Mutations in the nAChR β1 subunit and overexpression of P450 genes are associated with high resistance to thiamethoxam in melon aphid, Aphis gossypii Glover

The TMXR is a strain of melon aphids (Aphis gossypii Glover) that has extremely high resistance (resistance ratio > 2300 fold) to thiamethoxam. We explored the basis of this resistance by examining differences in nicotinic acetylcholine receptors (nAChRs) and cytochmme P450 monooxygenase (CYP450s) between the TMXR and the susceptible strain. The results showed that two mutation sites of nAChR beta 1 subunit, V62I and R81T, were found in TMXR, with the mutation frequencies of the two mutation sites as 93.75%. Meanwhile, compared with the susceptible strain, the expression level of nAChR beta 1 subunit gene in the TMXR decreased by 38%. In addition, piperonyl butoxide (PBO) showed a synergistic ratio of 17.78-fold on TMX toxicity against the TMXR, which suggested the involvement of CYP450s in the TMX resistance of melon aphid. Moreover, the expression levels of 4 P450s genes were significantly higher in the TMXR than the susceptible strain. Through RNAi, we verified that down-regulating CYP6DA1 increased the sensitivity of TMXR to TMX toxicity, demonstrating that a decrease in CYP6DA1 expression may reduce resistance in vivo. These results suggest that A. gossypii has the capacity to develop extremely high resistance to TMX through aggregated resistance mechanisms including enhancement of detoxification by upregulation of CYP450s, and target insensitivity caused by alteration of nAChR beta 1 subunit with mutation and low expression. These findings provide basic information for further clarifying the molecular mechanism of insecticide resistance in A. gossypii.

Automated summary

The highly resistant strain TMXR of melon aphids showed mutations in nAChR beta 1 subunit, decreased expression levels of the gene, and higher expression levels of P450s genes compared to susceptible strains. The involvement of nAChRs and CYP450s in the resistance mechanisms were highlighted, suggesting potential targets for overcoming insecticide resistance in Aphis gossypii.