Phylogenetic Placement of Rickettsiae from the TicksAmblyomma americanumandIxodes scapularis

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Abstract
A rickettsial isolate (isolate MOAa) belonging to the spotted fever group (SFG) was obtained from the lone star tickAmblyomma americanum. We used PCR to characterize the genes for the rickettsial outer membrane proteins rOmpA and rOmpB. We sequenced the PCR products (domains I of both therompAgene and therompBgene) of MOAa and WB-8-2, another rickettsial isolate fromA. americanum. To place MOAa and WB-8-2 and two other nonpathogenic isolates (Rickettsia rickettsiiHlp2 andRickettsia montanaM5/6) with respect to their putative sister species, we included them in a phylogenetic analysis of 9Rickettsiaspecies and 10Rickettsiastrains. Our phylogenetic results implied three evolutionary lineages of SFG rickettsiae and that WB-8-2 and MOAa were most closely related toR. montana. New World isolates were not the most closely related to each other (they did not form a clade). Rather, our results supported four independent origins (introductions) of rickettsiae into North America from different Old World regions. The results of our phylogenetic analysis did not support the hypothesis of a stable coevolution of rickettsiae and their tick hosts. Finally, we examined therompAgene of a nonpathogenic rickettsial symbiont isolated from the tickIxodes scapularis. In a phylogenetic analysis, the symbiont was placed as the sister toR. montanaand its isolates. The relationship of this symbiont toR. montanaraised questions as to the potential origin of pathogenic SFG rickettsiae from nonpathogenic tick symbionts, or vice versa.