Studies on speciation of Asian schistosomes

亚洲人的染色体似乎分为几个系统发育组。其中之一包括日本血吸虫、湄公河血吸虫和马来西亚血吸虫，它们似乎彼此密切相关。S.sinensium的位置仍有争议。它似乎具有介于日本血吸虫和曼氏血吸虫之间的某些特征（Greer等人，1989年）。斯平代尔似乎最接近非洲物种（巴克和布莱尔，1996年），也许最类似于牛。不同的寄生虫使用非常不同的蜗牛宿主，如日本钉螺（S.japonicum）、新锥虫（S.mekongi）、罗伯茨氏菌（S.malaynsis）、拟钉螺（S. sinensis）、印度扁螺（S.spindale）和南方扁螺（S.incognitum）。血吸虫宿主特异性极高。本研究利用线粒体（COI）和核（ITS 2）DNA序列分析了亚洲和非洲血吸虫的进化关系，发现湄公河血吸虫和马来西亚血吸虫是日本血吸虫的姐妹篇。我们的数据还表明，S. sinensis ITS2区域包含两个缺失，每个共享与日本血吸虫和曼氏血吸虫。中华血吸虫位于日本血吸虫群的基部。这一事实有力地表明，S.sinencium可能是日本血吸虫的一个等位种。最近，我们能够从印度尼西亚雅加达获得S.incognitum，并扩增COI和ITS2区域。用Neighbor joining和UPGMA法进行计算机分析，发现该血吸虫与其它亚洲血吸虫在许多碱基对位点上存在差异，属于曼氏血吸虫或血生血吸虫等非洲血吸虫。这表明这种寄生虫起源于非洲，最近（在过去的10万年内）迁移到亚洲。

Asian schistosomes appear to fall into several phylogenetic groups. One of these includes Schistosoma japonicum, S.mekongi, and S.malaynsis which appear closely related to one another. The placement of S.sinensium is still debaded. It seems to possess some characteristics intermediate between S.japonicum and S.mansoni (Greer et al., 1989). S.spindale seems closest to Afrian species (Barker & Blair, 1996), and perhaps most similar to S.bovis. Very different snail hosts are used by the different parasites, Oncomelania (S.japonicum), Neotricula (S.mekongi), Robertsiella (S.malaynsis), Tricula (S.sinensium), Indoplanorbis (S.spindale), and Lymnea (S.incognitum). Schistosoma host specificities are extremely high. The present study was done to examine evolutionay relationships among some of Asian and African species of Schistosoma using mitochondrial (COI) and nuclear (ITS 2) DNA.It was found that S.mekongi and S.malaynsis were sisters to S.japonicum, based on DNA sequences. Our data also indicate that the S.sinensium ITS2 region contains two deletions, each shared with S.japonicum and S.mansoni. S.sinensium was placed at the base of the S.japonicum group. This fact strongly suggests that S.sinencium could be an ancestal species of S.japonicum. Recently, we were able to obtain S.incognitum from Jakarta, Indonesia, and to amplify the COI and ITS2 region. Basedn the computor analysis using Neighbor joining and UPGMA method, it was found that this blood fluke differs in many base pair sites from the other Asian blood flukes, and belongs to African blood flukes like S.mansoni or S.haematobium. This suggests that this parasite is of African origin and has migrated to Asia very recently (within last 100 thousands years).