Abstract
Aedes aegypti is the major vector of yellow and dengue fevers. After 10 generations of adult selection, an A. aegypti strain (SP) developed 1650-fold resistance to permethrin, which is one of the most widely used pyrethroid insecticides for mosquito control. SP larvae also developed 8790-fold resistance following selection of the adults. Prior to the selections, the frequencies of V1016G and F1534C mutations in domains II and III, respectively, of voltage-sensitive sodium channel (Vssc, the target site of pyrethroid insecticide) were 0.44 and 0.56, respectively. In contrast, only G1016 alleles were present after two permethrin selections, indicating that G1016 can more contribute to the insensitivity of Vssc than C1534. In vivo metabolism studies showed that the SP strain excreted permethrin metabolites more rapidly than a susceptible SMK strain. Pretreatment with piperonyl butoxide caused strong inhibition of excretion of permethrin metabolites, suggesting that cytochrome P450 monooxygenases (P450s) play an important role in resistance development. In vitro metabolism studies also indicated an association of P450s with resistance. Microarray analysis showed that multiple P450 genes were over expressed during the larval and adult stages in the SP strain. Following quantitative real time PCR, we focused on two P450 isoforms, CYP9M6 and CYP6BB2. Transcription levels of these P450s were well correlated with the rate of permethrin excretion and they were certainly capable of detoxifying permethrin to 4′-HO-permethrin. Over expression of CYP9M6 was partially due to gene amplification. There was no significant difference in the rate of permethrin reduction from cuticle between SP and SMK strains.
Highlights
The yellow fever mosquito Aedes aegypti inhabits tropical and subtropical regions worldwide and is the major vector of dengue fever (DF) and yellow fever
To determine the degree of dominance of the resistance [51], the SP strain was crossed with a susceptible reference strain (SMK) and bioassays were performed on the progenies
We focused on P450 monooxygenases (P450s) and their related genes because our investigations including bioassays and in vivo and in vitro metabolism studies showed that this metabolic enzyme is the key factor in the development of resistance
Summary
The yellow fever mosquito Aedes aegypti inhabits tropical and subtropical regions worldwide and is the major vector of dengue fever (DF) and yellow fever. The disease is endemic in at least 112 countries, especially in south and Southeast Asia, and an estimated 2.5 billion people are currently living in risk areas [1]. Successful population control of vector insects is the key to prevent transmission and epidemics of infectious diseases. This strategy relies heavily on insecticides, pyrethroid, a popular class of insecticides with high and rapid toxic activity toward insects and low toxicity to mammals [6]. Many dengue endemic areas are facing the problem of pyrethroid resistance due to frequent and intensive use of these chemicals [7]. Resistance of A. aegypti to pyrethroids has been reported from various countries [8]. Understanding the level and mechanism of resistance to insecticides is essential for developing appropriate vector control measures
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