Fluctuating asymmetry (FA) of the body, reflecting developmental instability, has been linked to fitness traits in birds. However, its potential role in shaping susceptibility to ectoparasitism remains largely unexplored. We investigated the relationship between FA in tarsus length and tick parasitism in European Robins Erithacus rubecula during autumn migration on the Baltic Sea coast in northern Poland. We examined 198 robins for tick infestations. FA was quantified as the difference between lengths of the left and right tarsus. The prevalence of tick-infested birds was 49.5%, with ticks predominantly identified as Ixodes ricinus (98.5%). Negative binomial models revealed that migration period was the sole predictor of tick abundance, with early migrants carrying significantly more ticks than late migrants. We found no association between tarsus FA and either total tick load or body side-specific tick presence. Although our results are correlative, they suggest that moderate asymmetry in robin leg morphology may not influence susceptibility to ticks. However, this susceptibility might depend on the context, for example the condition, sex and age of birds, or their stopover length, which suggests directions for further research. Our results highlight the importance of seasonal factors in host-parasite dynamics.

Ixodes ricinus (Family Ixodidae), a widespread generalist tick in Europe, feeds on a large number of host species and acts as a major vector for various tick-borne pathogens. Immature stages of I. ricinus are often found feeding on various bird species, but questions remain open regarding ecological interactions between these organisms especially in relation to potential life-stage specific infestation of juvenile birds. Utilizing ongoing monitoring schemes during the breeding season in northern Germany, we studied the occurrence of ticks on birds caught in two independent forest areas during three subsequent seasons (2022-2024) to study relationships between tick infestation and bird-specific traits (i.e., age, body condition). Tick infestation differed significantly between bird species and we observed significant interactions with bird age and body condition. Adult birds showed lower tick burdens in comparison to juvenile conspecifics and tick infestation showed complex interactions with body condition; highlighting multiple mechanisms are most likely influencing our results. Through morphological identification of attached ticks, we could show that I. ricinus larva and nymphs differ in their interactions with bird traits, with only the number of larvae differing with bird age and neither life-stage alone correlating with bird body condition. Taken together, we supported life-stage specific infestation as a potential driver of higher tick burdens in juvenile birds and show how standardized monitoring can provide insight into these interactions.

Many migratory species have experienced severe population declines, but the genetic consequences of such declines are still rarely assessed. The last Central European population of gull-billed terns ( Gelochelidon nilotica ) has declined from 500 breeding pairs in the 1940s to 52 in 2025, whereas Mediterranean populations of this migratory waterbird still thrive. Here, we compare whole-genome sequencing (WGS) data among the declining population, two thriving populations and the ancestors of the declining population. We find comparable nucleotide diversity, but lower observed heterozygosity in the Central European population compared to the Mediterranean populations. The contemporary samples show some population structure as well, although admixture analyses and low genetic differentiation ( F ST ) still suggest potential population connectivity. Museum specimens from the historic population reveal an increased level of genetic diversity compared to the contemporary population, with effective population size estimates suggesting two past population declines. While inbreeding coefficients ( F ROH ) in the current Central European population are significantly higher than in the historic population, they are similar to those in the Mediterranean populations. These results suggest that population structure may be emerging, and that although inbreeding is not yet at worrisome levels in the last Central European population of gull-billed terns, it may be on the rise. If this endangered population remains small and isolation manifests, the effects of inbreeding depression may become more pronounced over time, potentially reducing fitness and increasing the risk of extinction.

European and Asian populations of Borrelia (B.) bavariensis, a causative agent of Lyme borreliosis, substantially differ in their infection dynamics. This is argued to be a byproduct of the unique demographic history of B. bavariensis in relation to colonizing Europe from a highly diverse, ancestral Asian population. Whether genetic factors related to human disease could be unique traits associated with the demographic history of the European population though remains largely unclear. European B. bavariensis possesses at least two anti-complement determinants, BGA66 and BGA71 encoding by genes of the PFam54 gene array. In Asian B. bavariensis populations, the composition of this gene array is highly diverse. To assess functional integrity of PFam54 orthologs, two Asian B. bavariensis isolates, NT24 and JHM1114, were investigated. Despite the substantial observed genetic diversity, the complement-inhibitory and cell-protective function of BGA66 and BGA71 orthologs are largely conserved among European and Asian populations. We also identified two novel PFam54 orthologs of Asian origin, BGA67b and BGA71b, both of which display anti-complement activity on the terminal pathway and confer serum resistance. Taken together, our findings highlight the importance of studying natural variation of proteins potentially involved in immune escape, pathogenesis, and host adaptation.

Lyme borreliosis is caused by species of the Borrelia burgdorferi sensu lato complex. Genospecies vary in human pathogenicity, reservoir host and tick vector adaptation. What causes these differences is currently unknown, but some answers are likely to be found within the fragmented genomes containing linear and circular plasmids (and even fusions). These plasmids are highly variable and challenging to reconstruct completely. We aimed to analyze plasmid presence/absence and to identify trends in plasmid numbers and/or types associated with specific ecological adaptations or pathogenic potentials. For this, we used the most complete, reliable, and currently available 86 B. burgdorferi s.l. genomes (30 newly sequenced, 56 publicly available) belonging to 21 of the 28 known species. The plasmid types per isolate vary from 6-21 (median: 12) and linear plasmid numbers (3-12, median: 7) are significantly higher than circular plasmids (2-11, median: 5). The only plasmids found in all isolates are lp54 and cp26 and only a few species- or population-specific plasmids are identified, demonstrating the high variability. Species with smaller plasmid counts (median < 11) tend to have fewer lp28 and cp32 plasmids than larger plasmid counts species (median ≥ 15). Significantly higher total and circular plasmid numbers are found in rodents-only compared to birds-only associated species. Interestingly, plasmid numbers are not significantly reduced in species associated with a single reservoir host/vector compared to multiple. Analyses of the plasmid-location of the vls/vlsE antigenic variation system highlighted the dynamic nature of B. burgdorferi s.l. plasmids. Surprisingly, whilst the plasmid repertoire of B. burgdorferi s.l. is very plastic and variable, our analyses rarely reveal plasmids associated with specific ecological adaptations or pathogenic potentials. However, some plasmids were more frequently found in one of the groups and will be of interest for gene-based analyses that will be required for further progress in this research area.

Ixodes ricinus and Ixodes inopinatus are closely related sister taxa which show high morphological similarity resulting in challenges to proper species determination. It is unclear how these two species are geographically distributed in areas where both species could occur (i.e., North Africa) and what this could mean towards potential pathogenic microorganisms. Here we report on Ixodes ticks (n = 31) collected from cattle in high plateau areas (Guelma province) of north-eastern Algeria. Three Ixodes species (I. ricinus, n = 10; I. inopinatus, n = 19; I. ventalloi, n = 2) were identified morphological and confirmed molecularly through analysis of the trospa gene sequence. Clear morphological markers between the three species were found in line with the original descriptions, although molecular analysis did not support morphological identification for all I. ricinus and I. inopinatus ticks (n = 7). With this we found no significant association between the molecular and morphological identification of I. inopinatus (phi coefficient of rϕ = 0.088, p = 0.71). Taking molecular identification as a standard, a literature search was performed to determine the geographic extent of I. inopinatus and associate this with Köppen and Geiger climate classifications. Geographically, I. inopinatus is likely restricted to the western Mediterranean Basin of North Africa and Europe and most likely adapted to the Mediterranean climate (Csa). The three investigated tick species were tested to be positive for DNA of Rickettsia helvetica and Rickettsia monacensis. This study confirmed the sympatric presence of I. inopinatus and I. ricinus as well as reported, for the first time, the occurrence of I. ventalloi in the high plateaus from north-eastern Algeria. In addition, the present study highlights the detection of R. helvetica and R. monacensis in I. inopinatus infesting cattle, which represent valuable findings of significant epidemiological interest, opening potential questions on the influence I. inopinatus could play in relation to rickettsioses in cattle.

Heligoland is an island located in the North Sea, where vegetation was almost destroyed as a result of heavy bombardment during and after the Second World War. However, over the past 70 years, the vegetation has developed from scrub towards bushy or even forested environments. This change has most likely altered habitat suitability for various organisms, including many species of ticks. Ticks can act as major vectors for various pathogens of humans and animals; thus, characterizing the occurrence of a tick population and associated microorganism on the island is of great importance in relation to public and animal health. For this characterization on Heligoland, we flagged ticks at four different locations during June 2023 and 2024. In 2024, ticks were opportunistically sampled from house pets living on the island and during the annual ringing of common murre (Uria aalge) fledglings. In total, 267 ticks were collected over the 2 years which were identified morphologically, and confirmed molecularly if needed, to four species: Ixodes ricinus (n = 132), Haemaphysalis punctata (n = 47), Ixodes uriae (n = 3), and Alectorobius maritimus (n = 85), which for the latter represents the first report in Germany. Questing tick samples positive for Borrelia burgdorferi sensu lato, Rickettsia spp., Anaplasma phagocytophilum, and Babesia spp. were found in one or both years. Subsequent sequencing showed the presence of two Rickettsia species (R. helvetica, R. aeschlimannii), multiple Borrelia species (B. garinii, B. valaisiana, B. bavariensis, B. afzelii), and two Babesia species (Ba. venatorum, Ba. capreoli). Our research highlights a diverse tick and associated microorganism population on the island, which could pose public and animal health risks that will need to be monitored in the future.

Ticks are blood-sucking arthropods which can vector various, pathogenic microorganisms between humans and domestic or wild animal hosts. In Cameroon, little is still known about the diversity of ticks and tick-borne pathogens found feeding on these various hosts. This study investigates the frequency of positive pools of Borrelia spp. and Rickettsia spp. in 415 DNA pools arising from 1148 collected ticks belonging to five genera and twenty-five tick species collected from both domestic and wild animals in Cameroon. Tick species were identified morphologically and confirmed molecularly when necessary. All tick pools were tested for Rickettsia spp. and Borrelia spp. using molecular methods of which 18.01 % and 10.38 % of tick pools tested positive for Rickettsia or Borrelia DNA, respectively. This is the first Borrelia spp. detection in ticks collected from wild animals in Cameroon. Three species of Rickettsia were found in ticks feeding on domestic animals, namely, Rickettsia africae, Rickettsia aeschlimannii, and Rickettsia massiliae. Borrelia spp. in Cameroon are closely related to Candidatus Borrelia javanensis from China, as well as Candidatus Borrelia africana and Candidatus Borrelia ivorensis from the Ivory Coast. Although the risk this Borrelia species could pose to humans or animals is currently not known, both Rickettsia species are known to cause human disease warranting continuous monitoring and future research to determine the overall public health risk these microorganisms could pose.

AbstractLyme borreliosis (LB) is the most common vector‐borne disease in the Northern Hemisphere caused by spirochetes belonging to the Borrelia burgdorferi sensu lato (Bbsl) complex. Borrelia spirochetes circulate in obligatory transmission cycles between tick vectors and different vertebrate hosts. To successfully complete this complex transmission cycle, Bbsl encodes for an arsenal of proteins including the PFam54 protein family with known, or proposed, influences to reservoir host and/or vector adaptation. Even so, only fragmentary information is available regarding the naturally occurring level of variation in the PFam54 gene array especially in relation to Eurasian‐distributed species. Utilizing whole genome data from isolates (n = 141) originated from three major LB‐causing Borrelia species across Eurasia (B. afzelii, B. bavariensis, and B. garinii), we aimed to characterize the diversity of the PFam54 gene array in these isolates to facilitate understanding the evolution of PFam54 paralogs on an intra‐ and interspecies level. We found an extraordinarily high level of variation in the PFam54 gene array with 39 PFam54 paralogs belonging to 23 orthologous groups including five novel paralogs. Even so, the gene array appears to have remained fairly stable over the evolutionary history of the studied Borrelia species. Interestingly, genes outside Clade IV, which contains genes encoding for proteins associated with Borrelia pathogenesis, more frequently displayed signatures of diversifying selection between clades that differ in hypothesized vector or host species. This could suggest that non‐Clade IV paralogs play a more important role in host and/or vector adaptation than previously expected, which would require future lab‐based studies to validate.

Many bird populations are declining globally, with migratory sea- and waterbirds among the most threatened groups. Increased isolation between populations, due to habitat loss leading to limited connectivity and reduced gene flow, may accelerate population declines or even cause local extinctions. To address this threat and inform conservation efforts, assessment of the genetic diversity of endangered populations is required. In Central Europe, gull-billed terns (Gelochelidon nilotica) have experienced a severe population decline in the past century, with only one last population of c. 40 breeding pairs remaining. Here, we aimed to characterise the genetic diversity of this population, and to compare it to that of two separate Mediterranean colonies to assess potential population connectivity. To achieve this, we developed a PCR protocol to amplify and sequence the complete mitochondrial genome (mtDNA). We found high genetic diversity in all three populations, and no population structure based on breeding location, as well as moderate population differentiation between Germany and Italy, and low differentiation between Germany and Spain, and between Italy and Spain. The German population, however, showed signs of population restriction, while both Mediterranean colonies showed evidence for population expansion. These results indicate that the last population of gull-billed terns in Central Europe is not acutely threatened by loss of genetic diversity, but instead shows signs of connectivity with other European colonies, which in turn suggests that conservation efforts should best focus on habitat and predation management.

Ticks are blood-feeding parasites which act as major vectors for various pathogenic microorganisms affecting both animal and human health. Hard ticks are known to “hitchhike” on migratory birds as they transit from breeding to overwintering grounds in spring and autumn, potentially leading to exchange and establishment of non-endemic tick species in novel environments. That said, it is not yet clear which migratory bird taxa play a role in movement of specific tick genera and what influence migratory season may have. To fill this knowledge gap, we performed a systematic literature review regarding primary data of ticks moving on migratory birds within the African-Western Palearctic flyways. In total, 34 studies were found which showed 123 bird species from 37 families connected to potential movement of 26 tick species representing six genera (Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Rhipicephalus). Statistical analysis showed bird families (Ixodes: Turdidae, Prunellidae; Hyalomma: Turdidae, Strigidae, Muscicapidae, Motacillidae, Emerizidae) which carried above average number of ticks were only found for interactions with Hyalomma and Ixodes ticks with below average estimates found for all tick genera besides Rhipicephalus. Contrary to expectation, no tick genus, which was found in both migratory seasons, was estimated to have increased numbers in one season or the other. In certain cases, tick genera (e.g., Amblyomma) were only found on birds during spring migration. This pattern could highlight that the assemblage of ticks present on a bird at capture does not represent the ticks present at the point of migratory departure, highlighting an understudied importance of stopover sites to potential tick introduction or turnover. Taken together, the results presented here provide guiding information for further analysis into species specific interactions which will allow for the integration of individual level variation into understanding the risk of tick movement with migratory birds and potential for emergent disease.

Ticks are important vectors of human and animal pathogens, but many questions remain unanswered regarding their taxonomy. Molecular sequencing methods have allowed research to start understanding the evolutionary history of even closely related tick species. Ixodes inopinatus is considered a sister species and highly similar to Ixodes ricinus, an important vector of many tick-borne pathogens in Europe, but identification between these species remains ambiguous with disagreement on the geographic extent of I. inopinatus. In 2018-2019, 1583 ticks were collected from breeding great tits (Parus major) in southern Germany, of which 45 were later morphologically identified as I. inopinatus. We aimed to confirm morphological identification using molecular tools. Utilizing two genetic markers (16S rRNA, TROSPA) and whole genome sequencing of specific ticks (n = 8), we were able to determine that German samples, morphologically identified as I. inopinatus, genetically represent I. ricinus regardless of previous morphological identification, and most likely are not I. ricinus/I. inopinatus hybrids. Further, our results showed that the entire mitochondrial genome, let alone singular mitochondrial genes (i.e., 16S), is unable to distinguish between I. ricinus and I. inopinatus. Our results suggest that I. inopinatus is geographically isolated as a species (northern Africa and potentially southern Spain and Portugal) and brings into question whether I. inopinatus exists in central Europe. Our results highlight the probable existence of I. inopinatus and the power of utilizing genomic data in answering questions regarding tick taxonomy.

Ixodes persulcatus, a hard-bodied tick species primarily found in Asia and Eastern Europe, is a vector of pathogens to human and livestock hosts. Little research has been done on the microbiome of this species, especially using individual non-pooled samples and comparing different geographical locations. Here, we use 16S rRNA amplicon sequencing to determine the individual microbial composition of 85 Borrelia-positive I. persulcatus from the Japanese islands of Hokkaido and Honshu. The resulting data (164 unique OTUs) were further analyzed to compare the makeup and diversity of the microbiome by sex and location, as well as to determine the presence of human pathogens. We found that, while location had little influence, the diversity of I. persulcatus microbiome was predominantly dependent on sex. Males were seen to have higher microbiome diversity than females, likely due to the high presence of endosymbiotic Candidatus Lariskella arthropodarum within the female microbial communities. Furthermore, high read counts for five genera containing potentially human pathogenic species were detected among both male and female microbiomes: Ehrlichia, Borrelia, Rickettsia, Candidatus Neoehrlichia and Burkholderia and co-infections between different pathogens were frequent. We conclude that the microbiome of I. persulcatus depends mainly on sex and not geographical location and that the major difference between sexes is due to the high abundance of Ca. L. arthropodarum in females. We also stress the importance of this tick species as a vector of potential human pathogens frequently found in co-infections.

BACKGROUND:Bacteria of the Borrelia burgdorferi sensu lato (s.l.) complex can cause Lyme borreliosis. Different B. burgdorferi s.l. genospecies vary in their host and vector associations and human pathogenicity but the genetic basis for these adaptations is unresolved and requires completed and reliable genomes for comparative analyses. The de novo assembly of a complete Borrelia genome is challenging due to the high levels of complexity, represented by a high number of circular and linear plasmids that are dynamic, showing mosaic structure and sequence homology. Previous work demonstrated that even advanced approaches, such as a combination of short-read and long-read data, might lead to incomplete plasmid reconstruction. Here, using recently developed high-fidelity (HiFi) PacBio sequencing, we explored strategies to obtain gap-free, complete and high quality Borrelia genome assemblies. Optimizing genome assembly, quality control and refinement steps, we critically appraised existing techniques to create a workflow that lead to improved genome reconstruction.RESULTS:Despite the latest available technologies, stand-alone sequencing and assembly methods are insufficient for the generation of complete and high quality Borrelia genome assemblies. We developed a workflow pipeline for the de novo genome assembly for Borrelia using several types of sequence data and incorporating multiple assemblers to recover the complete genome including both circular and linear plasmid sequences.CONCLUSION:Our study demonstrates that, with HiFi data and an ensemble reconstruction pipeline with refinement steps, chromosomal and plasmid sequences can be fully resolved, even for complex genomes such as Borrelia. The presented pipeline may be of interest for the assembly of further complex microbial genomes.

AbstractTicks are important vectors of human and animal pathogens, but many questions remain unanswered regarding their taxonomy. Molecular sequencing methods have allowed research to start understanding the evolutionary history of even closely related tick species.Ixodes inopinatusis considered a sister species and highly similar toIxodes ricinus, an important vector of many tick-borne pathogens in Europe, but identification between these species remains ambiguous with disagreement on the geographic extent ofI. inopinatus. In 2018-2019, 1583 ticks were collected from breeding great tits (Parus major) in southern Germany, of which 45 were later morphologically identified asI. inopinatus. We aimed to confirm morphological identification using molecular tools. Utilizing two genetic markers (16S rRNA, TROSPA) (n=37) and whole genome sequencing of specific ticks (n=8), we were able to determine that German samples morphologically identified asI. inopinatus, genetically representI. ricinusregardless of previous morphological identification and most likely are notI. ricinus/I. inopinatushybrids. Further, our results showed that the entire mitochondrial genome, let alone singular mitochondrial genes (i.e., 16S), is unable to distinguish betweenI. ricinusandI. inopinatus. As most examples ofI. inopinatusin Germany were based on morphology and mitochondrial sequences, the results of the current study brings into question whetherI. inopinatuswas properly identified in previous research and if this species exists in Central Europe. Our results highlight the power of utilizing genomic data in answering questions regarding tick taxonomy even when closely related species are considered.HighlightsGermanIxodes inopinatussamples representI. ricinusbased on genomic dataThe mitochondrial genome is not sufficient for delineation ofI. inopinatusandI. ricinusGerman samples most likely do not representI. ricinus/I. inopinatushybrids

Lyme borreliosis (LB) is the most common vector-borne disease in the Northern Hemisphere caused by spirochetes belonging to the Borrelia burgdorferi sensu lato (Bbsl) complex. Borrelia spirochetes circulate in obligatory transmission cycles between tick vectors and different vertebrate hosts. To successfully complete this complex transmission cycle, Bbsl encode for an arsenal of proteins including the PFam54 protein family with known, or proposed, influences to reservoir host and/or vector adaptation. Even so, only fragmentary information is available regarding the naturally occurring level of variation in the PFam54 gene array and its impact on Borrelia pathogenesis. Utilizing whole genome data from isolates (n=141) originated from the three major LB-causing Borrelia species across Eurasia (B. afzelii, B. bavariensis, and B. garinii), we aimed to characterize the diversity of the PFam54 gene array in these isolates to facilitate understanding the evolution of PFam54 orthologs on an intra- and interspecies level. We found an extraordinarily high level of variation in the PFam54 gene array with 39 PFam54 paralogs belonging to 23 orthologous groups including five novel paralogs. Even so, the gene array appears to have remained fairly stable over the evolutionary history of these Borrelia species. Interestingly, genes outside Clade IV previously associated with host or, proposed, vector adaptation more frequently displayed signatures of diversifying selection. Taken together, our findings support the idea that non-Clade IV orthologs could play a larger role in host and/or vector adaptation than previously thought.

Lyme borreliosis is the most common vector-borne disease in the Northern Hemisphere, caused by spirochetes belonging to the Berretta burgdorferi sensu lato species complex, which are transmitted by ixodid ticks. 8. burgdorferi sensu law species produce a family of proteins on the linear plasmid 54 (PFam54), some of which confer the functions of cell adhesion and inactivation of complement, the first line of host defense. However, the impact of PFam54 in promoting B. burgdorferi sensu lato pathogenesis remains unclear because of the hurdles to simultaneously knock out all PFam54 proteins in a spirochete. Here, we describe two Borrelia bavariensis strains, PBN and PNi, isolated from patients naturally lacking PFam54 but maintaining the rest of the genome with greater than 95% identity to the reference 8. bavariensis strain, PBi. We found that PBN and PNi less efficiently survive in human serum than PBi. Such defects were restored by introducing two B. bavariensis PFam54 recombinant proteins, BGA66 and BGA71, confirming the role of these proteins in providing complement evasion of 8. bavariensis. Further, we found that all three strains remain detectable in various murine tissues 21 days post-subcutaneous infection, supporting the nonessential role of B. bavariensis PFam54 in promoting spirochete persistence. This study identified and utilized isolates deficient in PFam54 to associate the defects with the absence of these proteins, building the foundation to further study the role of each PFam54 protein in contributing to B. burgdorferi sensu lato pathogenesis. IMPORTANCE To establish infections, Lyme borreliae utilize various means to overcome the host's immune system. Proteins encoded by the PFam54 gene array play a role in spirochete survival in vitro and in vivo. Moreover, this gene array has been described in all currently available Lyme borreliae genomes. By investigating the first two Borrelia bavariensis isolates naturally lacking the entire PFam54 gene array, we showed that both patient isolates display an increased susceptibility to human serum, which can be rescued in the presence of two PFam54 recombinant proteins. However, both isolates remain infectious to mice after intradermal inoculation, suggesting the nonessential role of PFam54 during the long-term, but may differ slightly in the colonization of specific tissues. Furthermore, these isolates show high genomic similarity to type strain PBi (>95%) and could be used in future studies investigating the role of each PFam54 protein in Lyme borreliosis pathogenesis.

Thermal food processing leads to the formation of advanced glycation end products (AGE) such as Nε -carboxymethyllysine (CML). Accordingly, these non-canonical amino acids are an important part of the human diet. However, CML is only partially decomposed by our gut microbiota and up to 30% are excreted via faeces and, hence, enter the environment. In frame of this study, we isolated a soil bacterium that can grow on CML as well as its higher homologue Nε -carboxyethyllysine (CEL) as sole source of carbon. Bioinformatic analyses upon whole-genome sequencing revealed a subspecies of Pseudomonas asiatica, which we named 'bavariensis'. We performed a metabolite screening of P. asiatica subsp. bavariensis str. JM1 grown either on CML or CEL and identified N-carboxymethylaminopentanoic acid and N-carboxyethylaminopentanoic acid respectively. We further detected α-aminoadipate as intermediate in the metabolism of CML. These reaction products suggest two routes of degradation: While CEL seems to be predominantly processed from the α-C-atom, decomposition of CML can also be initiated with cleavage of the carboxymethyl group and under the release of acetate. Thus, our study provides novel insights into the metabolism of two important AGEs and how these are processed by environmental bacteria.

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.

Vector-borne pathogens exist in obligate transmission cycles between vector and reservoir host species. Host shifts can lead to geographic expansion and the emergence of new diseases. Three etiological agents of human Lyme borreliosis (Borrelia afzelii, Borrelia bavariensis, and Borrelia garinii) predominantly utilize two distinct tick species as vectors in Asia (Ixodes persulcatus) and Europe (Ixodes ricinus) but how and in which order they colonized each continent remains unknown. Here, by reconstructing the evolutionary history of 142 Eurasian isolates, we show that all three Borrelia genospecies evolved from an Asian origin, suggesting that successful expansion into Europe resulted through invading a novel vector. The pattern of gene flow between continents is different between genospecies and most likely conditioned by reservoir host association and their dispersal. Our results highlight that Eurasian Lyme borreliosis agents are all capable of geographic expansion through vector shifts, but potentially differ in their capacity as emergent pathogens.