Strength of the pathogenicity caused by feminizing Wolbachia after transfer in a new host: strain or dose effect?

The alphaproteobacteria Wolbachia pipientis are among the most common and widespread symbionts in the animal world. Their vertical transmission mode is predicted to favour genotypes with low virulence. On the contrary, horizontal transfers of Wolbachia from one host to another have been shown to possibly increase the symbiont virulence. This situation has been previously described when two feminizing Wolbachia strains, wVulC and wVulM, from the ovaries of the woodlouse Armadillidium vulgare were introduced into another woodlouse named Porcellio dilatatus. These two Wolbachia strains induced severe symptoms and eventually caused the death of the recipient host. However, symptoms and death appeared sooner with wVulC than with wVulM. To know whether this difference was due to variation in the dose of infection or a difference in virulence between the two Wolbachia strains, we performed controlled and gradual doses of injection with wVulC and wVulM in P. dilatatus. We showed that the two strains differed intrinsically in their virulence against P. dilatatus and that their virulence is related to the injection dose. Moreover, we showed that wVulC reached higher concentrations in the recipient host than wVulM suggesting a potential link between the bacterial titers and the levels of virulence. We also addressed the impact of the tissue source of the Wolbachia used for the transinfection and demonstrated that Wolbachia transinfected via hemolymph colonized the body of the recipient more quickly and caused accelerated symptoms compared to Wolbachia introduced via a crushed ovaries suspension.