Abstract
BackgroundIn China, large amounts of chemical insecticides are applied in fields or indoors every year, directly or indirectly bringing selection pressure on vector mosquitoes. Culex pipiens complex has evolved to be resistant to all types of chemical insecticides, especially organophosphates, through carboxylesterases. Six resistant carboxylesterase alleles (Ester) were recorded previously and sometimes co-existed in one field population, representing a complex situation for the evolution of Ester genes.ResultsIn order to explore the evolutionary scenario, we analyzed the data from an historical record in 2003 and a recent investigation on five Culex pipiens pallens populations sampled from north China in 2010. Insecticide bioassays showed that these five populations had high resistance to pyrethroids, medium resistance to organophosphates, and low resistance to carbamates. Six types of Ester alleles, EsterB1, Ester2, Ester8, Ester9, EsterB10, and Ester11 were identified, and the overall pattern of their frequencies in geographic distribution was consistent with the report seven years prior to this study. Statistical correlation analysis indicated that Ester8 and Ester9 positively correlated with resistance to four insecticides, and EsterB10 to one insecticide. The occurrences of these three alleles were positively correlated, while the occurrence of EsterB1 was negatively correlated with Ester8, indicating an allelic competition.ConclusionOur analysis suggests that one insecticide can select multiple Ester alleles and one Ester allele can work on multiple insecticides. The evolutionary scenario of carboxylesterases under insecticide selection is possibly "one to many".
Highlights
In China, large amounts of chemical insecticides are applied in fields or indoors every year, directly or indirectly bringing selection pressure on vector mosquitoes
As one of seven major species of vector mosquitoes in China, the Culex pipiens complex has evolved to be resistant to all types of chemical insecticides, except carbamates, in many regions [5]
Twelve Ester alleles conferring OP resistance have been identified at the Ester locus: Ester1 (A1), Ester2 (A2-B2), Ester4 (A4-B4), Ester5 (A5B5), Ester8 (A8-B8), Ester9 (A9-B9), Ester11 (A11-B11), EsterB1 (B1), EsterB6 (B6), EsterB7 (B7), EsterB10 (B10) and EsterB12 (B12) [5,9,10,11]
Summary
In China, large amounts of chemical insecticides are applied in fields or indoors every year, directly or indirectly bringing selection pressure on vector mosquitoes. Six resistant carboxylesterase alleles (Ester) were recorded previously and sometimes co-existed in one field population, representing a complex situation for the evolution of Ester genes. Mosquitoes, due to their special behavior, physiology and close relationship with humans, act as ideal transmitters of a wide variety of human disease agents, including filariasis, Japanese encephalitis, West Nile virus, dengue and malaria [1]. Est-2 and Est-3, encode non-specific esterases through gene amplification or up-regulation in the C. pipiens complex [8,9]. Twelve Ester alleles conferring OP resistance have been identified at the Ester locus (the corresponding overproduced esterases are named in parentheses): Ester (A1), Ester (A2-B2), Ester (A4-B4), Ester (A5B5), Ester (A8-B8), Ester (A9-B9), Ester (A11-B11), EsterB1 (B1), EsterB6 (B6), EsterB7 (B7), EsterB10 (B10) and EsterB12 (B12) [5,9,10,11]
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