Evolution of Northeastern and Midwestern Borrelia burgdorferi

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 (online Technical Appendix) 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 (34). 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.

Four novel ospC major group alleles appear to be unique to the Midwest. In addition to these novel ospC major groups, a type C-like allele appears to have been generated independently in the Midwest and the Northeast. The group C allele in the Midwest shares 96.8% similarity with the group C allele in the Northeast. Whether the unique ospC major group alleles were generated recently in only 1 region or whether they were shared in the ancestral population and subsequently lost in only 1 region is not clear.

B. burgdorferi lineages exchange DNA, contrary to previous reports. We found at least some evidence of recombination between all genetic loci examined; even the ospAB operon has several homoplasious mutations, suggesting potential recombination (online Technical Appendix). However, recombination between ospA and ospB is not statistically supported and may have arisen from recurrent mutation (online Appendix Table). Despite evidence for recombination, the linkage relationships are similar in the Northeast and the Midwest, supporting the recent common ancestry of these populations.

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