Restriction of Wolbachia bacteria in early embryogenesis of neotropical Drosophila species via ER-mediated autophagy

Wolbachia bacteria are maternally transmitted intracellular microbes that are not only restricted to the reproductive organs but also found in various somatic tissues of their native hosts. The abundance of the endosymbiont in somatic tissues, usually a dead end for vertically transmitted bacteria, causes a multitude of effects on life history traits of their hosts, which are still not well understood. Thus, deciphering the host-symbiont interactions on a cellular level throughout a hosts lifecycle is of great importance to understand their homeostatic nature, persistence and spreading success. Using fluorescent and transmission electron microscopy, we conducted a comprehensive analysis of Wolbachia tropism in somatic and reproductive tissues of six Drosophila species at the intracellular level during host development. Our data uncovered diagnostic patterns of infections to embryonic primordial germ cells and to particular cells of somatic tissues in three different neotropical Drosophila species of the willistoni and saltans groups that have apparently evolved in both independently. We further found that restricted patterns of Wolbachia tropism are already determined in early fly embryogenesis. This is achieved via selective autophagy, and the restriction of infection is preserved through larval hatching and metamorphosis. We further uncovered tight interactions of Wolbachia with membranes of the endoplasmic reticulum, which might play a scaffolding role for autophagosome formation and subsequent elimination of the endosymbiont. Finally, by analyzing D. simulans lines transinfected with non-native Wolbachia, we uncovered that the host genetic background regulates tissue tropism of infection. Our data demonstrate a peculiar and novel mechanism to limit and spatially restrict bacterial infection in somatic tissues during a very early stage of host development.