Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 5, 2003
Publication Date: May 1, 2004
Citation: Scoles, G.A. 2004. Phylogenetic analysis of the Francisella-like endosymbionts of Dermacentor Ticks. 41:277-286. Interpretive Summary: Ticks contain symbiontic bacteria that are not pathogenic and are not transmitted by the bite of the tick. Symbionts that are closely related to the tick transmitted bacteria that causes tularemia ("rabbit fever", Francisella tularensis) are known as Francisella-like endosymbionts. These Francisella-like endosymbionts have been found in 5 different species of ticks, including 2 common North American Dermacentor species, the Rocky Mountain wood tick (D. andersoni) and the American dog tick (D. variabilis). The symbionts from different tick species and the tick transmitted pathogen F. tularensis appear to have evolved from a common ancestor, but the relationship is not well understood. A fragment of the ribosomal RNA gene and a fragment of a lipoprotein gene of the symbionts was amplified and sequenced from 6 different North American ticks in the same genus (Dermacentor): the Rocky Mountain wood tick, the American dog tick, the winter tick, the Pacific Coast tick, the bighorn sheep tick and the tropical horse tick (respectively: D. andersoni, D. variabilis, D. albipictus, D. occidentalis, D. hunteri, and D. nitens) as well as of the Gulf Coast tick (Amblyomma maculatum) and the warthog tick (Ornithodoros porcinus) for comparison. The DNA sequences of the symbionts were compared to determine how closely related symbionts from different tick species are. The relationships of the symbionts to one another was compared to the relationships of the tick species to one another to try to determine if the ticks and their symbionts had evolved in tandem (co-speciation). Although the Francisella-like endosymbionts from different tick genera (Dermacentor, Amblyomma, Ornithodoros) can be separated from on another the symbionts from different species of ticks within the genus Dermacentor cannot be differentiated. This suggests that there has not been close co-speciation between the symbionts and the Dermacentor ticks; this may mean that the relationship between Francisella-like endosymbiont and the Dermacentor ticks is of a relatively recent origin. Several ticks were infected with more than one symbiont: some had two different Francisella-like endosymbionts, others had a Francisella-like endosymbiont and an unidentified species of spotted fever group rickettsia, a distantly related group of symbionts related to the organism that causes Rocky Mountain spotted fever. Infection with the Francisella-like endosymbiont does not appear to have prevented infection with either a second closely related Francisella-like endosymbiont or with a second less closely related rickettsial symbiont.
Technical Abstract: Bacterial endosymbionts with significant homology to Francisella tularensis (gamma-proteobacteria) have been described from at least 5 species of ticks in 3 different genera, including 2 North American Dermacentor species (D. andersoni and D. variabilis). The evolutionary relationships among the Francisella-like endosymbionts (FLE) from different hosts and between FLE and the arthropod-borne pathogen F. tularensis are not known. A 1169 base fragment of the 16s rDNA and a 714 base fragment of the F. tularensis 17-kDa lipoprotein gene homolog of the FLE of 6 North American Dermacentor tick species (D. andersoni, D. variabilis, D. albipictus, D. occidentalis, D. hunteri, and D. nitens) and of Amblyomma maculatum and Ornithodoros porcinus as out groups, were subjected to phylogenetic analysis. These gene phylogenies were compared to a phylogeny of the same tick species constructed from a 435 base fragment of the tick mitochondrial 16s rDNA. Although the phylogenies of the FLE and their tick hosts are parallel at the genus level, the Dermacentor FLE are unresolved at the species level. There does not appear to have been close co-speciation between the FLE and the Dermacentor ticks, possibly indicating that the association between these endosymbiont and the Dermacentor ticks is of a relatively recent origin. Several ticks were co-infected, either with two FLE with divergent 17-kDa lipoprotein gene sequences or with FLE and an unidentified species of spotted fever group rickettsia (alpha-proteobacteria). Infection with FLE does not appear to have precluded infection with either a second closely related gamma-proteobacterial symbiont or with a second less closely related alpha-proteobacterial symbiont.