Abstract: 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 whether the esterase B1 identified by electrophoretic studies in Culex pipiens mosquitoes from different countries is over-produced 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. In addition to the highly amplified 2.1kb fragment, EcoRI digestion led to two other restriction fragments of low amplification in mosquitoes from the Americas but notfrom China. It is concluded that B1 amplification has a unique origin, either in America or in Asia, and has subsequently spread on a large scale by migration. This migration is more limited than that of A2-B2 esterases, since B1 is confined to the Americas, Carribean and part of China, whereas the A2-B2 distribution now includes the Americas, Carribean, Asia, Africa, Pacific islands and Europe.

Key words: Insecticides, resistant, Culex pipiens, esterase B1 amplification, RFLP analysis