Matching the genetics of released and local Aedes aegypti populations is critical to assure Wolbachia invasion

Abstract

Background: Traditional vector control approaches such as source reduction and insecticide spraying have limited effect on reducing Aedes aegypti population. The endosymbiont Wolbachia is pointed as a promising tool to mitigate arbovirus transmission and has been deployed worldwide. Models predict a rapid increase on the frequency of Wolbachia-positive Ae. aegypti mosquitoes in local settings, supported by cytoplasmic incompatibility (CI) and high maternal transmission rate associated with the wMelBr strain.

Methodology/principle findings: Wolbachia wMelBr strain was released for 20 consecutive weeks after receiving >87% approval of householders of the isolated community of Tubiacanga, Rio de Janeiro. wMelBr frequency plateued~40% during weeks 7-19, peaked 65% but dropped as releases stopped. A high (97.56%) maternal transmission was observed. Doubling releases and deploying mosquitoes with large wing length and low laboratory mortality produced no detectable effects on invasion trend. By investigating the lab colony maintenance procedures backwardly, pyrethroid resistant genotypes in wMelBr decreased from 68% to 3.5% after 17 generations. Therefore, we initially released susceptible mosquitoes in a local population highly resistant to pyrethroids which, associated with the over use of insecticides by householders, ended jeopardizing Wolbachia invasion. A new strain (wMelRio) was produced after backcrossing wMelBr females with males from field to introduce mostly pyrethroid resistance alleles. The new strain increased mosquito survival but produced relevant negative effects on Ae. aegypti fecundity traits, reducing egg clutche size and egg hatch. Despite the cost on fitness, wMelRio successful established where wMelBr failed, revealing that matching the local population genetics, especially insecticide resistance background, is critical to achieve invasion.

Conclusions/significance: Local householders support was constantly high, reaching 90% backing on the second release (wMelRio strain). Notwithstanding the drought summer, the harsh temperature recorded (daily average above 30°C) did not seem to affect the expression of maternal transmission of wMel on a Brazilian background. Wolbachia deployment should match the insecticide resistance profile of the wild population to achieve invasion. Considering pyrethroid-resistance is a widely distributed phenotype in natural Ae. aegypti populations, future Wolbachia deployments must pay special attention in maintaining insecticide resistance in lab colonies for releases.

Fig 1. The frequency of wMelBr and wMelRio strains during Wolbachia deployment in Tubiacanga.

Red area represents the wMelBr releases and Green area the wMelRio releases.

Fig 2. Pyrethroid resistance in two Wolbachia-infected strains (wMelBr and wMelRio) and three field Aedes aegypti populations (Tubiacanga, Jurujuba and Urca).

The susceptible strain Rockefeller was used as a calibration control. A) Mortality profile of Ae. aegypti adult females exposed to the pyrethroid deltamethrin. B) Allelic frequency of population samples; numbers above bars indicate the sum of ‘resistance genotypes’ to pyrethroids, In blue Na_VS (1016 Val⁺ + 1534 Phe⁺), in orange Na_VR1 (1016 Val⁺ + 1534 Cys^kdr) and in red Na_VR2 (1016 Ile^kdr + 1534 Cys^kdr).

Fig 3. Frequency of kdr alleles during Wolbachia releases in Tubiacanga.

At least 60 mosquitoes were analyzed per time point. Field-caught mosquitoes (A) without Wolbachia, and (B) infected with Wolbachia. In blue Na_VS (1016 Val⁺ + 1534 Phe⁺), in orange Na_VR1 (1016 Val⁺ + 1534 Cys^kdr) and in red Na_VR2 (1016 Ile^kdr + 1534 Cys^kdr).

Fig 4

Survival curves of Aedes aegypti females from: (A) seven field populations from the backcrossing to produce an insecticide resistant line, (B) populations infected and uninfected with Wolbachia. (C) Number of eggs laid per Aedes aegypti female from the seven populations tested and the frequency of kdr mutation, represented by the red line.

Fig 5. Mean and standard deviation of Aedes aegypti egg hatching during the first four clutches of the seven populations from the backcrossing.

Different letters indicate significant differences on egg hatch averages.

Fig 6. Genetic similarities and differences among wMelBr, wMelRio, Tubiacanga (Rio de Janeiro, Brazil) and Gordonvale (Cairns, Australia) Aedes aegypti mosquitoes as visualized by a DAPC analysis on >5000 SNPs, with data plotting samples for the two main PCs.