The influence of larval competition on Brazilian Wolbachia-infected Aedes aegypti mosquitoes

Abstract

Background: With field releases starting in Brazil, particular interest must be given to understanding how the endosymbiotic bacterium Wolbachia pipientis affects Aedes aegypti mosquitoes with a Brazilian genetic background. Currently, there is limited information on how the bacterium affects phenotypic traits such as larval development rate, metabolic reserves and morphometric parameters in Ae. aegypti. Here, we analyze for the first time, the effect of Wolbachia on these key phenotypes and consider how this might impact the potential of the bacterium as a disease control agent in Brazil.

Methods: We examined the influence of the wMel strain of Wolbachia in laboratory Ae. aegypti with a Brazilian genetic background, reared under different larval densities. Pupae formation was counted daily to assess differences in development rates. Levels of metabolic reserves and morphometric parameters were assessed in adults resulting from each larval condition.

Results: wMel infection led to more rapid larval development at higher densities for both males and females, with no effect under less crowded conditions in females. Infection also led to reduced body size at both high and low density, but not at intermediate density, although the scale of this difference was maintained regardless of larval density, in comparison to uninfected individuals. Wing shape also varied significantly between infected and uninfected mosquitoes due to larval density. Glycogen levels in uninfected mosquitoes decreased under higher larval density, but were consistently high with Wolbachia infection, regardless of larval density.

Conclusions: We demonstrate that the wMel Wolbachia strain can positively influence some important host fitness traits, and that this interaction is directly linked to the conditions in which the host is reared. Combined with previously published data, these results suggest that this Wolbachia strain could be successfully used as part of the Eliminate Dengue Program in Brazil.

Keywords: Aedes aegypti; Development time; Glycogen; Larval competition; Morphometrics; Wolbachia.

Fig. 1

Aedes aegypti landmarks. a The right wing of an adult Brazilian female Aedes aegypti mosquito with its scales manually removed, showing the position of the 18 landmarks (red dots). b Scheme of the imaginary links between the 18 landmarks used to depict the consensus wing size and shape

Fig. 2

wMel infection differentially affects the dynamics of pupation in male and female Brazilian Ae. aegypti reared under different larval densities. For females, low a and high larval densities c did not affect the pupation dynamics between wMel-infected and uninfected individuals (F _(0.05) = 3.072, P = 0.0119) and (F _(0.05) = 0.1425, P = 0.9344), respectively; b, with differences only occurring at the intermediate condition (F _(0.05) = 3.072, P = 0.0119). For males, d, low; e, intermediate conditions showed differences between groups (F _(0.05) = 5.899, P = 0.0008) and (F _(0.05) = 2.778, P = 0.0457), respectively, with peaks occurring earlier, while there was no difference at f high density (F _(0.05) = 2.072, P = 0.1054). Black lines represent uninfected mosquitoes. Colored lines represent Wolbachia-infected mosquitoes, with a different color for each density condition. Data were pooled from two independent biological replicates

Fig. 3

wMel infection influences the median wing size of adult Brazilian Ae. aegypti females. Box-and-whisker plots of median wing centroid size (mm) of wMel-infected (+) and uninfected (−) female mosquitoes under different crowding conditions. Green boxes represent the lower density, blue and red boxes the intermediate and higher densities, respectively. wMel infection led to reduced wing size in females at the low (Mann-Whitney U test, U = 1811, df = 1, P = 0.0009) and high densities (Mann-Whitney U test, U = 1668, df = 1, P < 0.0001); however, there was no difference from uninfected females at intermediate larval density (Mann-Whitney U test, U = 2198, df = 1, P = 0.1119). Data were pooled from two independent biological replicates. The total number of females (n_fem.) is indicated above treatment

Fig. 4

Rearing under different larval densities produced distinct wing shapes in adult Brazilian Ae. aegypti females. Scatterplot comparisons of wing shape for (a) wMel_Br and (b) wMel_BrTET female mosquitoes reared at either low (green dots), intermediate (blue dots) or high (red dots) larval densities, based on analysis of the main canonical variates predicting wing shape pattern. These data are depicted without allometry. Each circle represents a single adult female. Shape variations are depicted with the aid of thin-plate spline deformation grids (c) for PC1, PC2 and PC3, since 62.3 % of the observed variation was concentrated in these three principal components. Shape variation was scaled down 10 times in this analysis since it over-exaggerates the observed variation. Data were pooled from two independent biological replicates

Fig. 5

The influence of wMel infection on wing shape in adult Brazilian Ae. aegypti females. Histograms displaying the main canonical variates for (a) low, (b) intermediate and (c) high densities without allometry, for Wolbachia-infected (red) and uninfected (blue) mosquitoes. Data were pooled from two independent biological replicates

Fig. 6

wMel-infected Brazilian Ae. aegypti adult female mosquitoes have higher levels of glycogen after rearing at high larval density. Graphs depict the median glycogen levels of wMel-infected (+) and uninfected (−) adult Brazilian Ae. aegypti female mosquitoes. Each circle represents a single adult female, while the horizontal black lines indicate the median glycogen content in each treatment. Green dots represent the lower density where there was no difference in the levels of glycogen between infected and uninfected females (Mann-Whitney U test, U = 795, df = 1, P = 0.4627). Blue and red dots depict the intermediate and higher densities, respectively, where Wolbachia-infected females had a higher level of glycogen (Intermediate: Mann-Whitney U test, U = 863, df = 1, P < 0.0001; High: Mann-Whitney U test, U = 536, df = 1, P < 0.0001). The total number of females (n_fem.) is indicated above each treatment. Data were pooled from two independent biological replicates