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Journal of Clinical Microbiology, May 1998, p. 1305-1317, Vol. 36, No. 5
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Phylogenetic Placement of Rickettsiae from the Ticks Amblyomma americanum and Ixodes scapularisdagger

Susan J. Weller,1,2,* Gerald D. Baldridge,1,Dagger Ulrike G. Munderloh,1 Hiroaki Noda,3 Jason Simser,1 and Timothy J. Kurtti1

Department of Entomology1 and J. F. Bell Museum of Natural History,2 University of Minnesota, St. Paul, Minnesota 55108, and National Institute of Sericultural and Entomological Science, Tsukuba, Ibaraki 305, Japan3

Received 4 April 1997/Returned for modification 14 May 1997/Accepted 6 February 1998

A rickettsial isolate (isolate MOAa) belonging to the spotted fever group (SFG) was obtained from the lone star tick Amblyomma 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 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 two other nonpathogenic isolates (Rickettsia rickettsii Hlp2 and Rickettsia montana M5/6) with respect to their putative sister species, we included them in a phylogenetic analysis of 9 Rickettsia species and 10 Rickettsia strains. Our phylogenetic results implied three evolutionary lineages of SFG rickettsiae and that WB-8-2 and MOAa were most closely related to R. 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 the rompA gene of a nonpathogenic rickettsial symbiont isolated from the tick Ixodes scapularis. In a phylogenetic analysis, the symbiont was placed as the sister to R. montana and its isolates. The relationship of this symbiont to R. montana raised questions as to the potential origin of pathogenic SFG rickettsiae from nonpathogenic tick symbionts, or vice versa.

* Corresponding author. Mailing address: Department of Entomology, University of Minnesota, 219 Hodson Hall, 1980 Folwell Ave., St. Paul, MN 55108-6125. Phone: (612) 625-6253. Fax: (612) 625-5299. E-mail: welle008{at}maroon.tc.umn.edu.

dagger Paper no. 981170002 of the Minnesota Agricultural Experiment Station.

Dagger Present address: Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108.

Journal of Clinical Microbiology, May 1998, p. 1305-1317, Vol. 36, No. 5
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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