Clarification of the Taxonomic Position of Paramecium caudatum Micronucleus Symbionts.

Bacteria of genus Holospora (order Holosporales, class Alphaproteobacteria) are obligate intranuclear symbionts of ciliates Paramecium spp. with strict host species and nuclear (macronucleus or micronucleus) specificity. However, three species under study Holospora undulata, Holospora elegans and 'Holospora recta' occupy the same ecological niche-micronucleus of Paramecium caudatum and demonstrate some differences in morphology of infectious form. The genetic diversity of holosporas by rrs and rpoB sequence analysis was determined. Phylogenetic and phylogenomic analysis of Holospora spp., as well as some phenotypic features indicate that there is no distinctive difference supporting studied micronuclear endosymbionts as distinct species. Therefore, Holospora elegans and 'Holospora recta' should be considered subspecies of Holospora undulata (ex Haffkine 1890) Gromov and Ossipov 1981, which was described first. Thus, we confirmed the evolutionary aspects of the development of symbiotic relationships: holosporas have a strict specificity to the host species and the type of nucleus.