Abstract
The Borrelia burgdorferi sensu lato species complex currently consists of more than 20 species that utilize small vertebrate hosts as reservoirs and ticks of the genus Ixodes as vectors. The bacterial parasite species differ in their ecology (reservoir hosts, competent vectors) as well as geographical distribution. Several studies have defined reservoir host association as a driver for diversification, speciation and pattern of spatial occurrence of populations but in this review we focus on vector association and its role in diversification and speciation of Borrelia. Borrelia bavariensis, a member of the species complex, uses small mammals as reservoir hosts and can cause Lyme borreliosis in humans. Phylogeographic analysis employing both genetic and genomic data shows that this species has undergone a recent range expansion, invading Europe from its probable original range in Asia while undergoing a drastic genetic bottleneck, suggesting that the colonization of Europe was a single event. We hypothesize that this invasion-like range expansion coincided with a change in the parasite’s tick vector, switching from Ixodes persulcatus in Asia to I. ricinus in Europe. Making extensive use of the ecological concept of niche, we discuss the importance of host and vector associations in defining spatial range and their possible role in speciation in the Borrelia burgdorferi sensu lato species complex.
Highlights
PARASITE INVASIVENESSJust like their hosts, parasites may on occasion abruptly expand their geographic range in an invasion-like process (Hatcher et al, 2012; Poulin, 2017; Chalkowski et al, 2018)
A simple model for Borrelia adaptation to a new vector would be that co-infection of a single tick with a Borrelia species already adapted to transmission by I. ricinus (e.g., B. garinii, B. afzelii) would enable that species, and B. bavariensis to attach to the midgut of the new tick vector, and/or to be protected against immune attack of the new vector, and/or to be successfully transmitted by the new vector to the reservoir host (Figure 6)
In this review we have described ecological factors that are important for geographic invasion by tick-borne bacterial pathogens
Summary
An “unoccupied niche” is susceptible to colonization, with or without human assistance, and identifying that niche leads to predictions about which organisms will invade and how they will spread. The idea that invasiveness is correlated with overall “niche breadth” can be tested at a simple level; the hypothesis is not universally validated by evidence but has received strong support in some cases (e.g., Sol, 2016; Granot et al, 2017). It is not self-evident what is meant by overall “niche breadth” nor how it can be measured or compared. It may be the case that larger geographic range drives adaptation to greater variety of niches (Jahner et al, 2011)
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