Summary auto-generated
This study analyzed the ompA gene, which encodes an outer membrane protein (rOmpA), from 21 isolates of spotted fever group (SFG) rickettsiae to determine their phylogenetic relationships. The researchers amplified and sequenced nearly the entire ompA gene (excluding tandem repeat regions) from diverse Rickettsia species, then used multiple phylogenetic inference methods to construct evolutionary trees. The analysis revealed three strongly supported phylogenetic subgroups: the Rickettsia conorii complex (containing R. conorii strains and related species), a cluster with R. africae and R. sibirica, and a Mediterranean/North American cluster including R. massiliae and R. montanensis. Four species—R. rickettsii, R. japonica, R. slovaca, and Thai tick typhus rickettsia—did not group with other species. Analysis of synonymous versus non-synonymous substitution rates indicated the ompA gene undergoes neutral evolution rather than positive selection. The ompA gene proved more variable and useful for distinguishing SFG rickettsiae compared to previously used molecular markers like 16S rRNA and gltA genes, making it valuable for phylogenetic studies within this genus.
Key findings
- Three major phylogenetic subgroups were identified within SFG rickettsiae using ompA gene sequences, including a distinct R. conorii complex and a Mediterranean-associated cluster
- The ompA gene exhibits higher interspecies variation than 16S rRNA and citrate synthase genes, making it superior for resolving phylogenetic relationships among closely related Rickettsia species
- Analysis of substitution rates (dS/dN ratio of 0.99) demonstrates the ompA gene undergoes neutral evolution, indicating immune selection is not driving sequence diversity
- Four Rickettsia species (R. rickettsii, R. japonica, R. slovaca, Thai tick typhus) did not cluster with other species, suggesting their distinct evolutionary positions
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Abstract
Rickettsiae are classified in the order Rickettsiales and have been included in the alpha subclass of the class Proteobacteria on the basis of 16S rRNA gene sequence comparison. To estimate the evolutionary forces that have shaped the members of the spotted fever group (SFG) rickettsiae, the ompA gene (apart from the tandem repeat units), encoding an antigenic high-molecular-mass membrane protein specific for the group, was amplified and sequenced from 21 isolates. The phylogenetic relationship between SFG rickettsiae were inferred from the comparison of both the gene and derived protein sequences, using the parsimony, neighbor-joining and maximum-likelihood methods. Three strongly supported phylogenetic sub-groups were distinguished: first, the Rickettsia conorii complex (R. conorii Malish, R. conorii M1, R. conorii Moroccan, R. conorii Indian tick typhus, Astrakhan fever rickettsia and Israeli tick typhus rickettsia); second, a cluster including Rickettsia africae, strain S, Rickettsia parkeri, Rickettsia sibirica and 'Rickettsia mongolotimonae'; and, third, a cluster including Rickettsia aeschlimannii, Rickettsia rhipicephali, Rickettsia massiliae, Bar 29 and Rickettsia montanensis. Rickettsia rickettsii, Rickettsia japonica, Rickettsia slovaca and Thai tick typhus rickettsia did not cluster with any other Rickettsia species. To test whether positive selection was responsible for sequences diversity, rates of synonymous and nonsynonymous nucleotide substitutions were compared for Rickettsia ompA alleles and indicated that this gene is undergoing neutral evolution.