Thermal tolerance of mosquito eggs is associated with urban adaptation and human interactions

Abstract

Climate change is expected to profoundly affect mosquito distributions and their ability to serve as vectors for disease, specifically with the anticipated increase in heat waves. The rising temperature and frequent heat waves can accelerate mosquito life cycles, facilitating higher disease transmission. Conversely, higher temperatures could increase mosquito mortality as a negative consequence. Warmer temperatures are associated with increased human density, suggesting a need for anthropophilic mosquitoes to adapt to be more hardy to heat stress. Mosquito eggs provide an opportunity to study the biological impact of climate warming as this stage is stationary and must tolerate temperatures at the site of female oviposition. As such, egg thermotolerance is critical for survival in a specific habitat. In nature, Aedes mosquitoes exhibit different behavioral phenotypes, where specific populations prefer depositing eggs in tree holes and prefer feeding non-human vertebrates. In contrast, others, particularly human-biting specialists, favor laying eggs in artificial containers near human dwellings. This study examined the thermotolerance of eggs, along with adult stages, for Aedes aegypti and Ae. albopictus lineages associated with known ancestry and shifts in their relationship with humans. Mosquitoes collected from areas with higher human population density, displaying increased human preference, and having a human-associated ancestry profile have increased egg viability following high-temperature stress. Unlike eggs, thermal tolerance among adults showed no significant correlation based on the area of collection or human-associated ancestry. This study highlights that the egg stage is likely critical to mosquito survival when associated with humans and needs to be accounted when predicting future mosquito distribution.

Keywords: Aedes sp.; ecological correlation; egg hatching index; egg survival; heat tolerance; human density; mosquitoes.

Figure 1:. Deviation in adult Aedes aegypti survival in comparison to combined average following varied temperature exposures

Male and female adults were exposed to three extreme temperatures for 2 hours (Supplementary figures 1a and 1b) and 6 hours (Supplementary figures 1c and 1d). Following the exposure, the survival of the mosquitoes was evaluated after 2 hours (early) (Supplementary figures 1a and 1c) and 24 hours (later) (Supplementary figures 1b and 1d). For each treatment, 5 replicates were run, with 8 mosquitoes per replicate. The numbers within each heatmap tile represent the proportion survival of adult mosquitoes for a specified population, depending on the temperature treatment (Supplementary tables S2a and S2b). Figures were produced using R 4.2.3 and edited with Inkscape 1.3.

Figure 2:. Deviation in adult Aedes albopictus survival in comparison to combined average following various levels of temperature exposure

Male and female adults Ae. albopictus following exposure to three distinct temperatures for 2 hours (Supplementary figures 1e and 1f) and 6 hours (Supplementary figures 1g and 1h). After these exposures, mosquito survival rates were assessed at 2-hour intervals (early) (Supplementary figures 1e and 1g) and 24-hour intervals (later) (Supplementary figures 1f and 1h). The experimental setup consisted of 5 replicates for each treatment, with each replicate comprising 8 mosquitoes. The numbers on each tile of the heatmap depict the proportion survival for that specific population of mosquitoes following the respective temperature treatment (Supplementary tables S2c and S2d). Figures were produced using R 4.2.3 and edited with Inkscape 1.3.

Figure 3:. Deviation in egg hatching of Aedes aegypti populations in comparison to combined average following different temperature exposures

Egg hatching within the populations after exposure to experimental temperatures for 2 hours (Supplementary figure 4a) and 6 hours (Supplementary figure 4b). Each temperature-treatment combination was experimented with 10–12 replicates with 30–50 eggs. The numbers in each tile depict the proportion of hatching for the respective populations after each temperature exposure (Supplementary tables S4a and S4b). Figures were produced using R 4.2.3 and edited with Inkscape 1.3.

Figure 4:. Deviation in egg hatching of Aedes albopictus populations in comparison to combined average following varying temperature exposures

Egg hatching in the Aedes albopictus populations occurred following exposure to varying temperature regimens for 2 hours (Supplementary figure 4c) and 6 hours (Supplementary figure 4d). Each experimental condition consisted of 10–12 replicates, each replicate having 30–50 eggs. The numerical values within individual tiles of the heatmap denote the proportion of egg hatching for the corresponding populations following each discrete temperature exposure (Supplementary tables S4c and S4d). Figures were produced using R 4.2.3 and edited with Inkscape 1.3.

Figure 5:. Factors influencing egg hatching success in Aedes aegypti

(a) The influence of host preference on egg hatching with a focus on the duration of thermal stress reveals a robust linear trend (p = 0.004). (b) The significant positive effect of ancestral origin on the egg-hatching success of Ae. aegypti mosquito eggs (p = 0.002). (c) The positive influence of density on egg hatching rate reveals that eggs originating from areas with higher human density exhibit higher hatching rates (p = 1.38 × 10⁻⁸). (d) Variables in (a), (b) and (c) can be combined in an index of human specialization syndrome, which accounts for 24% of variation in the final model ([LRT] p = 0.04). (e) Precipitation regimes (p = 0.78) and (f) precipitation during the warmest months of the year (p = 0.24) had no significant association with egg thermotolerance. ‘r’ values denote the correlation coefficient between the egg thermotolerance and the biological factors. R² denotes the proportion of variance for egg thermotolerance associated with the specific biological factor(s). Figures were produced using R 4.2.3 and edited with Inkscape 1.3.