NGS population genetics analyses reveal divergent evolution of a Lyme Borreliosis agent in Europe and Asia.

Borrelia bavariensis is a recently described agent of Lyme disease within the B. burgdorferi sensu lato species complex and exhibits a strong capacity for human pathogenicity. B. bavariensis strains are widely distributed in Eurasia spanning the distribution range of the tick vectors Ixodes persulcatus and I. ricinus. It has been suggested that B. bavariensis forms two populations, one of which arose through vector adaptation and geographic expansion. We have performed phylogenetic and population genetic analyses with next-generation sequencing data of 26 strains of B. bavariensis targeting the main linear chromosome and two plasmids (lp54, cp26). A very low number of single nucleotide polymorphisms (SNPs) was found in the European population and a deep branching pattern between European and Asian B. bavariensis was observed in all phylogenies. The results confirm the population structure of B. bavariensis and strongly support the hypothesis of clonal expansion of the European population of B. bavariensis. In addition, signals of positive selection identified in the populations further support the hypothesis that the European population of B. bavariensis likely underwent vector adaptation in its recent evolutionary history. Identified genes represent promising candidates for experimental vector adaptation studies. Thus, this species forms a very good model to study vector adaptation, which is known to play an important role in the geographic distribution of B. burgdorferi. Analysis of well known virulence determinants that are attributed to severity of clinical manifestation in B. burgdorferi s.s. revealed no variation within the European population of B. bavariensis, underlining the importance of including various Borrelia species into investigations that aim to understand the pathogenesis of Lyme disease agents.