Wolbachia are intracellular bacteria found in the reproductive tissue of all major groups of arthropods. They are transmitted vertically from the female hosts to their offspring, in a pattern analogous to mitochondria inheritance. But Wolbachia phylogeny does not parallel that of the host, indicating that horizontal infectious transmission must also occur. Insect parasitoids are considered the most likely vectors, but the mechanism for horizontal transfer is largely unknown. Here we show that newly introduced Wolbachia cross several tissues and infect the germline of the adult Drosophila melanogaster female. Through investigation of bacterial migration patterns during the course of infection, we found that Wolbachia reach the germline through the somatic stem cell niche in the D. melanogaster germarium. In addition, our data suggest that Wolbachia are highly abundant in the somatic stem cell niche of long-term infected hosts, implying that this location may also contribute to efficient vertical transmission. This is, to our knowledge, the first report of an intracellular parasite displaying tropism for a stem cell niche.
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