High degree of specificity in the association between symbiotic betaproteobacteria and the host Euplotes (Ciliophora, Euplotia)

The Betaproteobacteria-Euplotes association is an obligatory symbiotic system involving a monophyletic group of ciliate species and two betaproteobacteria species which can be alternatively present. Recent data showed that this relationship has been established more than once and that several symbiont-substitution events took place, revealing a complex and intriguing evolutionary path. Due to the different evolutionary pathways followed by the different symbionts, each bacterial strain could have differentially evolved and/or lost functional traits. Therefore, we performed re-infection experiments, both by phagocytosis and by microinjection, to test the possible functional role of the different bacteria towards the ciliates. Our results confirm that the growth capacity of the host is indissolubly linked to the presence of its original symbionts. Results of the attempts of re-infection by phagocytosis showed that none of the bacteria is able to successfully colonize the host cytoplasm in this way, even if regularly ingested. Re-infection by microinjection succeed only in one case. Such results point to a high degree of specificity in the interactions between bacteria and Euplotes even after the invasion step. Due to a co-evolutive pathway of reciprocal adaptation, different degree of re-colonization ability could have been conserved by the different species and strains of the symbionts.