EVOLUTION OF RESISTANCE IN CULEX-PIPIENS - ALLELE REPLACEMENT AND CHANGING ENVIRONMENT

Fixation of adaptive mutations in populations is often constrained by pleiotropic fitness costs. The evolutionary pathways that compensate s uch fitness disadvantages are either the occurrence of modifier genes or replacement of the adaptive allele by less costly ones. In this con text, 23 years of evolution of insecticide resistance genes in the mos quito Culex pipiens from southern France are analyzed. The aim of this study is to answer the following points. Is there a fitness cost asso ciated with these resistance genes in natural populations? Does evolut ion proceed through allele replacement or through selection of modifie rs? And finally, how do environmental changes affect the evolution of resistance genes? Samples from the same transect, crossing the boundar y between an insecticide-treated and a nontreated area, are analyzed. Clinal analyses indicate a variable fitness cost among the resistance genes and show that allele replacement has been the primary mechanism of resistance evolution in this area. It is also shown that replacemen t was probably due to environmental changes corresponding to modificat ion in pesticide-treatment intensity.