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).