A decade of stability for wMel Wolbachia in natural Aedes aegypti populations

Mosquitoes carrying Wolbachia endosymbionts are being released in many countries for arbovirus control. The w Mel strain of Wolbachia blocks Aedes -borne virus transmission and can spread throughout mosquito populations by inducing cytoplasmic incompatibility. Aedes aegypti mosquitoes carrying w Mel were first released into the field in Cairns, Australia, over a decade ago, and with wider releases have resulted in the near elimination of local dengue transmission. The long-term stability of Wolbachia effects is critical for ongoing disease suppression, requiring tracking of phenotypic and genomic changes in Wolbachia infections following releases. We used a combination of field surveys, phenotypic assessments, and Wolbachia genome sequencing to show that w Mel has remained stable in its effects for up to a decade in Australian Ae. aegypti populations. Phenotypic comparisons of w Mel-infected and uninfected mosquitoes from near-field and long-term laboratory populations suggest limited changes in the effects of w Mel on mosquito fitness. Treating mosquitoes with antibiotics used to cure the w Mel infection had limited effects on fitness in the next generation, supporting the use of tetracycline for generating uninfected mosquitoes without off-target effects. w Mel has a temporally stable within-host density and continues to induce complete cytoplasmic incompatibility. A comparison of w Mel genomes from pre-release (2010) and nine years post-release (2020) populations show few genomic differences and little divergence between release locations, consistent with the lack of phenotypic changes. These results indicate that releases of Wolbachia -infected mosquitoes for population replacement are likely to be effective for many years, but ongoing monitoring remains important to track potential evolutionary changes. Author summary Wolbachia are endosymbionts that can block the transmission of arboviruses by mosquitoes. Aedes aegypti mosquitoes carrying the w Mel strain of Wolbachia have been released in ‘population replacement’ interventions, which aim to establish w Mel in mosquito populations, thereby reducing their ability to spread disease. Wolbachia population replacement programs began only a decade ago, raising uncertainty about their long-term effectiveness. Here we provide a comprehensive assessment of the long-term stability of w Mel from the very first Wolbachia population replacement release. We show that there is no evidence for changes in the phenotypic effects of w Mel in mosquitoes, and confirm that the w Mel genome has changed very little in the decade since field releases began. w Mel remains at high levels within mosquitoes, suggesting that its ability to block virus transmission has been retained. Our data provides confidence that Wolbachia population replacement releases will provide ongoing protection against arbovirus transmission.