Background

Insect pest control programs often use periods of insecticide treatment with intermittent breaks, to prevent fixing of mutations conferring insecticide resistance. Such mutations are typically costly in an insecticide free environment, and their frequency is determined by the balance between insecticide treatment and cost of resistance. Ace, a key gene in neuronal signaling, is a prominent target of many insecticides and across several species three amino acid replacements (I161V, G265A and F330Y) provide resistance against several insecticides. Because temperature disturbs neuronal signaling homeostasis, we reasoned that the cost of insecticide resistance could be modulated by ambient temperature.

Results

Experimental evolution of a natural Drosophila simulans population at hot and cold temperature regimes uncovered a surprisingly strong effect of ambient temperature. In the cold temperature regime, the resistance mutations were strongly counter selected (s= -0.055), but in a hot environment the fitness costs of resistance mutations were reduced by almost 50% (s=-0.031). We attribute this unexpected observation to the advantage of the reduced enzymatic activity of resistance mutations in hot environments.

Conclusion

We show that fitness costs of insecticide resistance genes are temperature-dependent, and suggest that duration of insecticide-free periods need to be adjusted for different climatic regions to reflect these costs. We suggest that such environment-dependent fitness effects may be more common than previously assumed and pose a major challenge for modeling climate change.

PHENOTYPIC ANALYSIS

• Fecundity_stats.R: Statistical analysis of the fecundity phenotypic assay
• Insecticides_stats.R: Statistical analysis of the insecticide resistance phenotypic assay
• Viability_stats.R: Statistical analysis of the viability phenotypic assay

GENOMIC ANALYSIS

• HaplotypeStructure.R: Analysis of the ancestral haplotypes (structure, resistance mutation status, marker SNPs, and diversity measurements); output = marker SNPs between haplotype class 1, and 2 (n=166)
• Pool-seq.R: Analysis of the experimental population in the hot and cold regime (raw allele frequency trajectories, resistance mutation trajectories, haplotype class frequency trajectories, selection coefficients (with poolSeq v0.3.2; https://github.com/popgenvienna)); output = replicate-specific haplotype class frequencies (1a, 1b, and 2) for hot, and cold regime

AVAILABLE DATA

• AF-DsimPort-ColdRegime-Ace.txt: allele frequencies of SNPs (+/- 200 kb around the Ace locus) in the experimental D.simulans Portugal populations evolving in the cold regime; frequency is polarised for the rising allele; used in Pool-seq.R
• AF-DsimPort-HotRegime-Ace.txt: allele frequencies of SNPs (+/- 200 kb around the Ace locus) in the experimental D.simulans Portugal populations evolving in the hot regime; frequency is polarised for the rising allele; used in Pool-seq.R
• Class-Markers.txt: output of HaplotypeStructure.R ; Chromosome/Postition/Allele information of the marker SNPs distinguishing class 1 from class 2
• CMH-DsimPort-ColdRegime-Ace.txt: CMH test results (+/- 200 kb around the Ace locus) of generation 51 vs. generation 0 of the experimental D.simulans Portugal populations evolving in the cold regime; used in Figure 4
• CMH-DsimPort-HotRegime-Ace.txt: CMH test results (+/- 200 kb around the Ace locus) of generation 59 vs. generation 0 of the experimental D.simulans Portugal populations evolving in the hot regime; used in Figure 4
• dsimM252.1.1.exon_CDS_utr.clean.gtf: genome annotation D.simulans (M252 reference strain); used in HaplotypeStructure.R
• dsimPort*: SNP calls of the ancestral haplotypes; used in HaplotypeStructure.R
• Fecundity.txt: raw Fecundity data
• HaploClasses-DsimPort-ColdRegime-Ace.txt: output of Pool-seq.R; haplotype class trajectories in the experimental D.simulans Portugal populations evolving in the cold regime
• HaploClasses-DsimPort-HotRegime-Ace.txt: output of Pool-seq.R; haplotype class trajectories in the experimental D.simulans Portugal populations evolving in the hot regime
• Malathion.txt: raw Insecticide resistance data, Malathion
• Ne-DsimPort-ColdRegime.txt: used in Pool-seq.R; replicate-specific effective population size estimates between generation 0 and generation 51 for the experimental D.simulans populations evolving in the cold regime
• Ne-DsimPort-HotRegime.txt: used in Pool-seq.R; replicate-specific effective population size estimates between generation 0 and generation 59 for the experimental D.simulans populations evolving in the hot regime
• Propoxur.txt: raw insecticide resistance data, Propoxur
• Viability.txt: raw Viability data