Cytochrome P450s and Their Roles in Insecticide Resistance in the Mosquito, Culex quinquefasciatus
Type of Degreedissertation
Entomology and Plant Pathology
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The southern house mosquito, Culex quinquefasciatus, is an important disease vector of West Nile virus (WNV), Saint Louis encephalitis virus (SLEV), and filariasis. Insecticides are the most important component in the vector-control effort. Pyrethroids are currently the most widely used insecticides for indoor sprays of mosquitoes worldwide. However, mosquitoes have developed resistance to insecticides. An improved understanding of the mechanisms governing insecticide resistance is necessary and would be helpful to develop novel strategies for preventing resistance development, and controlling resistant mosquitoes. A mosquito strain of Cx. quinquefasciatus, HAmCqG0, was collected from Huntsville, Alabama, and established in the laboratory in 2002. The level of resistance to permethrin in the HAmCqG0 strain is 10-fold, compared with an insecticide susceptible mosquito strain S-Lab, which has served as reference strain. The strain, HAmCqG8, was the 8th generation of permethrin-selected offspring of HAmCqG0, with a resistance level to permethrin at 2700-fold compared with S-Lab strain. There are multiple mechanisms reported to be involved in pyrethroid insecticide resistance, including two major ones, cytochrome P450-mediated detoxification and voltage-gated sodium channel insensitivity. In order to gain insight in understanding the mechanisms conferring pyrethroid resistance in the Culex mosquito, I investigated the distribution of the resistance levels in the field strain HAmCqG0 to permethrin by isolating 104 single-egg-raft colonies. I further characterized both the frequency of the L-to-F kdr mutation allelic expression of sodium channels and the expression levels of multiple up-regulated cytochrome P450 genes in single-egg-raft colonies with different levels of resistance to determine their possible roles in resistance. To better understand the mechanism of cytochrome P450-mediated detoxification involved in insecticide resistance, I demonstrated the expression profiles for the total 204 P450 genes that are reported in the genome of Cx. quinquefasciatus from different strains bearing different pyrethroid resistance phenotypes (susceptible, intermediate and highly resistant). My objectives were to pinpoint the key P450 genes involved in insecticide resistance. With those key P450 genes identified, I then examined their functions in conferring resistance using double-stranded RNA-mediated gene interference (RNAi) techniques, and revealed that silencing the expression of up-regulated genes in mosquitoes resulted in a reduction of resistance. Furthermore, characterization of transgenic lines of Drosophila melanogaster with the mosquito P450 genes showed an induction of the levels of tolerance to permethrin in these transgenic lines. Taken together, my research indicated that multiple up-regulated P450 genes are co-responsible for detoxification of insecticides and insecticide selection, and also that multiple mechanisms co-work on the development of insecticide resistance in mosquitoes.