Effects of Btt Exposure and Pesticide Selection on Gene Expression in the Absence of Pesticide Exposure
We investigated the interaction between pesticide selection regime andBtt exposure on gene expression in the absence of pesticides, as this provides information on the main host response to oral Bttinfection as well as whether the evolution of pesticide resistance modifies the host response to Btt . Using only the data from individuals never exposed to pesticides, we identified 104 genes that were differentially expressed in Btt -exposed individuals compared to unexposed individuals (Fig. 3A; Bt Tx +). This gene set largely concurred with previously published results on Bt infection in T. castaneum , and included the upregulation of AMPs (two attacins and two defensins), a pathogenesis-related protein, and a cytochrome P450 (Fig. 4, no pesticide model BtTx). GO enrichment analyses revealed the importance of immune system processes and defense against bacteria, among other immune related terms, in the genes differentially expressed with Btt treatment in the absence of pesticide exposure (Suppl. Fig. 4A, no pesticide model BtTx).
Only two transcripts (an elastin and one unannotated transcript) were upregulated upon Btt infection in OP-selection regime individuals relative to susceptible ones (Fig. 3A, no pesticide model OP Reg:BtTx interaction), indicating that evolved resistance to OP minimally affects the baseline response to infection in the absence of pesticides. Fourteen transcripts were differentially expressed in Pyr-selected individuals relative to susceptible-regime individuals after Bttinfection (Fig. 3A; no pesticide model Pyr Reg:BtTx interaction), including the downregulation of two defensins, a pathogenesis-related protein, and a histidine-rich glycoprotein (Fig. 4B, no pesticide model Pyr Reg:BtTx). Notably, these genes were differentially expressed in the opposite direction in Pyr regime-Btt infected larvae compared to gene expression induced by the main effect of Btt exposure, indicative of dampened expression of these genes (Fig. 4B, no pesticide model Pyr Reg:BtTx vs. BtTx, e.g. Def: TC006250). GO enrichment analysis revealed significant enrichment of humoral immune response and defense response among the differentially expressed genes from this interaction term supporting the impact on immune processes associated with the interaction effect of Pyr-selection regime with Bttinfection (Suppl. Fig. 4A, Pyr Reg:BtTx).