Rickettsiae are classified in the order and have been included in the α subclass of the class 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 gene (apart from the tandem repeat units), encoding an antigenic high-molecularmass membrane protein specific for the group, was amplified and sequenced from 21 isolates. The phylogenetic relationships between SFG rickettsiae were inferred from the comparison of both the gene and derived protein sequences, using the parsimony, neighbour-joining and maximum-likelihood methods. Three strongly supported phylogenetic sub-groups were distinguished: first, the conorii complex (R. conorii Malish, R. conorii M1, R. conorii Moroccan, Indian tick typhus, Astrakhan fever rickettsia and Israeli tick typhus rickettsia); second, a cluster including , strain S, and ‘’; and, third, a cluster including , Bar 29 and 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 alleles and indicated that this gene is undergoing neutral evolution.


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