MacLeod A, Tait A, Turner C M
Wellcome Centre of Molecular Parasitology, The Anderson College, University of Glasgow, 56 Dumbarton Road, Glasgow G11 6NU, UK.
Philos Trans R Soc Lond B Biol Sci. 2001 Jul 29;356(1411):1035-44. doi: 10.1098/rstb.2001.0892.
The African trypanosome, Trypanosoma brucei, is a zoonotic parasite transmitted by tsetse flies. Two of the three subspecies, T. brucei gambiense and T.b. rhodesiense, cause sleeping sickness in humans whereas the third subspecies, T.b. brucei, is not infective to humans. We propose that the key to understanding genetic relationships within this species is the analysis of gene flow to determine the importance of genetic exchange within populations and the relatedness of populations. T.brucei parasites undergo genetic exchange when present in infections of mixed genotypes in tsetse flies in the laboratory, although this is not an obligatory process. Infections of mixed genotype are surprisingly common in field isolates from tsetse flies such that there is opportunity for genetic exchange to occur. Population genetic analyses, taking into account geographical and host species of origin, show that genetic exchange occurs sufficiently frequently in the field to be an important determinant of genetic diversity, except where particular clones have acquired the ability to infect humans. Thus, T. brucei populations have an 'epidemic' genetic structure, but the better-characterized human-infective populations have a 'clonal' structure. Remarkably, the ability to infect humans appears to have arisen on multiple occasions in different geographical locations in sub-Saharan Africa. Our data indicate that the classical subspecies terminology for T. brucei is genetically inappropriate. It is an implicit assumption in most infectious disease biology that when a zoonotic pathogen acquires the capability to infect humans, it does so once and then spreads through the human population from that single-source event. For at least one major pathogen in tropical medicine, T. brucei, this assumption is invalid.
非洲锥虫，即布氏锥虫，是一种通过采采蝇传播的人畜共患寄生虫。三个亚种中的两个，即冈比亚布氏锥虫和罗德西亚布氏锥虫，可导致人类昏睡病，而第三个亚种，即布氏布氏锥虫，不会感染人类。我们认为，理解该物种内遗传关系的关键在于分析基因流动，以确定种群内基因交换的重要性以及种群之间的亲缘关系。在实验室条件下，当采采蝇感染混合基因型的布氏锥虫寄生虫时，它们会进行基因交换，不过这并非一个必然发生的过程。混合基因型感染在采采蝇的野外分离株中出奇地常见，因此存在发生基因交换的机会。考虑到地理和宿主物种来源的种群遗传分析表明，在野外基因交换发生得足够频繁，足以成为遗传多样性的一个重要决定因素，但特定克隆获得感染人类能力的情况除外。因此，布氏锥虫种群具有“流行”的遗传结构，但特征更明显的人类感染种群具有“克隆”结构。值得注意的是，感染人类的能力似乎在撒哈拉以南非洲的不同地理位置多次出现。我们的数据表明，布氏锥虫的经典亚种术语在遗传上并不恰当。在大多数传染病生物学中，一个隐含的假设是，当一种人畜共患病原体获得感染人类的能力时，它只会发生一次，然后从那个单一来源事件开始在人群中传播。对于热带医学中的至少一种主要病原体布氏锥虫来说，这个假设是不成立的。
