Infertility and fecundity loss of Wolbachia-infected Aedes aegypti hatched from quiescent eggs is expected to alter invasion dynamics

Abstract

The endosymbiotic bacterium Wolbachia shows viral blocking in its mosquito host, leading to its use in arboviral disease control. Releases with Wolbachia strains wMel and wAlbB infecting Aedes aegypti have taken place in several countries. Mosquito egg survival is a key factor influencing population persistence and this trait is also important when eggs are stored prior to releases. We therefore tested the viability of mosquitoes derived from Wolbachia wMel and wAlbB-infected as well as uninfected eggs after long-term storage under diurnal temperature cycles of 11-19°C and 22-30°C. Eggs stored at 11-19°C had higher hatch proportions than those stored at 22-30°C. Adult Wolbachia density declined when they emerged from eggs stored for longer, which was associated with incomplete cytoplasmic incompatibility (CI) when wMel-infected males were crossed with uninfected females. Females from stored eggs at both temperatures continued to show perfect maternal transmission of Wolbachia, but storage reduced the fecundity of both wMel and wAlbB-infected females relative to uninfected mosquitoes. Furthermore, we found a very strong negative impact of the wAlbB infection on the fertility of females stored at 22-30°C, with almost 80% of females hatching after 11 weeks of storage being infertile. Our findings provide guidance for storing Wolbachia-infected A. aegypti eggs to ensure high fitness adult mosquitoes for release. Importantly, they also highlight the likely impact of egg quiescence on the population dynamics of Wolbachia-infected populations in the field, and the potential for Wolbachia to suppress mosquito populations through cumulative fitness costs across warm and dry periods, with expected effects on dengue transmission.

Fig 1. Quiescent egg viability depends on Wolbachia infection type and storage temperature.

Survival of eggs based on hatch proportions of uninfected, wMel-infected and wAlbB-infected Aedes aegypti eggs stored under cycling temperatures of 11–19°C or 22–30°C for 0, 1, 4, 8, 11, 14 or 16 weeks. Shaded areas represent 95% confidence intervals.

Fig 2. Wolbachia density declines with egg storage.

Box plots of log relative Wolbachia density of wMel-infected or wAlbB-infected 2-day-old female mosquitoes hatched from eggs stored under temperature cycles of 11–19°C or 22–30°C for 0, 8, 11, 14 or 16 weeks. Points represent individuals.

Fig 3. Reduced fertility of Wolbachia-infected warm-stored females crossed to Wolbachia-infected unstored males.

(A) Number of eggs laid by females and (B) their hatch proportions. Wolbachia-infected females were derived from eggs stored under 26°C for less than three weeks (control) or a cycling temperature of 11–19°C for 14 weeks or 22–30°C for 11 (wMel-infected) or 10 (wAlbB-infected) weeks. Each data point represents the total number of eggs laid and their hatch proportion from a replicate cage of five females and five males.

Fig 4. Female fertility loss when wAlbB-infected females derived from stored eggs.

Females hatched at each time point were crossed to males from the same treatment (Control), crossed to males with the same Wolbachia infection type that stored under 26°C for less than three weeks (CI rescue) and crossed to uninfected males stored under 26°C for less than three weeks (Maternal). Each data point is based on the proportion of 22–42 tested females.

Fig 5. Fecundity and egg hatch of fertile females derived from stored eggs crossed to different male lines.

(A) Fecundity and (B) egg hatch proportions of eggs from female mosquitoes that were derived from eggs stored under a cycling temperature of 22–30°C for 0, 6, 9 or 11 weeks. Females hatched at each time point were crossed to males from the same treatment (Control), crossed to males with the same Wolbachia infection type that stored under 26°C for less than three weeks (CI rescue) and crossed to uninfected males stored under 26°C for less than three weeks (Maternal). The data exclude females that did not lay eggs (see Fig 4). Error bars represent means ± standard errors.