EVOLUTIONARY ANALYSIS OF THE SPOTTED-FEVER AND TYPHUS GROUPS OF RICKETTSIA USING 16S RIBOSOMAL-RNA GENE-SEQUENCES

The bacterial genus Rickettsia is traditionally divided into three biotypes, the spotted fever group (SFG), the typhus group (TG), and the scrub typhus group (STG) based on vector host and antigenic cross-reactivity. DNA sequence data were gathered from the 16S ribosomal RNA gene of several SFG and TG species. Comparative sequence analysis shows that: i) all species of Rickettsia are closely related, exhibiting 0.3-2.6% sequence divergence; ii) although there are identifiable clusters corresponding to the SFG and TG, species of Rickettsia fall into more than two distinct phylogenetic groups; iii) the tick-borne species Rickettsia bellii and Rickettsia canada diverged prior to the schism between the spotted fever and typhus groups; iv) the newly described AB bacterium is clearly a member of Rickettsia, but its phylogenetic placement within the genus is problematic; v) the mite-borne Rickettsia akari, the tick-borne Rickettsia australis and the recently described flea-borne ELB agent form a loose cluster that cannot be definitively associated with either the TG or the traditional SFG cluster. This latter clade may represent a unique group(s) distinct from the main cluster of spotted fever and typhus group species. The divergence of Rickettsia was an ancient event within the alpha-subclass of the proteobacteria. The sequence divergence between Rickettsia and Ehrlichia, the closest known genus to Rickettsia, is nearly equal to the sequence divergence between Rickettsia and all other alpha-subclass proteobacterial taxa included in the analysis. When Rickettsia was compared to a representative group of the alpha-subclass, twenty-eight nucleotide sites were identified which uniquely characterize the 16S rRNA sequences of all species of Rickettsia. The approximate lime of divergence between the various species of Rickettsia, estimated from the bacterial 16S rRNA molecular clock, coincides with the approximate divergence time of the hard body ticks which are the arthropod hosts of many Rickettsia, Thus, the possibility of coevolution between these intracellular bacteria and their tick hosts exists.