The effect of insecticide synergist treatment on genome-wide gene expression in a polyphagous pest

Abstract

Synergists can counteract metabolic insecticide resistance by inhibiting detoxification enzymes or transporters. They are used in commercial formulations of insecticides, but are also frequently used in the elucidation of resistance mechanisms. However, the effect of synergists on genome-wide transcription in arthropods is poorly understood. In this study we used Illumina RNA-sequencing to investigate genome-wide transcriptional responses in an acaricide resistant strain of the spider mite Tetranychus urticae upon exposure to synergists such as S,S,S-tributyl phosphorotrithioate (DEF), diethyl maleate (DEM), piperonyl butoxide (PBO) and cyclosporin A (CsA). Exposure to PBO and DEF resulted in a broad transcriptional response and about one third of the differentially expressed genes (DEGs), including cytochrome P450 monooxygenases and UDP-glycosyltransferases, was shared between both treatments, suggesting common transcriptional regulation. Moreover, both DEF and PBO induced genes that are strongly implicated in acaricide resistance in the respective strain. In contrast, CsA treatment mainly resulted in downregulation of Major Facilitator Superfamily (MFS) genes, while DEGs of the DEM treatment were not significantly enriched for any GO-terms.

Figure 1

Gene expression relationships among synergist treatments and controls. PCA plot of gene expression levels in untreated adult T. urticae females (CON), adult T. urticae females sprayed with formulation only (FORM) or adult T. urticae females exposed to synergist compounds CsA, DEF, DEM or PBO.

Figure 2

qPCR validation of differentially expressed genes in adult T. urticae females after PBO, DEF or CsA treatment. Eight up- and five downregulated genes as assessed by differential gene expression (DESeq2) analysis of RNA-seq data were selected for qPCR analysis. Error bars represent the standard error of the calculated mean. Except for tetur40g00030 (CsA treatment), each gene was significantly differentially expressed (based on an unpaired t-test) compared to the reference condition (FORM). The ‘‘tetur’’ prefix was removed from T. urticae gene IDs for figure clarity.

Figure 3

Venn diagrams depicting overlap among differentially expressed genes of adult T. urticae females exposed to either PBO, DEF, DEM or CsA compared to adult T. urticae females treated with formulation only. Left panel: downregulated genes, right panel: upregulated genes.

Figure 4

Expression heatmaps of genes coding for P450s, UGTs or MFS members in adult T. urticae females exposed to either DEF, PBO, DEM or CsA. The log₂ transformed fold changes are relative to adult T. urticae females treated with formulation only and were clustered using a Euclidean distance metric and Ward’s method. T. urticae gene IDs are shown on the right.