Resistance Gene Replacement in the Mosquito Culex pipiens: Fitness Estimation From Long-Term Cline Series

Abstract

How adaptation appears and is later refined by natural selection has been the object of intense theoretical work. However, the testing of these theories is limited by our ability to estimate the strength of natural selection in nature. Using a long-term cline series, we estimate the selection coefficients acting on different alleles at the same locus to analyze the allele replacement observed in the insecticide resistance gene Ester in the mosquito Culex pipiens in the Montpellier area, southern France. Our method allows us to accurately account for the resistance allele replacement observed in this area since 1986. A first resistance allele appeared early, which was replaced by a second resistance allele providing the same advantage but at a lower cost, itself being replaced by a third resistance allele with both higher advantage and cost. It shows that amelioration of the adaptation (here resistance to insecticide) through allele replacement was successively achieved by selection of first a generalist allele (i.e., with a low fitness variance across environments) and later a specialist allele (i.e., with a large fitness variance across environments). More generally, we discuss how precise estimates of the strength of selection obtained from field data help us understand the process of amelioration of adaptation.

Adaptive deletion in resistance gene duplications in the malaria vector Anopheles gambiae

“…It was simultaneously maximized ( L max ) for each sample using a simulated annealing algorithm (Labbé, Sidos, Raymond, & Lenormand, 2009; Lenormand, Guillemaud, Bourguet, & Raymond, 1998; Milesi, Lenormand, Lagneau, Weill, & Labbé, 2016). For each allele frequency, the support limits (SL) were calculated as the minimum and maximum values that it could take without significantly decreasing the likelihood (Labbé et al., 2009; Milesi et al., 2016); SL are roughly equivalent to 95% confidence intervals. Recursions and likelihood maximization algorithms were written and compiled with Lazarus v1.0.10 (http://www.lazarus.freepascal.org/).…”

Role of dispersal in resistance evolution and spread

“…Case study: Culex pipiens resistance allele clines An especially well studied case of the spatial and temporal distribution of resistance alleles in established populations comes from the mosquito Culex pipiens in the south of France [3][4][5][6]. In this case, the dynamics of resistance were studied at a small geographical scale relative to dispersal distances.…”

“…This rate of dispersal was sufficient to rapidly reestablish selectionmigration equilibrium each year at both loci [4]. In the case of the Ester locus in southern France, the Ester 2 [ 6 7 _ T D $ D I F F ] resistance allele increased in frequency between 1999 and 2002 but did not replace Ester 4 despite a higher level of resistance, because it also imposed a higher fitness cost in the absence of insecticide [5]. After the use of OPs was discontinued in 2007, the Ester 2 allele was rapidly lost from the population.…”

A unified approach to the estimation and interpretation of resistance costs in plants

“…In the case of resistance, the shape of the cline step depends on the fitness difference among the SS, RS and RR genotypes in the regions with the selective pressure and the regions without the selective pressure, as well as the amount of gene flow between the two types of regions (Lenormand et al, 1999;Lenormand and Raymond, 2000;Labbé et al, 2009).…”