Adaptive radiation of Borrelia in relation to cospeciation with vector ticks on geographical isolation

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An internal transcribed spacer (ITS2) sequence between the 5.8S and 28S rRNA genes and mitochondrial rrs sequence were used to estimate the phyletic relationships among Ixodes spp. tick vectors of Lyme disease-causing Borrelia spirochetes in geographically isolated regions. Analysis indicates that Borrelia burgdorferi sensu lato species associated with Lyme disease are found mainly in ticks of the Ixodes ricinus species complex and adapted to their own tick vectors. Other closely related tick species are not known to transmit the Borrelia-that cause Lyme disease in humans, but they appear to have a specific association with other closely related Borrelia species. There is a high degree of concordance in the phylogenetics of Borrelia taxa and the phylogenetic relationships among Ixodes ticks, which suggests a history of cospeciation in the tick-spirochete assemblage. We also investigated genetic variations of borrelial spirochetes isolated from two geographically separated sites. Seuqnce alignment using flagellin gene and outer sufface protein gene sequences showed that two genetic groups evolved independently in two separated region from borrelial species, and adapted to their own vector ticks. These results suggest that the occurrence and distribution of Borrelia in nature are depend on the vector competence of particular tick species.