The same esterase B1 haplotype is amplified in insecticide-resistant mosquitoes of the Culex pipiens complex from the Americas and China.

In Culex pipiens, overproduction of nonspecific esterases is a common mechanism of resistance to organophosphate insecticides. The esterases are attributed to closely linked loci named A and B, and overproduction of all esterases B is due to gene amplification. In order to determine if the esterase B1 identified by electrophoretic studies in Culex pipiens mosquitoes from different countries is overproduced due to the amplification of the same DNA haplotype, the amplified region encompassing the structural esterase B1 gene was characterized by restriction mapping and RFLP. The same amplified haplotype was found in mosquitoes with an esterase B1 protein, independently of their geographical origin: French Guiana, Venezuela, Puerto Rico, California and China. Large variations in amplification levels were observed. It is concluded that B1 amplification has a unique origin, either in America or in Asia, and has subsequently spread by migration. This migration is more limited than that of A2-B2 esterases, since B1 is confined to the Americas, the Caribbean and part of China, whereas the A2-B2 distribution now includes the Americas, the Caribbean, Asia, Africa, the Pacific Islands and Europe.