PHYLOGENETIC PLACEMENT OF RICKETTSIAE FROM THE TICKS AMBLYOMMA-AMERICANUM AND IXODES-SCAPULARIS

A rickettsial isolate (isolate MOAa) belonging to the spotted fever gr oup (SFG) was obtained from the lone star tick Amblyomma americanum. W e used PCR to characterize the genes for the rickettsial outer membran e proteins rOmpA and rOmpB. We sequenced the PCR products (domains I o f both the rompA gene and the rompB gene) of MOAa and WB-8-2, another rickettsial isolate from A. americanum. To place MOAa and WB-8-2 and t wo other nonpathogenic isolates (Rickettsia rickettsii Hlp2 and Ricket tsia montana M5/6) with respect to their putative sister species, we i ncluded them in a phylogenetic analysis of 9 Rickettsia species and 10 Rickettsia strains. Our phylogenetic results implied three evolutiona ry lineages of SFG rickettsiae and that WB-8-2 and MOAa were most clos ely related to R. montana. New World isolates were not the most closel y related to each other (they did not form a clade), Rather, our resul ts supported four independent origins (introductions) of rickettsiae i nto North America from different Old World regions. The results of our phylogenetic analysis did not support the hypothesis of a stable coev olution of rickettsiae and their tick hosts. Finally, we examined the rompA gene of a nonpathogenic rickettsial symbiont isolated from the t ick Ixodes scapularis. In a phylogenetic analysis, the symbiont was pl aced as the sister to R. montana and its isolates. The relationship of this symbiont to R. montana raised questions as to the potential orig in of pathogenic SFG rickettsiae from nonpathogenic tick symbionts, or vice versa.