Evolutionarily conserved Wolbachia -encoded factors control pattern of stem-cell niche tropism in Drosophila ovaries and favor infection

Abstract

Wolbachia are intracellular bacteria that infect invertebrates at pandemic levels, including insect vectors of devastating infectious diseases. Although Wolbachia are providing novel strategies for the control of several human pathogens, the processes underlying Wolbachia's successful propagation within and across species remain elusive. Wolbachia are mainly vertically transmitted; however, there is also evidence of extensive horizontal transmission. Here, we provide several lines of evidence supporting Wolbachia's targeting of ovarian stem cell niches-referred to as "niche tropism"-as a previously overlooked strategy for Wolbachia thriving in nature. Niche tropism is pervasive in Wolbachia infecting the Drosophila genus, and different patterns of niche tropism are evolutionarily conserved. Phylogenetic analysis, confirmed by hybrid introgression and transinfection experiments, demonstrates that bacterial factors are the major determinants of differential patterns of niche tropism. Furthermore, bacterial load is increased in germ-line cells passing through infected niches, supporting previous suggestions of a contribution of Wolbachia from stem-cell niches toward vertical transmission. These results support the role of stem-cell niches as a key component for the spreading of Wolbachia in the Drosophila genus and provide mechanistic insights into this unique tissue tropism.endosymbiont | maternal transmission | microbial tissue tropism | germline stem cell niche | somatic stem cell niche

Variable Inhibition of Zika Virus Replication by Different Wolbachia Strains in Mosquito Cell Cultures

“…Fluorescent in situ hybridization was performed with minor modifications to a method previously described (51). Briefly, cells were fixed with 4% paraformaldehyde in serum-free medium with 0.1% Triton X-100 and 0.1% Tween 20 for 1 h. Cells were then incubated in hybridization buffer (50% formamide, 5ϫ SSC [1ϫ SSC is 0.15 M NaCl plus 0.015 M sodium citrate], 250 mg/liter of salmon sperm DNA, 0.5ϫ Denhardt's solution, 20 mM Tris HCl, 0.1% SDS) at 37°C for 1 h. Probes were added in in situ buffer at a concentration of 1 ng/l (probe 1) or 7.5 pg/l (probe 2).…”

Wolbachia Associations with Insects: Winning or Losing Against a Master Manipulator

“…The second route is through stem-cell tissue tropism, where Wolbachia targets and colonizes two groups of stem cell niches in the ovary, the germline stem cell niche at the anterior end of the ovariole and the somatic stem cell niche at the germarium. Evidence suggests that transmission of Wolbachia from stem cell niches into the developing oocyte is conserved in Drosophila and may even be the most prevalent mechanism of egg colonization by Wolbachia (Frydman et al, 2006;Toomey et al, 2013).…”

“…Intriguingly, only the posterior localization pattern directly targets Wolbachia to the site of germ cell formation (germ plasm), which indicates that strains with a different localization pattern must rely on other mechanisms to reach embryonic germ cells, such as the previously described stem cell niche tropism. This has important evolutionary implications as such mechanisms would allow both the passage of bacteria from mother to offspring and the establishment of stable infection of symbionts transmitted horizontally (Toomey et al, 2013).…”

Native microbiome impedes vertical transmission of Wolbachia in Anopheles mosquitoes

“…After microinjection into Drosophila, Wolbachia infects the stem cell niches in the germ line (16,17), and both Wolbachiaderived and host factors appear to influence tropism and bacterial density during oogenesis (17)(18)(19)(20). Alternatively, somatic tissue may act as a reservoir for Wolbachia infection of the developing oocyte (20)(21)(22)(23). Although pathways of horizontal transmission have been characterized in some species, identification of barriers to vertical transmission of the acquired Wolbachia infection remains elusive.…”

“…Multiple mechanisms of Wolbachia horizontal transmission have been proposed, including cohabitation, hemolymph transfer, predation, and parasitoid infection (11)(12)(13)(14)(15). After microinjection into Drosophila, Wolbachia infects the stem cell niches in the germ line (16,17), and both Wolbachiaderived and host factors appear to influence tropism and bacterial density during oogenesis (17)(18)(19)(20). Alternatively, somatic tissue may act as a reservoir for Wolbachia infection of the developing oocyte (20)(21)(22)(23).…”