Comparative analysis of Wolbachia maternal transmission and localization in host ovaries

Abstract

Many insects and other animals carry microbial endosymbionts that influence their reproduction and fitness. These relationships only persist if endosymbionts are reliably transmitted from one host generation to the next. Wolbachia are maternally transmitted endosymbionts found in most insect species, but transmission rates can vary across environments. Maternal transmission of wMel Wolbachia depends on temperature in natural Drosophila melanogaster hosts and in transinfected Aedes aegypti, where wMel is used to block pathogens that cause human disease. In D. melanogaster, wMel transmission declines in the cold as Wolbachia become less abundant in host ovaries and at the posterior pole plasm (the site of germline formation) in mature oocytes. Here, we assess how temperature affects maternal transmission and underlying patterns of Wolbachia localization across 10 Wolbachia strains diverged up to 50 million years-including strains closely related to wMel-and their natural Drosophila hosts. Many Wolbachia maintain high transmission rates across temperatures, despite highly variable (and sometimes low) levels of Wolbachia in the ovaries and at the developing germline in late-stage oocytes. Identifying strains like closely related wMel-like Wolbachia with stable transmission across variable environmental conditions may improve the efficacy of Wolbachia-based biocontrol efforts as they expand into globally diverse environments.

Fig. 1. Divergent Wolbachia strains found in Drosophila host species.

a Estimated Bayesian phylogram of the 10 A- and B-group Wolbachia strains included in the study. The phylogram was estimated using 170 single-copy genes of identical length in all genomes, spanning 135,105 bp. b Estimated Bayesian phylogram of the eight Drosophila species using 20 single-copy genes. All nodes on both trees are supported with Bayesian posterior probabilities of 1. Estimates of Wolbachia and Drosophila divergence reported in millions of years ago (MYA) are reproduced from Meany et al. and Suvorov et al., respectively. The Wolbachia and host trees are generally discordant, as expected with frequent Wolbachia host switching^,,,,.

Fig. 2. Maternal transmission is high and stable across temperatures for many Wolbachia strains.

Mean maternal transmission rates (±BC_a confidence intervals) of different Wolbachia strains in their naturally associated host species (N = 364 sublines). Asterisks indicate the rates of maternal transmission differ between 25° and 20 °C for a given Wolbachia strain according to a Wilcoxon rank-sum test at P < 0.05. Below, the cladogram depicts evolutionary relationships among Wolbachia strains.

Fig. 3. Wolbachia densities in host tissues vary by Wolbachia-host system and temperature.

Wolbachia densities are shown for dissected female ovaries and the remaining somatic carcass tissue (N = 183 F0 females). Asterisks indicate the Wolbachia density differs between 25° and 20 °C for a given Wolbachia strain according to a Wilcoxon rank-sum test at P < 0.05. Boxplots show medians, first and third quartiles (hinges), and the smallest/largest values within 1.5*IQR of the hinges (whiskers).

Fig. 4. Cellular Wolbachia abundance in late-stage oocytes generally does not align with maternal transmission rates.

Cellular Wolbachia abundance in stage 10 oocytes (measured as fluorescence due to propidium iodide; CTCF), measured in a the whole oocyte, b the posterior region, and c the posterior cortex (N = 283 oocytes). Asterisks indicate that Wolbachia abundance differs between 25° and 20 °C for a given Wolbachia strain according to a Wilcoxon rank-sum test at P < 0.05. d Representative confocal images of Wolbachia strains that have decreased abundance at the posterior cortex in the cold (wMel), increased abundance at the cortex in the cold (wRi), and no change in the cold (wCha). Confocal micrographs are DNA-stained with PI (red) and actin-stained with phalloidin (green). The second column depicts a single channel image of the PI stain and the third column depicts an enlarged PI-stained image at the posterior cortex of each oocyte. Scale bars are set to 25 µm.