Forty percent of the world's population is at risk of contracting dengue virus, which produces dengue fever with a potentially fatal hemorrhagic form. The wMelPop Wolbachia infection of Drosophila melanogaster reduces life span and interferes with viral transmission when introduced into the mosquito Aedes aegypti, the primary vector of dengue virus. Wolbachia has been proposed as an agent for preventing transmission of dengue virus. Population invasion by Wolbachia depends on levels of cytoplasmic incompatibility, fitness effects, and maternal transmission. Here we characterized these traits in an outbred genetic background of a potential target population of Ae. aegypti using two crossing schemes. Cytoplasmic incompatibility was strong in this background, and the maternal transmission rate of Wolbachia was high. The infection substantially reduced longevity of infected adult females, regardless of whether adults came from larvae cultured under high or low levels of nutrition or density. The infection reduced the viability of diapausing and nondiapausing eggs. Viability was particularly low when eggs were laid by older females and when diapausing eggs had been stored for a few weeks. The infection affected mosquito larval development time and adult body size under different larval nutrition levels and densities. The results were used to assess the potential for wMelPop-CLA to invade natural populations of Ae. aegypti and to develop recommendations for the maintenance of fitness in infected mosquitoes that need to compete against field insects.

Abstract. Wolbachia endosymbionts are potentially useful tools for suppressing disease transmission by Aedes aegypti mosquitoes because Wolbachia can interfere with the transmission of dengue and other viruses as well as causing deleterious effects on their mosquito hosts. Most recent research has focused on the wMel infection, but other infections also influence viral transmission and may spread in natural populations. Here, we focus on the wAlbB infection in an Australian outbred background and show that this infection has many features that facilitate its invasion into natural populations including strong cytoplasmic incompatibility, a lack of effect on larval development, an equivalent mating success to uninfected males and perfect maternal transmission fidelity. On the other hand, the infection has deleterious effects when eggs are held in a dried state, falling between wMel and the more virulent wMelPop Wolbachia strains. The impact of this infection on lifespan also appears to be intermediate, consistent with the observation that this infection has a titer in adults between wMel and wMelPop. Population cage experiments indicate that the wAlbB infection establishes in cages when introduced at a frequency of 22%, suggesting that this strain could be successfully introduced into populations and subsequently persist and spread.

Abstract. There is increasing interest in rearing modified mosquitoes for mass release to control vector-borne diseases, particularly Wolbachia-infected Aedes aegypti for suppression of dengue. Successful introductions require release of high quality mosquitoes into natural populations. Potential indicators of quality are body size and shape. We tested to determine if size, wing/thorax ratio, and wing shape are associated with field fitness of Wolbachia-infected Ae. aegypti. Compared with field-collected mosquitoes, released mosquitoes were larger in size, with lower size variance and different wing shape but similar in wing-thorax ratio and its associated variance. These differences were largely attributed to nutrition and to a minor extent to wMel Wolbachia infection. Survival potential of released female mosquitoes was similar to those from the field. Females at oviposition sites tended to be larger than those randomly collected from BG-Sentinel traps. Rearing conditions should thus aim for large size without affecting wing/thorax ratios.

BackgroundRecent releases have been carried out with Aedes aegypti mosquitoes infected with the wMelPop mosquito cell-line adapted (wMelPop-CLA) strain of Wolbachia. This infection introduced from Drosophila provides strong blockage of dengue and other arboviruses but also has large fitness costs in laboratory tests. The releases were used to evaluate the fitness of released infected mosquitoes, and (following termination of releases) to test for any effects of wMelPop-CLA on wing size and shape when mosquitoes were reared under field conditions.MethodsWe monitored gravid females via double sticky traps to assess the reproductive success of wMelPop-CLA-infected females and also sampled the overall mosquito population post-release using Biogent Sentinel traps. Morphometric analyses were used to evaluate infection effects on wing shape as well as size.ResultsOviposition success as assessed through double sticky traps was unrelated to size of released mosquitoes. However, released mosquitoes with lower wing loading were more successful. Furthermore, wMelPop-CLA-infected mosquitoes had 38.3% of the oviposition success of uninfected mosquitoes based on the predicted infection frequency after release. Environmental conditions affected wing shape and particularly size across time in uninfected mosquitoes, but not in naturally-reared wMelPop-CLA-infected mosquitoes. Although the overall size and shape do not differ between naturally-reared wMelPop-CLA-infected and uninfected mosquitoes, the infected mosquitoes tended to have smaller wings than uninfected mosquitoes during the cooler November in comparison to December.ConclusionThese results confirm the lower fitness of wMelPop-CLA infection under field conditions, helping to explain challenges associated with a successful invasion by this strain. In the long run, invasion may depend on releasing strains carrying insecticide resistance or egg desiccation resistance, combined with an active pre-release population suppression program.

Abstract. The intracellular endosymbiont Wolbachia has been artificially transinfected into the dengue vector Aedes aegypti, where it is being investigated as a potential dengue biological control agent. Invasion of Wolbachia in natural populations depends upon the fitness of Wolbachia-infected Ae. aegypti relative to uninfected competitors. Although Wolbachia infections impose fitness costs on the adult host, effects at the immature stages are less clear, particularly in competitive situations. We look for effects of two Wolbachia infections, wMel and wMelPop, on intra-strain and interstrain larval competition in Ae. aegypti. Development of Wolbachia-infected larvae is delayed in mixed cohorts with uninfected larvae under crowded-rearing conditions. Slow developing wMelPop-infected larvae have reduced adult size compared with uninfected larvae, and larvae with the wMel infection are somewhat larger and have greater viability relative to uninfected larvae when in mixed cohorts. Implications for successful invasion by these Wolbachia infections under field conditions are considered.