Heritable symbionts that protect their hosts from pathogens have been described in a wide range of insect species. By reducing the incidence or severity of infection, these symbionts have the potential to reduce the strength of selection on genes in the insect genome that increase resistance. Therefore, the presence of such symbionts may slow down the evolution of resistance. Here we investigated this idea by exposing Drosophila melanogaster populations to infection with the pathogenic Drosophila C virus (DCV) in the presence or absence of Wolbachia, a heritable symbiont of arthropods that confers protection against viruses. After nine generations of selection, we found that resistance to DCV had increased in all populations. However, in the presence of Wolbachia the resistant allele of pastrel—a gene that has a major effect on resistance to DCV—was at a lower frequency than in the symbiont-free populations. This finding suggests that defensive symbionts have the potential to hamper the evolution of insect resistance genes, potentially leading to a state of evolutionary addiction where the genetically susceptible insect host mostly relies on its symbiont to fight pathogens.
Genotype data of populations selected for virus resistance
Columns indicate the generation during selection, the Wolbachia infection status, the number of females with a given genotype (N_CC, N_CT, N_TT), the frequency of pastrel resistant allele (freq_C), the frequency of each genotype (freq_CC, freq_CT, freq_TT) and the number of individuals genotyped (N_individuals).
Genotype_summary.txt
Genotype data of control populations not exposed to the virus
Columns indicate the generation during selection, the Wolbachia infection status, the number of females with a given genotype (N_CC, N_CT, N_TT), the frequency of pastrel resistant allele (freq_C), the frequency of each genotype (freq_CC, freq_CT, freq_TT) and the number of individuals genotyped (N_individuals).
Genotype_summary_control.txt
Genotype data of populations tested for DCV resistance after selection
Columns indicate the generation during selection, the Wolbachia infection status, the number of females with a given genotype (N_CC, N_CT, N_TT), the frequency of pastrel resistant allele (freq_C), the frequency of each genotype (freq_CC, freq_CT, freq_TT) and the number of individuals genotyped (N_individuals).
Pastrel_Genotype_Phenotypic_assay.txt
Phenotypic data on DCV resistance of populations after selection
Columns indicate the selection treatment, the Wolbachia infection status, the tetracycline treatment after selection, the infection treatment (stabbing with Ringer's solution or DCV), the replicate population, the replicate vial, the number of infected flies in a vial (N_flies) and the cumulative number of dead flies post-infection on a given day.
Pastrel_phenotypic_assay.txt
Phenotypic data on effect of infection procedure in control populations
Columns indicate the selection treatment, the Wolbachia infection status, the infection treatment (no stabbing, stabbing with Ringer's solution or DCV), the population, the replicate vial, the number of infected flies in a vial (N_flies) and the cumulative number of dead flies post-infection on a given day.
pastrel_genotype_phenotype_survival.txt
Genotype data of surviving flies 15 days post-infection in control populations (test of the infection procedure)
Columns indicate the infection treatment, the Wolbachia infection status, the population, the number of females with a given genotype (N_CC, N_CT, N_TT), the frequency of pastrel resistant allele (freq_C), the frequency of each genotype (freq_CC, freq_CT, freq_TT) and the number of individuals genotyped (N_individuals).
Genotype_summary_infection_procedure.txt