Mechanism of resistance of the varroa mite to Apistan

Sodium channels are “gates” on cell membranes that allow sodium ions (Na+) to move into cells when cells are excited. Sodium channels must then close quickly to allow the cells to be prepared for the next nerve impulse. Sodium pumps move the sodium inside cells to outside to maintain the gradient. Pyrethroid insecticides bind to sodium channels so that they do not close properly and insects die from overstimulation. Most insects have specific mutations that confers resistance to pyrethroids, such as kd-r (knowdown resistant), and skdr (super knockdown resistance). Our studies showed that none of the previously known pyrethroid resistance-associated insect sodium channel mutations were found in pyrethroid-resistant varroa mite (Varroa destructor). Instead, four novel mutations were found in pyrethroid resistant mite populations in Michigan and Florida. These mutations include a phenylalanine (F) to leucine (L) mutation in the segment 6 of domain III (IIIS6), a leucine (L) to proline (P) mutation in the linker connecting domains III and IV, and an isoleucine (I) to valine (V) mutation in IVS5 and a methionine (M) to isoleucine (I) mutation in IVS6. In a followup study, we examined the effect of these mutations on the sensitivity of a cockroach sodium channel variant (BgNa_v1-1) expressed in Xenopus oocytes to fluvalinate and another potent pyrethroid deltamethrin. We found that the I to V mite mutation in IVS5 did not alter the channel sensitivity to the pyrethroids. The BgNa_v1-1 channel carrying the F to L mutation in IIIS6 was not functional. Interestingly, at the position corresponding to the L to P mutation, all known insect sodium channel proteins naturally contain a P residue (P1577 in BgNa_v1-1). The P1577L substitution in BgNa_v1-1 caused a 2-fold or 5-fold increase in the channel sensitivity to deltamethrin and fluvalinate, respectively. Corresponding to the mite M to I mutation in IVS6, a valine (V1804) was found in BgNa_v. Both V1804M and V1804I channels were 4- to 8-fold more sensitive to deltamethrin and fluvalinate, respectively, compared to the wild-type BgNa_v1-1 channel. These results suggest that the L to P mutation confers varroa mite resistance to fluvalinate.

This study is a collaboration with Dr. Ke Dong, whose lab is internationally renowed for studying sodium channels.

Funding provided by project GREEEN, MSU, and USDA-NRI.

Publications:

Wang, R., Z. Liu, K. Dong, P.J. Elzen, J. Pettis, Z.Y. Huang. 2002. Association of novel mutations in a sodium channel gene with fluvalinate resistance in the varroa mite, Varroa destructor. Journal of Apicultural Research 40: 17-25 30.

Wang, R., Z.Y. Huang & K. Dong. 2003. Molecular characterization of an arachnid sodium channel gene from the varroa mite (Varroa destructor). Insect Biochemistry and Molecular Biology 33: 733-739.

Liu, Z., J. Tan, Z.Y. Huang, K. Dong. 2006. Effect of a fluvalinate-resistance-associated sodium channel mutation from varroa mites on cockroach sodium channel sensitivity to fluvalinate, a pyrethroid insecticide. Insect Biochemistry and Molecular Biology 36: 885-889

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