Ciliates host a huge variety of bacteria, carried either as epibionts onto the cell body surface or as endosymbionts in the cell cytoplasm. Endosymbionts include the gamma-proteobacterium Francisella, which may colonize and harm a variety of hosts, mammals included. Our interest is focused on a new species of Francisella, F. adeliensis, originally isolated from a population of an Antarctic marine species of Euplotes, E. petzi, and subsequently identified also in sympatric populations of three other congeneric Antarctic species, E. euryhalinus, E. focardii and E. nobilii. In consideration of the bipolar distribution of E. nobilii, we inquired whether the F. adeliensis/E. nobilii symbiotic association is limited to the Antarctica or radiates to the Arctic, by screening E. nobilii strains isolated from the coasts of southern Patagonia, Svalbard, Greenland and northern Alaska with a F. adeliensis-specific16S gene amplification and in-situ hybridization. All E. nobilii strains resulted positive to F. adeliensis endosymbionts. By gene and genome sequence comparisons, we are currently analyzing how the E. nobilii/F. adeliensis symbiotic association evolved in time and space
