Genome scan in the mosquito Aedes rusticus: population structure and detection of positive selection after insecticide treatment

Abstract

Identification of genes involved in local adaptation is particularly challenging for species functioning as a network of interconnected populations undergoing frequent extinctions-recolonizations, because populations are submitted to contrasted evolutionary pressures. Using amplified fragment length polymorphism markers, population genetic structure of the mosquito Aedes rusticus was analysed in five geographical areas of the French Rhône-Alpes region. We included a number of sites that were treated with the bio-insecticide Bacillus thuringiensis israelensis (Bti) for more than 15 years. Analysis of molecular variance revealed that most of the genetic variability was found within populations (96%), with no significant variation among geographical areas, although variation among populations within areas (4%) was significant. The global genetic differentiation index F(ST) was low (0.0366 +/- 0.167). However, pairwise F(ST) values were significant and no isolation-by-distance at the regional level was observed, suggesting a metapopulation structure in this species. Bti-treatment had no effect on genetic structure and on within-population genetic diversity. Potential signatures of positive selection associated with Bti-treatment were detected for five loci, even though toxicological bioassays performed on field-collected larvae showed no significant difference in mortality between Bti-treated and nontreated sites. The difficulty of detecting moderate resistance in field-collected larvae together with possible differential persistence of toxins in the environment may explain our inability to detect a toxicological response to Bti in treated sites. The evidence for positive selection occurring at several genomic regions suggests a first step towards Bti resistance in natural mosquito populations treated with this bio-insecticide. Furthermore, this signal was detectable using genomic tools before any toxicological evidence for resistance could be identified.

Recent range expansion and agricultural landscape heterogeneity have only minimal effect on the spatial genetic structure of the plant pathogenic fungus Mycosphaerella fijiensis

“…As no decrease in the expected heterozygosity and genotypic diversity was detected in the commercial area, we argue that the population size inside this area remains large enough to avoid the occurrence of a genetic drift effect. Alternatively, gene flow from the outside to the inside of the commercial plantation might continuously restore genetic diversity, as was suggested in the case of the mosquito Aedes rusticus when sites treated with bioinsecticides were compared with non-treated sites (Paris et al, 2010). The occurrence of sexual reproduction in M. fijiensis in commercial plantations is limited, mainly because of regular elimination of the necrotic tissues where sexual ascospores are produced.…”

Using the Bio-Insecticide Bacillus Thuringiensis Israelensis in Mosquito Control

“…A major point to address today is thus how to further improve the product and even more importantly how to make it sustainable. In natural mosquito populations, no consistent resistance has been detected even after long periods of repeated treatment with Bti toxins, but recent studies suggested that moderate Bti resistance may occur locally (Paul et al, 2005;Boyer et al, 2007;Paris et al, 2010). Furthermore, we obtained in only 22 generations of selection using environmental Bti (i.e., field collected leaf litters containing Bti toxins), a strain of Ae.…”

“…Resistance to Bti can involve both mutations (in specific proteases, specific membrane receptors), transcriptional regulation (up or down regulation of proteases, of membrane receptors and other co-factors involved in toxin binding, of immune genes…) and post-transcriptional changes (alternative splicing of mRNA…). Given the variety of mechanisms involved in resistance to the multiple toxin contained in Bti, a global screening of genomic /transcriptomic/ proteomic changes in resistant insects should be favoured to a candidate gene approach, in order to tackle all the genes simultaneously involved and their interactions (Bonin et al, 2008, 2009, Paris et al, 2010. Bti containing a mixture of toxins with various modes of action, the resistance is likely to involve many loci with various levels of dominance and various degrees of epistatic interactions.…”

“…Then, new mutations which decrease fitness costs may appear and accumulate. Finally, the replacement of the costly resistant allele by a less costly resistant allele may occur at the same or at another locus (Roush & McKenzie, 1987;Labbe et al, 2009 (Becker & Ludwig, 1993).The lack of evidence for Bti resistance in treated mosquito populations does not mean that resistance to individual toxins is not arising in the field (Paris et al, 2010). Indeed, resistance to individual toxins might evolve in field populations if toxins have different persistence in the environment.…”

Combining Genotype, Phenotype, and Environment to Infer Potential Candidate Genes

“…The first step of an investigation of genetic markers that may signal local adaptation is usually to determine if some loci in a dataset are correlated with local habitat or climatic conditions (Eckert et al 2010a;Paris et al 2010;Poncet et al 2010;Nunes et al 2011). While population genomics can give us valuable clues for which genes are of ecological relevance, a more specific approach, using phenotype, can indicate which genes may be associated with fitnessrelated phenotypic traits, such as in using QTLs (Shaw et al 2007;Lacaze et al 2009;Du et al 2016;McCouch et al 2016).…”