Evolution of Northeastern and Midwestern Borrelia burgdorferi, United States
Figure. Phylogeny of Borrelia burgdorferi isolates in the northeastern and midwestern United States based on intergenic spacer (IGS) sequence. operational taxanomic unit names beginning with IGS were isolated in the northeastern United States (10); all other isolates are from patients in the Midwest. The letter after period designates the outer surface protein C (ospC) major allele of the isolate. Colored isolate names highlight isolates with the same ospC major group that cluster in different clades, which suggests horizontal gene transfer. The ospC of several strains is not linked to the IGS ribosomal spacer type (RST) to which it is commonly linked in the Northeast (10,34). AB indicates differences between the ospAB tree and the IGS tree. This tree is midpoint rooted. Scale bar indicates number of substitutions per site.