Borrelia bavariensis: Vector Switch, Niche Invasion, and Geographical Spread of a Tick-Borne Bacterial Parasite

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 B. burgdorferi sensu lato species complex.

The need to unravel the twisted nature of the Borrelia burgdorferi sensu lato complex across Europe

“…A good example of this is B. bavarensis, which was designated as its own species a few years back and not part of the B. garinii clade [86,87]. It is suggested that it diverged at some point from B. garinii, which is known to be bird-specific in terms of reservoir hosts and made a switch that enabled B. bavarensis to be associated with small mammals [5]. Genetic analysis is a valuable tool that has been employed extensively to determine important factors responsible for the pathogenesis of many bacterial species and could certainly answer questions regarding how genetic composition, including the identified CRASP genes, could lead to differences in reservoir host association [81,88].…”

“…B. burgdorferi sensu lato complex (referred to as B. burgdorferi) comprises at least 20 named species [4,5], with most human Lyme cases in Europe being caused by B. burgdorferi ss, Borrelia afzelii, Borrelia garinii, Borrelia bavariensis and B. spielmanii [6]. In the USA, only B. burgdorferi ss and Borrelia mayonii [7] are commonly associated with human infection.…”

High conservation combined with high plasticity: genomics and evolution of Borrelia bavariensis

“…However, the Russian isolates are new to the present paper and allow us to see that the Asian clade shows no detectable geographic clustering. Asian B. bavariensis are vectored by I. persulcatus, whereas the European vector is I. ricinus (see [9] for a review). As these two tick species co-occur and can even hybridize in their overlapping zone in Estonia, Latvia and Western Russia [56] , we expected that Russian B. bavariensis samples, might be genetically closer to the European isolates than the Japanese isolates, perhaps even showing that the European population might have diverged from a Russian lineage, but this was not the case.…”

“…7, 8 and 9) within or between the two B. bavariensis populations. Because all Asian B. bavariensis isolates are vectored by I. persulcatus, whereas European isolates are found only in I. ricinus, it has been hypothesized that it is the adaptation to a new vector species that caused the strong bottleneck observed in the European population (see [9] for a review). Genes that show a high differentiation between the two populations are particularly interesting candidates for playing a role in the adaptation to speci c tick vector species.…”

“…Both species are present across Eurasia; their main vectors are Ixodes persulcatus in Asia and I. ricinus in Europe and both are pathogenic to humans. However, the main hosts of the two species differ, with B. bavariensis being found in rodents, while its sister species B. garinii is found only in birds [7][8][9] . Originally, the two species were differentiated genetically by their so-called OspA type (i.e.…”