The spread of Wolbachia through mosquito populations

Abstract

In many regions of the world, mosquito-borne viruses pose a growing threat to human health. As an alternative to traditional control measures, the bacterial symbiont Wolbachia has been transferred from Drosophila into the mosquito Aedes aegypti, where it can block the transmission of dengue and Zika viruses. A recent paper has reported large-scale releases of Wolbachia-infected Ae. aegypti in the city of Cairns, Australia. Wolbachia, which is maternally transmitted, invaded and spread through the populations due to a sperm-egg incompatibility called cytoplasmic incompatibility. Over a period of 2 years, a wave of Wolbachia infection slowly spread out from 2 release sites, demonstrating that it will be possible to deploy this strategy in large urban areas. In line with theoretical predictions, Wolbachia infection at a third, smaller release site collapsed due to the immigration of Wolbachia-free mosquitoes from surrounding areas. This remarkable field experiment has both validated theoretical models of Wolbachia population dynamics and demonstrated that this is a viable strategy to modify mosquito populations.

Fig 1. Cytoplasmic incompatibility.

(A) When a Wolbachia-infected male (red) mates with an uninfected female (black), a sperm–egg incompatibility means that some or all of the embryos die. Therefore, infected females produce more offspring than uninfected females (red versus black mosquitoes). (B) This reproductive advantage depends on the prevalence of Wolbachia in the population, because when Wolbachia is rare, females are unlikely to mate with infected males. The Wolbachia strain in Aedes aegypti carries a physiological cost, reducing the fecundity of infected females. If this cost exceeds the advantage of cytoplasmic incompatibility, then the infection is lost from the population. This creates a threshold prevalence below which Wolbachia is lost and above which it invades the population. This cartoon assumes infected females transmit Wolbachia to all their offspring. Image credit: https://doi.org/10.1093/gbe/evw018.