| Summary: | <p>Abstract</p> <p>Background</p> <p>In China, large amounts of chemical insecticides are applied in fields or indoors every year, directly or indirectly bringing selection pressure on vector mosquitoes. <it>Culex pipiens </it>complex has evolved to be resistant to all types of chemical insecticides, especially organophosphates, through carboxylesterases. Six resistant carboxylesterase alleles (<it>Ester</it>) were recorded previously and sometimes co-existed in one field population, representing a complex situation for the evolution of <it>Ester </it>genes.</p> <p>Results</p> <p>In order to explore the evolutionary scenario, we analyzed the data from an historical record in 2003 and a recent investigation on five <it>Culex pipiens pallens </it>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 <it>Ester </it>alleles, <it>Ester<sup>B1</sup></it>, <it>Ester<sup>2</sup></it>, <it>Ester<sup>8</sup></it>, <it>Ester<sup>9</sup></it>, <it>Ester<sup>B10</sup></it>, and <it>Ester<sup>11 </sup></it>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 <it>Ester<sup>8 </sup></it>and <it>Ester<sup>9 </sup></it>positively correlated with resistance to four insecticides, and <it>Ester<sup>B10 </sup></it>to one insecticide. The occurrences of these three alleles were positively correlated, while the occurrence of <it>Ester<sup>B1 </sup></it>was negatively correlated with <it>Ester<sup>8</sup></it>, indicating an allelic competition.</p> <p>Conclusion</p> <p>Our analysis suggests that one insecticide can select multiple <it>Ester </it>alleles and one <it>Ester </it>allele can work on multiple insecticides. The evolutionary scenario of carboxylesterases under insecticide selection is possibly "one to many".</p>
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