Ancestral Divergence of Rickettsia bellii from the Spotted Fever and Typhus Groups of Rickettsia and Antiquity of the Genus Rickettsia

Abstract

The eubacterial genus Rickettsia belongs to the cu subgroup of the phylum Proteobacteriu. This genus is usually divided into three biotypes on the basis of vector host and antigenic cross-reactivity characteristics. However, the species Rickettsia bellii does not fit into this classification scheme; this organism has characteristics common to both the spotted fever group and the typhus group biotypes and also exhibits some unique features. Sequences of the 16s rRNA and 23s rRNA genes from Rickettsia rickettsii (spotted fever group), Rickettsia prowazekii (typhus group), and R. bellii were studied to determine the position of R. bellii in the rickettsia1 classification scheme. The 23s rRNA gene sequences described in this paper are the first 23s rRNA sequences reported for any member of the Rickettsiuceue. The 23s rRNA gene contains substantially more phylogenetic information than is contained in the 16s rRNA sequences, and the 23s rRNA gene sequence has diverged about 1.9 times faster in the three Rickettsia species which we studied. Taken together, the molecular data obtained from the two genes indicate that R. bellii is not a member of either the spotted fever group or the typhus group; rather, this organism appears to be the product of a divergence which predates the separation of the genus into the spotted fever group and the typhus group. Consequently, different combinations of the ancestral characteristics retained by R. bellii have been retained in the more derived lineages of the genus. A comparison of the 16s rRNA and 23s rRNA gene sequences of Rickettsia strains with other proteobacterial sequences confirmed that the genus Rickettsia is a unique deeply branching member of the cu subgroup of the Proteobacteria and that the Rickettsia species form a monophyletic cluster. While divergence of the contemporary members of the genus Rickettsia occurred recently, the unique evolutionary line represented by this genus appears to be very old.Rickettsia strains are small, gram-negative bacteria that are intimately associated with eukaryotic cells and are members of a diverse family, the Rickettsiaceae, which also includes other intracellular organisms, including the genera Ehrlichia, Wolbachia, Anaplasma, and Cowdria (4,36). Rickettsiae have natural arthropod hosts (either ticks, mites, or insects) and can be pathogenic for humans and other vertebrates. The obligately intracellular lifestyle and fastidious nature of these organisms have made them difficult to study. A number of genotypic and phenotypic characteristics indicate that the Rickettsia species are closely related, and this genus is now recognized as the sole member of a branch of the cx subgroup Proteobacteriu (8) on the basis of 16s rRNA cataloging and partial 16s rRNA gene sequence data (34).The genus Rickettsia is divided into three biotypes on the basis of immunological cross-reactivity and ecological characteristics (36). Rickettsia species are placed into the spotted fever group (SFG), the typhus group (TG), or the scrub typhus group on the basis...

Conservation of the unique rickettsial rRNA gene arrangement in Anaplasma.

“…Interestingly, two species in the family Rickettsiaceae, Rickettsia bellii and Orientia tsutsugamushi, also lack a recognizable fmt gene in this position (Andersson et al, 1999). This is consistent with O. tsutsugamushi representing a separate genus (Tamura et al, 1995) and with the suggestion, based on analysis of 16S and 23S rRNA genes (Stothard et al, 1994 ;Stothard & Fuerst, 1995) and the citrate synthase gene (Roux et al, 1997), that R. bellii diverged relatively early in the speciation of the genus Rickettsia.…”

Characterization of a tandem repeat polymorphism in Rickettsia strains

“…Interestingly, the other species that did not yield amplification results were those previously described as being more distant phylogenetically (Sekeyova et al, 2001;Roux et al, 1997;Fournier et al, 1998;Vitorino et al, 2003). Moreover, it has been suggested that R. canadensis and R. bellii, as well as an unidentified bacterium from Adalia bipunctata, were the first representatives of the genus Rickettsia to diverge, and hence they have major genetic differences (Stothard et al, 1994). In view of this fact, the unsuccessful PCR amplification of the Rc-65 locus in these rickettsiae species could mean that they may have different nucleotide sequences in the primer regions.…”

Sequence transformation to a complex signature form for consistent phylogenetic tree using Extensible Markov Model

“…Second, another experiment was performed using a random sample consisting of 5 phyla, 7 classes and 13 genera and the phylogeny shown in Figure 6 confirms topological accuracy. The only exception seen is due to Rickettsia-bellii which is found to have diverged much earlier from the related organisms [30] and hence its isolation from the rest in the figure. Third, another experiment with a sample consisting of three eukaryotic RNA sequences from brown dog tick, mouse and sand fly as well as 15 organisms of microbial origin was performed.…”

“…The Rickettsi on the other hand isolates one organism from the other two. Stothard et al proved [30] that the isolated organism Rickettsi1 or Rickettsia-bellii-RML369-C has diverged out of the other genera of Rickettsi much earlier. The distance matrix for this group alone (Figure 7) clearly shows the distinction in the Rickettsi sample.…”

“…Except for Rickettsi group, all groupings are fully recovered. The Rickettsi1 or Rickettsia-bellii-RML369-C organism is known to have diverged out of the more common Rickettsi genera for spotted fever and typhus [30] and hence its relative disassociation. It is evident that similar organisms are grouped together in the phylogeny shown in Figure 6.…”