Out of Asia? Expansion of Eurasian Lyme borreliosis causing genospecies display unique evolutionary trajectories

Abstract

Vector‐borne pathogens exist in obligate transmission cycles between vector and reservoir host species. Host and vector shifts can lead to geographic expansion of infectious agents and the emergence of new diseases in susceptible individuals. Three bacterial genospecies (Borrelia afzelii, Borrelia bavariensis, and Borrelia garinii) predominantly utilize two distinct tick species as vectors in Asia (Ixodes persulcatus) and Europe (Ixodes ricinus). Through these vectors, the bacteria can infect various vertebrate groups (e.g., rodents, birds) including humans where they cause Lyme borreliosis, the most common vector‐borne disease in the Northern hemisphere. Yet, how and in which order the three Borrelia genospecies colonized each continent remains unclear including the evolutionary consequences of this geographic expansion. Here, by reconstructing the evolutionary history of 142 Eurasian isolates, we found evidence that the ancestors of each of the three genospecies probably have an Asian origin. Even so, each genospecies studied displayed a unique substructuring and evolutionary response to the colonization of Europe. The pattern of allele sharing between continents is consistent with the dispersal rate of the respective vertebrate hosts, supporting the concept that adaptation of Borrelia genospecies to the host is important for pathogen dispersal. Our results highlight that Eurasian Lyme borreliosis agents are all capable of geographic expansion with host association influencing their dispersal; further displaying the importance of host and vector association to the geographic expansion of vector‐borne pathogens and potentially conditioning their capacity as emergent pathogens.

Unprecedented genetic variability of PFam54 paralogs among Eurasian Lyme borreliosis‐causing spirochetes

“…For reconstruction of the lp54 sequences, on which the PFam54 gene array is located, we utilized the isolate library described in Rollins et al. ( 2023 ) containing MiSeq sequencing data for 136 Eurasian Borrelia isolates: B. afzelii (total n = 33, Asian n = 20, European n = 13), B. bavariensis (total n = 46, Asian n = 27, European n = 19), and B. garinii (total n = 57, Asian n = 25; European n = 32). Asian or European here refers to an isolate hypothesized to be maintained either in the I. persulcatus or I. ricinus transmission cycle, respectively, based on the geographic origin of isolation.…”

“…Sequences of the linear plasmid lp54 were assembled based on the mapping protocol outlined previously (Becker et al., 2020 ; Rollins et al., 2023 ). In brief, Illumina reads (paired‐end reads of 250 bp) were first trimmed for Illumina MiSeq adapter sequences using Trimmomatic v.0.38 (Bolger et al., 2014 ) before being assembled using SPAdes v.3.13.0 (Bankevich et al., 2012 ).…”

“…In brief, Illumina reads (paired‐end reads of 250 bp) were first trimmed for Illumina MiSeq adapter sequences using Trimmomatic v.0.38 (Bolger et al., 2014 ) before being assembled using SPAdes v.3.13.0 (Bankevich et al., 2012 ). SPAdes contigs were then mapped to reference lp54 sequences (PacBio Sequences: PBi, A104S, NT24, PHei, PBr, and NT31 (Margos et al., 2018 ; Rollins et al., 2023 ); GenBank: PKo (CP002950.1), K78 (CP009059.1), and ACA‐1 (CP001247)) using NUCmer v.3.23 from the package MUMmer (Delcher et al., 2002 ; Kurtz et al., 2004 ). Lp54 identity was confirmed by searching for plasmid partitioning genes belonging to PFam32, 49, 50, and 57.62 families using BLAST v.2.8.1 (Altschul et al., 1990 ; Camacho et al., 2009 ) (algorithm: blastn ) as outlined in (Becker et al., 2020 ; Rollins et al., 2023 ).…”

“…Currently, three major Borrelia species act as LB‐causing agents across Eurasia namely B. afzelii , B. bavariensis , and B. garinii (Kurtenbach et al., 2006 ; Radolf et al., 2012 ; Stanek et al., 2011 ; Steere et al., 2016 ). All three Borrelia species share an Asian origin and have expanded into at least two tick transmission cycles ( I. persulcatus , I. ricinus ) with this expansion being adaptive toward colonizing a new tick vector ( I. ricinus ) in B. bavariensis (Becker et al., 2020 ; Gatzmann et al., 2015 ; Margos et al., 2019 ; Rollins et al., 2023 ). In both transmission cycles, these Borrelia species additionally utilize variable reservoir hosts either infecting rodent ( B. afzelii , B. bavariensis ) or avian ( B. garinii ) species (Comstedt et al., 2011 ; Kurtenbach et al., 2006 ; Margos et al., 2009 ; Munro et al., 2017 ).…”

A high fidelity approach to assembling the complex Borrelia genome

“…Previous Borrelia genome sequencing projects have demonstrated that the use of short-read NGS sequencing alone, is insu cient for Borrelia plasmid assembly (15,29,38,39,40,41,42,43). Long-read sequencing technologies have greatly improved the complete reconstruction of plasmids and chromosome including challenging areas (21,44,45); however, incomplete plasmid sequences have still been reported (22,46).…”