SHORT COMMUNICATION Insecticide Resistance Genes in Culex pipiens (Diptera: Culicidae) from Italy: Esterase B Locus at the DNA Level CARLO SEVERINI, MARINO MARINUCCI, AND MICHEL RAYMOND 1 Department of Parasitology, Istituto Superiore di Sanita, Rome, Italy J. Med. Entomol. 31(3): 496-499 (1994) ABSTRACT Restriction fragment length polymorphism (RFLP) analysis at the esterase B locus was performed in samples from four populations of Italian Culex pipiens pipiens L. to identify correctly, at the molecular level, the resistance gene. The absence of the amplified B2 haplotype was confirmed, indicating that it has not increased in frequency after its detection in continental Italy around 1985. The B4 haplotype was detected in all populations at the same frequencies as a previous starch gel survey. The possibility that B4 amplification has occurred only once and has migrated across western Mediterranean region is supported by the data. Al also was detected indirectly by the presence of its closely linked B allele. The B5 haplotype apparently was absent. No amplification at the esterase B locus was detected, except B4, indicating a low rate of occurrence of new amplification at this locus. KEY WORDS mosquitoes, insecticide resistance genes, amplification ORGANOPHOSPHORUS INSECTICIDE (OP) resis- tance in the Culex pipiens L. mosquito complex is always associated with overproduced es- terases. This does not exclude the existence of other resistance mechanisms, such as the insen- sitive acetylcholinesterase found in southern Eu- rope and the Caribbean Islands (Raymond et al. 1986, 1987; Bisset et al. 1990; Severini et al. 1993). However, whenever there is only one OP resistance mechanism, it is always an overpro- duced esterase. For example, in southern France, the first OP resistance mechanism dis- covered was the overproduced esterase Al, first named Est-3 A (Pasteur et al. 1981a, b; Raymond et al. 1992). In Africa, the associated overpro- duced esterases A2 and B2 are very common and are the oldest OP resistance mechanisms de- scribed on this continent (Curtis & Pasteur 1981). In North America, the rise of OP resis- tance was explained by the occurrence of the overproduced Bl esterase, first named Est-B A (Georghiou et al. 1980, Pasteur et al. 1981b). All of the known overproduced esterases in- volved in OP resistance are coded at two loci, A and B. For B esterases, overproduction is ex- plained by the stable amplification of the gene plus flanking sequences (Mouche's et al. 1986; Raymond et al. 1989, 1993; Poirie et al. 1992). 1 Laboratoire G6n6tique et Environnement, Institut des Sci- ences de l'Evolution (URA), Universite Montpellier II. Mont- pellier, France. The various alleles at each locus are designated by a number. Apparently, some OP resistance genes of C. pipiens have been disseminated circumglobally. The associated A2 and B2 overproduced es- terases have spread worldwide and are still in- vading new territories (Raymond et al. 1991, Rivet et al. 1993). A2 and B2 were detected in an OP treated area in Italy (Callaghan 1989, Bon- ning et al. 1991), but apparently disappeared when OP treatment was discontinued (Severini et al. 1993). Other overproduced esterases have a more restricted geographical distribution. Al along with the associated A4-B4 and A5-B5 only are found in the Mediterranean region (Poiri6 et al. 1992, Ben Cheikh & Pasteur 1993, Raymond & Marquine 1994). A survey of OP resistance genes in continental Italy (Severini et al. 1993) recently detected several overproduced es- terases by starch electrophoresis (Al, A4-B4, or A5-B5) and an insensitive acetylcholinesterase. As a result of the identical electrophoretical mi- gration of A4-B4 or A5-B5 esterases (Poiri6 et al. 1992), their correct identification is not possible by electrophoresis. Because of the OP resistance provided by the A4-B4 or A5-B5 is very different (Poirie et al. 1992), this point is important for field resistance management. Moreover, they represent the most common overproduced ester- ase in continental Italy. The present study was undertaken to identify definitively the various Italian overproduced es- terases by establishing the DNA RFLP pattern at 0022-2585/94/0496-0499$02.00/0 © 1994 Entomological Society of America Downloaded from https://academic.oup.com/jme/article/31/3/496/850301 by guest on 11 April 2022