Distinct Wolbachia localization patterns in oocytes of diverse host species reveal multiple strategies of maternal transmission

Abstract

A broad array of endosymbionts radiate through host populations via vertical transmission, yet much remains unknown concerning the cellular basis, diversity and routes underlying this transmission strategy. Here we address these issues, by examining the cellular distributions of Wolbachia strains that diverged up to 50 million years ago in the oocytes of 18 divergent Drosophila species. This analysis revealed three Wolbachia distribution patterns: 1) a tight clustering at the posterior pole plasm (the site of germline formation); 2) a concentration at the posterior pole plasm, but with a significant bacteria population distributed throughout the oocyte; 3) and a distribution throughout the oocyte, with none or very few located at the posterior pole plasm. Examination of this latter class indicates Wolbachia accesses the posterior pole plasm during the interval between late oogenesis and the blastoderm formation. We also find that one Wolbachia strain in this class concentrates in the posterior somatic follicle cells that encompass the pole plasm of the developing oocyte. In contrast, strains in which Wolbachia concentrate at the posterior pole plasm generally exhibit no or few Wolbachia in the follicle cells associated with the pole plasm. Taken together, these studies suggest that for some Drosophila species, Wolbachia invade the germline from neighboring somatic follicle cells. Phylogenomic analysis indicates that closely related Wolbachia strains tend to exhibit similar patterns of posterior localization, suggesting that specific localization strategies are a function of Wolbachia-associated factors. Previous studies revealed that endosymbionts rely on one of two distinct routes of vertical transmission: continuous maintenance in the germline (germline-to-germline) or a more circuitous route via the soma (germline-to-soma-to-germline). Here we provide compelling evidence that Wolbachia strains infecting Drosophila species maintain the diverse arrays of cellular mechanisms necessary for both of these distinct transmission routes. This characteristic may account for its ability to infect and spread globally through a vast range of host insect species.

Comparative analysis ofWolbachiamaternal transmission and localization in host ovaries

“…Finally, we examined how temperature impacts cellular Wolbachia abundance at the site of germline formation in stage 10 oocytes during host oogenesis. At this stage, wMel Wolbachia localization at the posterior cortex of the oocyte is considered to be a critical step in the process of germline-to-germline Wolbachia maternal transmission 14,15,21,36,[59][60][61][62][63] . We imaged a total of 283 stage 10 oocytes from eight-day-old females reared at the two different temperatures and quantified cellular Wolbachia abundance (measured as corrected total cell fluorescence; CTCF) in the whole oocytes, the posterior region, and the posterior cortex (Figure S2) 14,15,21 .…”

“…At this stage, wMel Wolbachia localization at the posterior cortex of the oocyte is considered to be a critical step in the process of germline-to-germline Wolbachia maternal transmission 14,15,21,36,[59][60][61][62][63] . We imaged a total of 283 stage 10 oocytes from eight-day-old females reared at the two different temperatures and quantified cellular Wolbachia abundance (measured as corrected total cell fluorescence; CTCF) in the whole oocytes, the posterior region, and the posterior cortex (Figure S2) 14,15,21 . We defined the posterior region as the posterior 1/8 th portion of the oocyte and the posterior cortex as the narrow cortical region of Vasa expression (a germline protein component) 15,63 .…”

“…Two Wolbachia strains (wAura and B-group wMau) exhibited minor temperature-dependent changes in transmission, but wMel stood out with a relatively large decrease in transmission as temperature dropped from 25° to 20°C (Figure 2). Maternal transmission is generally expected to depend on Wolbachia density in host reproductive tissue 20,34,43,71 , and specifically within developing oocytes 14,15,21,36,[59][60][61][62][63] . Our analyses uncovered considerable, significant variation in Wolbachia densities at the tissue level and cellular abundances in oocytes, which also often varied depending on temperature.…”

“…This includes introducing the pathogen-blocking wMel Wolbachia strain from Drosophila melanogaster into mosquito populations to reduce the incidence of dengue and other diseases on multiple continents [29][30][31][32] . Nonetheless, it is largely unknown why maternal transmission may breakdown under certain circumstances, especially for non-model systems 15,21,[33][34][35][36] .…”

“…What might underlie variable maternal transmission rates? Maternal transmission breakdown is predicted to stem from reductions in Wolbachia abundance and localization throughout host development and specifically at the critical stages of oogenesis thought to facilitate germline-to-germline maternal transmission 14,15,21,36,[59][60][61][62][63] . wMel cells are present in the germline stem cells of adult females, and over the course of oogenesis, wMel localize via kinesin-mediated transport to the pole plasm-the site of germline formation-at the posterior cortex of the developing oocyte.…”