Molecular Mechanisms of Pyrethroid Resistance in the Mosquito, Culex quinquefasciatus (Say)

Abstract

Mosquitoes have been the ideal transmitters of a wide variety of disease agents due to their repeated blood feedings over their life span. Culex quinquefasciatus (Say) is an important disease vector in the Southeastern U.S.A. Mechanisms of resistance in mosquitoes have attracted much attention since they help in the elucidation of pathways of resistance development and in the design of novel strategies to prevent or minimize the spread and evolution of resistance. A mosquito strain of Cx. quinquefasciatus, HAmCqG0, from Huntsville, Alabama, has been established and further selected with permethrin, a pyrethroid insecticide, in the laboratory. The level of resistance to permethrin in the HAmCqG0 strain is 100-fold compared with an insecticide-susceptible mosquito strain S-Lab. Following permethrin selection for 8 generations, the level of resistance to permethrin in HAmCqG8 strain is 3100-fold compared with the S-Lab strain. High levels of resistance to pyrethroid insecticides in HAmCq parental and selected mosquito strains are conferred by multiple mechanisms, including P450 monooxygenase-, hydrolase- and/or GST-mediated detoxification. Incomplete suppression of pyrethroid resistance by synergists, PBO, DEF and/or DEM, suggests that one or more additional mechanisms are involved in overall resistance of HAmCq. The L-to-F kdr mutation of the sodium channel, the target site of pyrethroid insecticides, plays a very important role in pyrethroid resistance. My study has, for the first time, revealed that the transcriptional regulation of the L-to-F kdr mutation of the sodium channel gene through RNA allelic variation and RNA editing is an important mechanism involved in the evolution of kdr-mediated pyrethroid resistance in mosquitoes. Both Southern Blot and DNA sequencing analyses indicate multiple sodium channel genes presented in mosquitoes. The first full-length mosquito sodium channel gene has been cloned and sequenced from mosquitoes. Sequence comparison of the sodium channel gene between resistant and susceptible mosquitoes has revealed several non-silent and silent mutations that correspond with the levels of pyrethroid resistance in mosquitoes. This result suggests the importance of these mutations in altering the sensitivity of the sodium channels to insecticides and in insecticide resistance. Taken together, my research indicates a complexity of the sodium channel gene expression and its role in pyrethroid resistance of insects and provides a new framework to study the sodium channel gene expression regulation in insecticide resistance of mosquitoes.