Rich S M, Licht M C, Hudson R R, Ayala F J
Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697-2525, USA.
Proc Natl Acad Sci U S A. 1998 Apr 14;95(8):4425-30. doi: 10.1073/pnas.95.8.4425.
We have analyzed DNA sequences from world-wide geographic strains of Plasmodium falciparum and found a complete absence of synonymous DNA polymorphism at 10 gene loci. We hypothesize that all extant world populations of the parasite have recently derived (within several thousand years) from a single ancestral strain. The upper limit of the 95% confidence interval for the time when this most recent common ancestor lived is between 24,500 and 57,500 years ago (depending on different estimates of the nucleotide substitution rate); the actual time is likely to be much more recent. The recent origin of the P. falciparum populations could have resulted from either a demographic sweep (P. falciparum has only recently spread throughout the world from a small geographically confined population) or a selective sweep (one strain favored by natural selection has recently replaced all others). The selective sweep hypothesis requires that populations of P. falciparum be effectively clonal, despite the obligate sexual stage of the parasite life cycle. A demographic sweep that started several thousand years ago is consistent with worldwide climatic changes ensuing the last glaciation, increased anthropophilia of the mosquito vectors, and the spread of agriculture. P. falciparum may have rapidly spread from its African tropical origins to the tropical and subtropical regions of the world only within the last 6,000 years. The recent origin of the world-wide P. falciparum populations may account for its virulence, as the most malignant of human malarial parasites.
我们分析了来自全球各地恶性疟原虫地理菌株的DNA序列，发现在10个基因位点完全不存在同义DNA多态性。我们推测，该寄生虫目前所有的全球种群最近（在几千年内）都源自单一的祖先菌株。这个最近的共同祖先生活时间的95%置信区间上限在24,500至57,500年前之间（取决于核苷酸替代率的不同估计）；实际时间可能更近。恶性疟原虫种群的近期起源可能是由于种群扩张（恶性疟原虫最近才从一个地理范围有限的小种群传播到全世界）或选择性清除（一个受自然选择青睐的菌株最近取代了所有其他菌株）。选择性清除假说要求恶性疟原虫种群实际上是克隆性的，尽管该寄生虫的生命周期中有 obligate 有性阶段。几千年前开始 的种群扩张与末次冰期之后的全球气候变化、蚊媒嗜人性增加以及农业传播是一致的。恶性疟原虫可能仅在过去6000年内就从其非洲热带起源迅速传播到世界的热带和亚热带地区。全球恶性疟原虫种群的近期起源可能解释了它作为人类最恶性疟原虫的毒力。
