Negative cross-resistance of a pyrethroid-resistant Drosophila mutant to Phryma leptostachya-derived haedoxan A.

Voltage-gated sodium channels are the primary target of pyrethroid insecticides. Mutations in sodium channel confer knockdown resistance (kdr) to pyrethroids in various arthropod pests. Haedoxan A (HA) is the major insecticidal component from Phryma leptostachya. It has been shown that HA alters electrical responses at the Drosophila neuromuscular junction and modifies the gating properties of cockroach sodium channels expressed in Xenopus oocytes. However, whether sodium channel mutations that confer pyrethroid resistance also affect the action of HA is unknown. In this study, we conducted bioassays using HA and permethrin in two Drosophila melanogaster strains: w , an insecticide-susceptible strain, and para , a pyrethroid-resistant strain due to a I265N mutation in the sodium channel, and identified a new case of negative cross-resistance (NCR) between permethrin and HA. Both para larvae and adults were more resistant to permethrin, as expected. However, both para larvae and adults were more sensitive to HA compared to w . We confirmed that the I265N mutation reduced the sensitivity to permethrin of a Drosophila sodium channel variant, DmNa 22, expressed in Xenopus oocytes. Interestingly, the I265N mutation also abolished the effect of HA on sodium channels. Further characterization showed that I265 on the sodium channels is critical for the action of both pyrethroids and HA on sodium channels, pointing to an overlapping mode of action between pyrethroids and HA on the sodium channel. Overall, our results suggest an I265N-independnt mechanism(s) in para flies that is responsible for the NCR between permethrin and HA at the whole insect level.