Range-wide genetic analysis of Dermacentor variabilis and its Francisella-like endosymbionts demonstrates phylogeographic concordance between both taxa

Authors: KAUFMAN, EMILY - Northern Arizona University; STONE, NATHAN - Northern Arizona University; Scoles, Glen; HEPP, CRYSTAL - Northern Arizona University; BUSCH, JOSEPH - Northern Arizona University; WAGNER, DAVID - Northern Arizona University

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2018
Publication Date: 5/18/2018
Citation: Kaufman, E.L., Stone, N.E., Scoles, G.A., Hepp, C.M., Busch, J.D., Wagner, D.M. 2018. Range-wide genetic analysis of Dermacentor variabilis and its Francisella-like endosymbionts demonstrates phylogeographic concordance between both taxa. Parasites & Vectors. https://doi.org/10.1186/s13071-018-2886-5.
DOI: https://doi.org/10.1186/s13071-018-2886-5

Interpretive Summary: The American dog tick, Dermacentor variabilis, is an important vector of several pathogens to humans, wildlife, and domestic animals in North America. Although this tick is widely distributed in the U.S. and Canada, there is very little information about the genetic differences between populations in different parts of its range. A total of 1,053 ticks collected from 26 U.S. states and five Canadian provinces were analyzed to determine their genetic relationships. The ticks were also screened for a Francisella-like endosymbiont and other ecologically important bacterial species. The data shows that there are two major genetic groups of ticks with distinct geographic distributions: one group from the eastern U.S./Canada, and the other from the west coast of the U.S. (California and Washington). However the genetic subdivisions within these two major groups were not correlated with geography. We also found that there was a similar pattern for Francisella-like endosymbionts, showing a perfect one-to-one correlation between symbiont groups and the two major groups of ticks. These findings suggest that there are low levels of genetic exchange between the eastern and western tick groups, but that within the groups there is a high level of genetic mixing. The analysis of the symbiont provides evidence that there is co-evolution between the bacterial symbionts and the ticks. This work helps us to understand the distribution and spread of this tick species and will be also help or understanding of the role the symbionts may play in transmission of the vector-borne diseases associated with this tick.

Technical Abstract: Background: The American dog tick, Dermacentor variabilis, is an important vector of pathogens to humans, wildlife, and domestic animals in North America. Although this tick is widely distributed in the US and Canada, knowledge of its range-wide phylogeographic patterns remains incomplete. Methods: We carried out a phylogenetic analysis of D. variabilis using samples collected from 26 US states and five Canadian provinces. Tick samples (n=1,053 total) originated from two main sources: existing archives, and new collections made from 2012-2013. We sequenced a 691 bp fragment of the COI gene from a subset (n=332) of geographically diverse D. variabilis. DNA extracted from individual ticks (n=1,053) was also screened for a Francisella-like endosymbiont and other important bacterial species using a targeted 16S rRNA sequencing approach and two species-specific quantitative PCR assays. Results: Maximum parsimony analysis of COI sequences revealed two major groups within D. variabilis with distinct geographic distributions: one from the eastern US/Canada (Group 1) and one from the west coast of the US (California and Washington; Group 2). However, genetic subdivisions within both of these two major groups were weak to moderate and not tightly correlated with geography. We found molecular signatures consistent with Francisella-like endosymbionts, Coxiella burnetii, and Rickettsia spp. within the DNA extracts from the 1,053 individual ticks. Phylogenetic patterns for Francisella-like endosymbionts, constructed using sequence data from the bacterial 16S rRNA locus, were similar to those for D. variabilis, with two major groups that had an almost perfect one-to-one correlation with the two major groups within D. variabilis. Conclusions: Our findings suggest high levels of genetic mixture among widely separated geographic localities within each of the two major D. variabilis phylogenetic groups, while simultaneously implying little to no mixture between these two groups. Furthermore, our phylogenetic analyses provide evidence of long-term tick/symbiont co-evolution. This work has implications for understanding the dispersal and evolutionary ecology of this tick and associated vector-borne diseases.