Cross-Generational Effects of Heat Stress on Fitness and Wolbachia Density in Aedes aegypti Mosquitoes

Abstract

Aedes aegypti mosquitoes infected with Wolbachia symbionts are now being released into the field to control the spread of pathogenic human arboviruses. Wolbachia can spread throughout vector populations by inducing cytoplasmic incompatibility and can reduce disease transmission by interfering with virus replication. The success of this strategy depends on the effects of Wolbachia on mosquito fitness and the stability of Wolbachia infections across generations. Wolbachia infections are vulnerable to heat stress, and sustained periods of hot weather in the field may influence their utility as disease control agents, particularly if temperature effects persist across generations. To investigate the cross-generational effects of heat stress on Wolbachia density and mosquito fitness, we subjected Ae. aegypti with two different Wolbachia infection types (wMel, wAlbB) and uninfected controls to cyclical heat stress during larval development over two generations. We then tested adult starvation tolerance and wing length as measures of fitness and measured the density of wMel in adults. Both heat stress and Wolbachia infection reduced adult starvation tolerance. wMel Wolbachia density in female offspring was lower when mothers experienced heat stress, but male Wolbachia density did not depend on the rearing temperature of the previous generation. We also found cross-generational effects of heat stress on female starvation tolerance, but there was no cross-generational effect on wing length. Fitness costs of Wolbachia infections and cross-generational effects of heat stress on Wolbachia density may reduce the ability of Wolbachia to invade populations and control arbovirus transmission under specific environmental conditions.

Keywords: Aedes aegypti; Wolbachia; cross-generational effects; fitness; heat stress; starvation tolerance.

Figure A1

Diurnal fluctuating temperatures in water containers in the first (A) and second (B) generation. Black lines indicate the mean temperature recorded by 6 iButtons, with the shaded area representing 95% confidence intervals.

Figure 1

Adult starvation tolerance of female (A,B) and male (C,D) Ae. aegypti that were reared at either 26 °C (A,C) or 26–37 °C (B,D). Mosquitoes were either uninfected (gray lines) or infected with the wMel Wolbachia strain (yellow lines) or the wAlbB Wolbachia strain (blue lines). Solid lines represent the proportion of adults alive each day, while shaded areas are 95% confidence intervals.

Figure 2

Adult starvation tolerance of female (A) and male (B) Ae. aegypti that were reared under different temperature regimes across two generations. Mosquitoes were reared at 26 °C in both generations (gray lines), 26–37 °C in both generations (purple lines), 26–37 °C in the first generation and 26 °C in the second generation (red lines) or 26 °C in the first generation and 26–37 °C in the second generation (yellow lines). Data for all Wolbachia infection types were pooled in these comparisons. Solid lines represent the proportion of adults alive each day, while shaded areas are 95% confidence intervals.

Figure 3

Adult starvation tolerance of female (A) and male (B) Ae. aegypti with different Wolbachia infection types in the second generation. Mosquitoes were either uninfected (gray lines) or infected with the wMel Wolbachia strain (yellow lines) or the wAlbB Wolbachia strain (blue lines). Data for all rearing temperature regimes were pooled in these comparisons. Solid lines represent the proportion of adults alive each day, while shaded areas are 95% confidence intervals.

Figure 4

Wing length of female (A) and male (B) Ae. aegypti from three Wolbachia infection types (uninfected, wMel-infected or wAlbB-infected) that were reared under different temperature regimes across two generations. Mosquitoes were reared at 26 °C in both generations (gray dots), 26–37 °C in both generations (purple dots), 26–37 °C in the first generation and 26 °C in the second generation (red dots) or 26 °C in the first generation and 26–37 °C in the second generation (yellow dots). Error bars are medians and 95% confidence intervals.

Figure 5

Relative Wolbachia density of wMel-infected male and female Ae. aegypti that were reared under different temperature regimes across two generations. Mosquitoes were reared at 26 °C in both generations (gray dots), 26–37 °C in both generations (purple dots), 26–37 °C in the first generation and 26 °C in the second generation (red dots) or 26 °C in the first generation and 26–37 °C in the second generation (yellow dots). Error bars are medians and 95% confidence intervals.