the resistant populations were correlated with higher fecundity, suggesting a positive effect of the resistance on resistant populations. This research provides a novel genetic resource for onion thrips and a comprehensive molecular examination of resistant populations to spinosad. Those resources are important for future studies concerning thrips and resistance in insect pests regarding agriculture.