Multilocus genetic analysis indicates taxonomic status of Candidatus Rickettsia mendelii as a separate basal group

To date, the phylogeny of Rickettsia spp. from basal groups is based on the small number of identified species. Thus, the finding of Candidatus Rickettsia mendelii in 2016 is of great interest. In this study, Ca. R. mendelii was first identified in the Asian region in a new carrier, Ixodes pavlovskyi. Candidatus R. mendelii, along with Candidatus Rickettsia tarasevichiae, were found in Ixodes ticks collected on Russky Island (the Far East), where I. pavlovskyi coexists with I. persulcatus. To establish the taxonomic position of Ca. R. mendelii, a detailed genetic study was carried out. Candidatus R. mendelii was genotyped by five genetic fragments (16S rRNA, gltA, and ompB genes, groESL operon, and 23S-5S IGS region); among them, the ompB gene, groESL operon and 23S-5S IGS region were sequenced for the first time. In addition, Ca. R. tarasevichiae was genetically characterized by eight genetic loci (16S rRNA, gltA, ompA, ompB, sca4, htrA genes, groESL operon, and 23S-5S IGS region), of which the sca4 gene was first determined. Phylogenetic analysis indicated that regardless of analyzed genetic loci, Ca. R. mendelii formed a separate well-supported cluster on each phylogenetic tree. Phylogenetic analysis based on concatenated sequences of gltA, ompB, and groEL gene fragments (total length of 3191 bp) demonstrated that Ca. R. mendelii, like Rickettsia bellii, is a basal group of Rickettsia.

Automated summary

The discovery of Candidatus Rickettsia mendelii in 2016 in the Asian region has significant implications for the phylogeny of basal groups in Rickettsia spp.