Asmundsson Ingrid M, Dubey J P, Rosenthal Benjamin M
Animal Parasitic Disease Laboratory, Animal and Natural Resources Institute, Agricultural Research Service, United States Department of Agriculture, Beltsville, MD 20705, USA.
Infect Genet Evol. 2006 Sep;6(5):352-60. doi: 10.1016/j.meegid.2006.01.001. Epub 2006 Feb 20.
The population genetics and systematics of most coccidians remain poorly defined despite their impact on human and veterinary health. Non-recombinant parasite clones characterized by distinct transmission and pathogenesis traits persist in the coccidian Toxoplasma gondii despite opportunities for sexual recombination. In order to determine whether this may be generally true for tissue-cyst forming coccidia, and to address evolutionary and taxonomic problems within the genus Sarcocystis, we characterized polymorphic microsatellite markers in Sarcocystis neurona, the major causative agent of equine protozoal myeloencephalitis (EPM). Bayesian statistical modeling, phylogenetic reconstruction based on genotypic chord distances, and analyses of linkage disequilibrium were employed to examine the population structure within S. neurona and closely related Sarcocystis falcatula isolates from North and South America. North American S. neurona were clearly differentiated from those of South America and also from isolates of S. falcatula. Although S. neurona is characterized by substantial allelic and genotypic diversity typical of interbreeding populations, one genotype occurs with significantly excessive frequency; thus, some degree of asexual propagation of S. neurona clones may naturally occur. Finally, S. neurona isolated from disparate North American localities and diverse hosts (opossums, a Southern sea otter, and horses) comprise a single genetic population. Isolates associated with clinical neurological disease bear no obvious distinction as measured by these presumably neutral genetic markers.
尽管球虫对人类和兽医健康有影响，但其群体遗传学和系统发育学仍未得到很好的界定。尽管有性重组的机会存在，但以不同传播和发病特征为特点的非重组寄生虫克隆仍存在于球虫弓形虫中。为了确定这对于形成组织囊肿的球虫是否普遍成立，并解决肉孢子虫属内的进化和分类问题，我们对马原生动物脑脊髓炎（EPM）的主要病原体神经肉孢子虫中的多态微卫星标记进行了表征。采用贝叶斯统计建模、基于基因型弦距离的系统发育重建以及连锁不平衡分析，来研究神经肉孢子虫以及来自北美和南美的密切相关的镰状肉孢子虫分离株的群体结构。北美神经肉孢子虫与南美神经肉孢子虫以及镰状肉孢子虫分离株明显不同。尽管神经肉孢子虫具有杂交群体典型的大量等位基因和基因型多样性，但有一种基因型出现的频率明显过高；因此，神经肉孢子虫克隆可能自然会发生某种程度的无性繁殖。最后，从北美不同地区和不同宿主（负鼠、一只南海獭和马）分离出的神经肉孢子虫构成一个单一的遗传群体。通过这些大概是中性的遗传标记衡量，与临床神经疾病相关的分离株没有明显区别。
