Evolution of Northeastern and Midwestern Borrelia burgdorferi

The divergence in human Lyme disease incidence between the Northeast and Midwest does not result from independent evolution of human invasiveness because of geographic isolation. Although pathogen populations in the Midwest appear geographically isolated from those in the Northeast, evolutionary and demographic analyses indicate that they share a recent common ancestor. Both populations have little standing genetic variation, as indicated by the limited number of polymorphic sites, suggesting small effective population sizes and similar life-history strategies. The combination of linked alleles also is similar in both regions, supporting the recent shared ancestor hypothesis. B. burgdorferi strains isolated in the Midwest are interleaved with northeastern strains on phylogenetic trees evincing their close evolutionary relationship. However, there is some genetic divergence and differing linkage groups between the regions, intimating that gene flow is limited between these populations, allowing them to differentiate. The recent common ancestor in the northeastern and midwestern B. burgdorferi and limited genetic divergence suggests that human Lyme disease incidence cannot be explained by fundamentally different evolutionary histories resulting in differing degrees of human infectiousness.

None of the loci investigated show substantial genetic divergence between regions, suggesting a recent common ancestor and similar phenotypes. Northeastern haplotypes are interleaved with midwestern haplotypes such that the time to coalescence of alleles within a region is equivalent to the time to coalescence for alleles from both regions. These data suggest that northeastern and midwestern strains have a recent common ancestor. The limited genetic diversity in B. burgdorferi in the Midwest and Northeast suggests that the populations have retained the life-history strategy of their common ancestor. However, isolation by distance and subsequent divergence resulted in unique alleles in each region.

The IGS gene tree reconstructed from midwestern and northeastern data broadly supports the RST system described using northeastern populations. RST types 1 and 2 form strongly supported monophyletic groups. RST3 is polyphyletic and should be split into 3 groups as defined by the strongly supported clades. Supporting this suggestion, RST3 is diverse genetically and phenotypically. Interestingly, this division would separate ospC major group I bearing strains, a particularly invasive group in humans, from the other RST3 strains that rarely cause disseminated infections in humans.
The ospC data support the hypothesis that the strains from the Northeast and Midwest have a common ancestor but are currently isolated and have begun to diverge. Most ospC major groups are found in both regions. Given the genetic distance between major group alleles, the exact set of alleles is unlikely to have occurred twice independently. Additionally, the linkage relationships between ospC alleles and IGS alleles are similar in both regions. Both lines of evidence suggest that most of the diversity at ospC originated before the northeastern and midwestern populations diverged. Differences in invasiveness between B. burgdorferi in the Northeast and Midwest do not result from fundamentally different evolutionary histories.

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