Betaproteobacterial symbionts of the ciliate Euplotes: origin and tangled evolutionary path of an obligate microbial association

The PolynucleobacterEuplotes association is an obligatory symbiotic system between a monophyletic group of ciliate species belonging to the genus Euplotes and bacteria of the species Polynucleobacter necessarius (Betaproteobacteria). Both organisms are unable to survive independently. Several studies revealed the existence of free-living populations of Polynucleobacter bacteria which are phylogenetically closely related to the endosymbiotic ones, but never share associations with Euplotes in the natural environment. Hence, following the most parsimonious explanation on the origin of the association, this symbiosis should represent a synapomorphic character for the hosts' clade. Nevertheless, phylogenetic analyses performed on an increased number of strains here presented suggest that Euplotes species, during their evolution, recruited Polynucleobacter bacteria as symbionts more than once. Moreover, in three cases, we observed different bacteria as obligate symbionts. These symbionts are the first characterized representatives of a phylogenetic lineage branching in a basal position with respect to the genus Polynucleobacter. The hypothesis that the original obligate symbionts belonged to this newly discovered clade and that, only subsequently, in most cases they have been replaced by Polynucleobacter bacteria recruited from the environment is proposed and discussed. The evolutionary path of this association seems anyway to have been more complex than so far supposed.