Gene expression patterns and sequence polymorphisms associated with mosquito resistance to Bacillus thuringiensis israelensis toxins

Abstract

BackgroundDespite the intensive use of Bacillus thuringiensis israelensis (Bti) toxins for mosquito control, little is known about the long term effect of exposure to this cocktail of toxins on target mosquito populations. In contrast to the many cases of resistance to Bacillus thuringiensis Cry toxins observed in other insects, there is no evidence so far for Bti resistance evolution in field mosquito populations. High fitness costs measured in a Bti selected mosquito laboratory strain suggest that evolving resistance to Bti is costly. The aim of the present study was to identify transcription level and polymorphism variations associated with resistance to Bti toxins in the dengue vector Aedes aegypti. We used RNA sequencing (RNA-seq) for comparing a laboratory-selected strain showing elevated resistance to Bti toxins and its parental non-selected susceptible strain. As the resistant strain displayed two marked larval development phenotypes (slow and normal), each phenotype was analyzed separately in order to evidence potential links between resistance mechanisms and mosquito life-history traits.ResultsA total of 12,458 genes were detected of which 844 were differentially transcribed between the resistant and susceptible strains. Polymorphism analysis revealed a total of 68,541 SNPs of which 12,571 SNPs exhibited more than 40% frequency difference between the resistant and susceptible strains, affecting 2,953 genes. Bti resistance is associated with changes in the transcription level of enzymes involved in detoxification and chitin metabolism. Among previously described Bti-toxin receptors, four alkaline phosphatases (ALPs) were differentially transcribed between resistant and susceptible larvae, and non-synonymous changes affected the protein sequence of one cadherin, six aminopeptidases (APNs) and four α-amylases. Other putative Cry receptors located in lipid rafts, such as flotillin and glycoside hydrolases, were under-transcribed and/or contained non-synonymous substitutions. Finally, immunity-related genes showed contrasted transcription and polymorphisms patterns between the two developmental resistant phenotypes, suggesting the existence of trade-offs between Bti-resistance, life-history traits and immunity.ConclusionsThe present study is the first to analyze the whole transcriptome of Bti-resistant mosquitoes by RNA-seq, shedding light on the importance of studying both transcription levels and sequence polymorphism variations to get a comprehensive view of insecticide resistance.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-926) contains supplementary material, which is available to authorized users.

The genetic architecture of a complex trait: Resistance to multiple toxins produced by Bacillus thuringiensis israelensis in the dengue and yellow fever vector, the mosquito Aedes aegypti

“…In total, 10 (4.3%) of the studied supercontigs were found 22 within or near robust QTL regions for Bti resistance, or harbor markers statistically associated with this trait. Half of these displayed candidate genes as suggested by their function or by prior evidence from expression studies (Després et al, 2014;Tetreau et al, 2012; Table 4) and/or outlier analyses (Bonin et al, 2009;Paris et al, 2013; Table 4). For example, supercontigs 1.25 and 1.39 were detected twice independently, i.e.…”

“…This category included 41 genes coding for putative toxin receptors (cadherins, aminopeptidases, alpha-amylases) and 84 enzyme-coding genes potentially mediating toxin activation or degradation (serine proteases, chymotrypsins, trypsins). The second category corresponded to genes displaying altered transcription levels in the LiTOX strain and thus potentially associated with resistance to Bti (Després et al, 2014;Tetreau et al, 2012). These two categories brought together 231 candidate genes distributed over 107 supercontigs.…”

“…Although Cry toxins have a highly conserved structure, they are known to bind to different midgut membrane receptors such as cadherins, aminopeptidases N (APNs), alkaline phosphatases (ALPs) and alpha-amylases (Fernandez-Luna et al, 2010;Likitvivatanavong et al, 2011;Pigott and Ellar, 2007). Based on the mode of action of Cry toxins, it can be predicted that mosquito resistance to Bti is a complex phenotype involving several genes simultaneously (Ben-Dov, 2014;Després et al, 2014;.…”

“…Genomic scans based on markers associated with transposable elements (Bonin et al, 2009), on AFLPs sequenced with the 454 technology and on SNPs (Paris et al, 2013) detected several genomic regions associated with resistance which contain candidate genes (one cadherin and two aminopeptidases) showing patterns of sequence variability consistent with the action of positive selection. Recently, RNAseq analyses revealed that the transcriptome of resistant mosquitoes differs dramatically from that of their susceptible counterparts, not only in the expression levels of many gene families (including detoxication and immunity genes), but also in the sequence polymorphism of expressed genes (Després et al, 2014). Finally, the resistant strain showed an increase in larval midgut proteolytic 6 activities (Tetreau et al, 2013).…”

“…Finally, the resistant strain showed an increase in larval midgut proteolytic 6 activities (Tetreau et al, 2013). However, it remains to be investigated whether these changes in physiology and metabolism associated with Bti resistance are due to pleiotropic effects of the resistance alleles, or to resource reallocation among different metabolic pathways (Després et al, 2014).…”