Center for Microbial Genetics and Genomics, Northern Arizona University, Flagstaff, AZ 86011-4073, USA.
BMC Microbiol. 2011 Jun 17;11:139. doi: 10.1186/1471-2180-11-139.
Francisella tularensis, the causative agent of tularemia, displays subspecies-specific differences in virulence, geographic distribution, and genetic diversity. F. tularensis subsp. holarctica is widely distributed throughout the Northern Hemisphere. In Europe, F. tularensis subsp. holarctica isolates have largely been assigned to two phylogenetic groups that have specific geographic distributions. Most isolates from Western Europe are assigned to the B.Br.FTNF002-00 group, whereas most isolates from Eastern Europe are assigned to numerous lineages within the B.Br.013 group. The eastern geographic extent of the B.Br.013 group is currently unknown due to a lack of phylogenetic knowledge about populations at the European/Asian juncture and in Asia. In this study, we address this knowledge gap by describing the phylogenetic structure of F. tularensis subsp. holarctica isolates from the country of Georgia, and by placing these isolates into a global phylogeographic context.
We identified a new genetic lineage of F. tularensis subsp. holarctica from Georgia that belongs to the B.Br.013 group. This new lineage is genetically and geographically distinct from lineages previously described from the B.Br.013 group from Central-Eastern Europe. Importantly, this new lineage is basal within the B.Br.013 group, indicating the Georgian lineage diverged before the diversification of the other known B.Br.013 lineages. Although two isolates from the Georgian lineage were collected nearby in the Ukrainian region of Crimea, all other global isolates assigned to this lineage were collected in Georgia. This restricted geographic distribution, as well as the high levels of genetic diversity within the lineage, is consistent with a relatively older origin and localized differentiation.
We identified a new lineage of F. tularensis subsp. holarctica from Georgia that appears to have an older origin than any other diversified lineages previously described from the B.Br.013 group. This finding suggests that additional phylogenetic studies of F. tularensis subsp. holarctica populations in Eastern Europe and Asia have the potential to yield important new insights into the evolutionary history and phylogeography of this broadly dispersed F. tularensis subspecies.
土拉弗朗西斯菌是土拉热的病原体，其毒力、地理分布和遗传多样性存在亚种特异性差异。F. tularensis 亚种 holarctica 广泛分布于北半球。在欧洲，F. tularensis 亚种 holarctica 的分离株主要分为两个具有特定地理分布的系统发育群。来自西欧的大多数分离株被归入 B.Br.FTNF002-00 群，而来自东欧的大多数分离株则归入 B.Br.013 群内的许多谱系。由于缺乏对欧洲/亚洲交界处和亚洲种群的系统发育知识，B.Br.013 群的东部地理范围目前尚不清楚。在这项研究中，我们通过描述来自格鲁吉亚的 F. tularensis 亚种 holarctica 分离株的系统发育结构，并将这些分离株置于全球系统地理背景下，来填补这一知识空白。
我们从格鲁吉亚鉴定出一个新的 F. tularensis 亚种 holarctica 遗传谱系，该谱系属于 B.Br.013 群。该新谱系在遗传和地理上与先前描述的来自中东欧的 B.Br.013 群谱系不同。重要的是，该新谱系在 B.Br.013 群内是基础的，表明格鲁吉亚谱系在其他已知 B.Br.013 谱系多样化之前就已经分化了。尽管来自格鲁吉亚谱系的两个分离株是在附近的乌克兰克里米亚地区收集的，但所有其他归入该谱系的全球分离株都是在格鲁吉亚收集的。这种有限的地理分布以及谱系内高水平的遗传多样性，与相对较老的起源和局部分化相一致。
我们从格鲁吉亚鉴定出一个新的 F. tularensis 亚种 holarctica 谱系，该谱系的起源似乎比之前描述的任何其他来自 B.Br.013 群的多样化谱系都要早。这一发现表明，对东欧和亚洲的 F. tularensis 亚种 holarctica 种群进行更多的系统发育研究，有可能为这一广泛分布的 F. tularensis 亚种的进化历史和系统地理学提供重要的新见解。
