Does stem cell niche tropism favor the evolutionary success of specific Wolbachia strains? Michelle E. Toomey, Mark Deehan, Kanchana Panaram, Horacio Frydman. Biology, Boston University, Boston, MA.

   The intracellular bacteria Wolbachia infect most insect species, including the vectors of devastating infectious diseases such as Dengue and malaria. Although Wolbachia is being targeted as a novel control mechanism for the spread of these diseases, very little is known about the cellular and molecular mechanisms of the host-microbe interactions. Wolbachia are mainly vertically transmitted, however there is also evidence of extensive horizontal transmission. In order to successfully invade a new host species, once Wolbachia crosses the species barrier, it must avoid the hosts innate immune system and colonize the germline. We hypothesize that targeting of stem cell niches in the gonad offers a facilitated route to access the germline, aiding subsequent vertical transmission after invasion of a new host species. If this hypothesis is true, related Wolbachia strains that infect diverse host species (grouped by sequence type clonal complexes) will be efficient at colonizing stem cell niches. To address this hypothesis, we first looked at clonal complex ST-13, which has the highest number of diverse host species infected by closely relate Wolbachia strains. Many strains of Wolbachia infecting the Drosophila genus are represented in ST-13. We show that tropism for the stem cell niches in the ovary across the Drosophila genus is evolutionarily conserved in all Wolbachia strains investigated, and that this tropism may promote germline transmission. We also began looking at species that are infected with Wolbachia not belonging to the ST-13 complex, i.e. Culex pipiens pipiens infected with Wolbachia wPip. The limitation to this analysis is that the stem cells and associated niches in the gonads are not unequivocally identified in many other insect species. We have, however, identified the stem cells and niches in the ovary of Cx. pipiens, and there is no Wolbachia tropism present. These observations support our hypothesis that niche tropism, present in the Wolbachia strains in the ST-13 clonal complex, may promote successful spreading of Wolbachia in nature.