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

利用5.8S和28S rRNA基因之间的内部转录间隔区(ITS2)序列和线粒体RRS序列对硬蜱属(Ixodes spp.)地理隔离地区引起莱姆病的螺旋体滴虫媒介。分析表明，与莱姆病相关的伯氏疏螺旋体主要存在于莱姆病硬蜱复合体的硬蜱中，并适应其自身的媒介。其他关系密切的扁虱物种尚不知道会传播导致人类莱姆病的疏螺旋体，但它们似乎与其他密切相关的疏螺旋体物种有特定的联系。疏螺旋体分类群的系统发育与硬蜱之间的系统发育关系高度一致，这表明硬蜱-螺旋体组合中存在着一段共种的历史。我们还研究了从两个地理分离地点分离的带状螺旋体的遗传变异。鞭毛蛋白基因和外膜蛋白基因序列的比对表明，两个遗传群体在疏螺旋体物种的两个分离区域独立进化，并适应各自的媒介蚤。这些结果表明，疏螺旋体在自然界中的发生和分布取决于特定种类的媒介能力。

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.