Phylogenetic relationships among endosymbiotic R-body producer: Bacteria providing their host the killer trait

R-body producing bacterial endosymbionts of Paramecium spp. transform their hosts into “killer” paramecia and provide them a selective advantage. This killer trait is connected to the presence of R-bodies, which are peculiar, tightly coiled protein ribbons capable of rapid unrolling. Based mainly on those two characteristics the respective obligate intracellular bacteria have been comprised in the genus Caedibacter and additional traits such as host species, subcellular localization, and R-body dimensions and mode of unrolling were used for species discrimination. Previous studies applying the full-cycle rRNA approach demonstrated the polyphyly of this assemblage. Following this approach, we obtained new sequences and in situ hybridizations for five strains of Caedibacter taeniospiralis and four strains associated to Caedibacter varicaedens and Caedibacter caryophilus. Detailed phylogenetic reconstructions confirm the association of C. taeniospiralis to Fastidiosibacteraceae and to Holosporales in case of the others. Therefore, we critically revise the taxonomy of the latter group. The high 16S rRNA gene sequence similarity among the type strains of Caedibacter varicaedens and C. caryophilus indicate that they should be classified within a single species for which we propose Caedimonas varicaedens comb. nov. owing to the priority of Caedibacter varicaedens. Moreover, we propose to establish the new family Caedimonadaceae fam. nov. to encompass Caedimonas varicaedens, “Ca. Paracaedimonas acanthamoebae” comb. nov. and “Ca. Nucleicultrix amoebiphila” within the order Holosporales.