Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, USA.
mBio. 2011 Nov 8;2(6):e00228-11. doi: 10.1128/mBio.00228-11. Print 2011.
Toxoplasma gondii is a common parasite of animals that also causes a zoonotic infection in humans. Previous studies have revealed a strongly clonal population structure that is shared between North America and Europe, while South American strains show greater genetic diversity and evidence of sexual recombination. The common inheritance of a monomorphic version of chromosome Ia (referred to as ChrIa*) among three clonal lineages from North America and Europe suggests that inheritance of this chromosome might underlie their recent clonal expansion. To further examine the diversity and distribution of ChrIa, we have analyzed additional strains with greater geographic diversity. Our findings reveal that the same haplotype of ChrIa* is found in the clonal lineages from North America and Europe and in older lineages in South America, where sexual recombination is more common. Although lineages from all three continents harbor the same conserved ChrIa* haplotype, strains from North America and Europe are genetically separate from those in South America, and these respective geographic regions show limited evidence of recent mixing. Genome-wide, array-based profiling of polymorphisms provided evidence for an ancestral flow from particular older southern lineages that gave rise to the clonal lineages now dominant in the north. Collectively, these data indicate that ChrIa* is widespread among nonclonal strains in South America and has more recently been associated with clonal expansion of specific lineages in North America and Europe. These findings have significant implications for the spread of genetic loci influencing transmission and virulence in pathogen populations.
Understanding parasite population structure is important for evaluating the potential spread of pathogenicity determinants between different geographic regions. Examining the genetic makeup of different isolates of Toxoplasma gondii from around the world revealed that chromosome Ia is highly homogeneous among lineages that predominate on different continents and within genomes that were otherwise quite divergent. This pattern of recent shared ancestry is highly unusual and suggests that some gene(s) found on this chromosome imparts an unusual fitness advantage that has resulted in its recent spread. Although the basis for the conservation of this particularly homogeneous chromosome is unknown, it may have implications for the transmission of infection and spread of human disease.
刚地弓形虫是一种常见的动物寄生虫，也会导致人类的动物传染病。先前的研究揭示了北美的人群和欧洲的人群具有强烈的克隆种群结构，而南美菌株则表现出更大的遗传多样性和性重组的证据。北美和欧洲的三个克隆谱系共同遗传单态版本的 1 号染色体（称为 ChrIa*），这表明该染色体的遗传可能是它们最近克隆扩张的基础。为了进一步研究 ChrIa 的多样性和分布，我们分析了具有更大地理多样性的额外菌株。我们的发现表明，北美的克隆谱系和南美的古老谱系中都发现了相同的 ChrIa单倍型，而在性重组更为常见的地区。尽管来自所有三个大陆的谱系都携带相同的保守 ChrIa单倍型，但来自北美的菌株和欧洲的菌株在遗传上与来自南美的菌株不同，而这些各自的地理区域显示出最近混合的证据有限。基于全基因组的基于阵列的多态性分析提供了证据，表明来自特定古老南部谱系的祖先流产生了现在在北部占主导地位的克隆谱系。总的来说，这些数据表明 ChrIa*在南美的非克隆菌株中广泛存在，并且最近与北美的特定克隆谱系的克隆扩张相关联。这些发现对评估影响病原体种群传播和毒力的遗传基因座在不同地理区域之间的传播具有重要意义。
