Symbiosis represents a major evolutionary force leading to many adaptations of the partners, especially in response to pathogens. The endosymbiotic bacteria Wolbachia ( -proteobacteria) confer resistance to pathogens for vector insects. In the isopod crustacean Armadillidium vulgare, Wolbachia modify the host immunocompetence by decreasing total hemocyte count (THC) and increasing the natural septicemia. The presence of the bacteria in hemocytes and hematopoietic organs raises many questions about consequences of this presence on immune system functions and on immunocompetence of A. vulgare. We thus studied the impact of the symbiosis on hemocytes, immunocompetence and immune gene expressions. We showed by fluorescent in situ hybridization that Wolbachia infect more than one third of the hemocytes and by flow cytometry that the proportion of circulating granular hemocytes is lower in oneyear old infected animals, without any difference in THC at this age. Phenoloxidase activity decreases with age and symbiotic status. However, the Wolbachia presence seems to protect hemocytes against apoptosis and to increase host immunocompetence during infection with Listeria ivanovii. Finally, the quantification of the expression of immune genes, identified after establishing the first reference transcriptome of isopods (ANR EndoSymbArt project), revealed a trend to under-expression in whole animal and ovaries but an over-expression in immune tissues. Therefore, the Wolbachia presence modifies immune system characteristics at cellular and humoral levels and the immunocompetence of A. vulgare. The study of new parameters will clarify if the Wolbachia presence is a real benefit to its host or if the bacteria do have a significant cost to its host.
