Leander B S, Harper J T, Keeling P J
Canadian Institute for Advanced Research, Program in Evolutionary Biology, Department of Botany, #3529-6270 University Boulevard, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.
J Parasitol. 2003 Dec;89(6):1191-205. doi: 10.1645/GE-3155.
Many aseptate gregarines from marine invertebrate hosts are thought to have retained several plesiomorphic characteristics and are instrumental in understanding the early evolution of intracellular parasitism in apicomplexans and the phylogenetic position of cryptosporidians. We sequenced the small-subunit (SSU) ribosomal RNA genes from 2 archigregarines, Selenidium terebellae and Selenidium vivax, and 2 morphotypes of the marine eugregarine Lecudina polymorpha. We also used scanning electron microscopy to investigate the surface morphology of trophozoites from Lecudina tuzetae, Monocystis agilis, the 2 species of Selenidium, and the 2 morphotypes of L. polymorpha. The SSU ribosomal DNA sequences from S. vivax and L. polymorpha had long branch lengths characteristic of other gregarine sequences. However, the sequence from S. terebellae was not exceptionally divergent and consistently emerged as 1 of the earliest 'true' gregarines in phylogenetic analyses. Statistical support for the sister relationship between Cryptosporidium spp. and gregarines was significantly bolstered in analyses including the sequence from S. terebellae but excluding the longest branches in the alignment. Eugregarines formed a monophyletic group with the neogregarine Ophryocystis, suggesting that trophozoites with elaborate cortex folds and gliding motility evolved only once. The trophozoites from the 2 species of Selenidium shared novel transverse striations but differed from one another in overall cell morphologies and writhing behavior.
许多来自海洋无脊椎动物宿主的无隔簇虫被认为保留了几个原始特征，有助于理解顶复门细胞内寄生的早期进化以及隐孢子虫的系统发育位置。我们对两种原簇虫（ terebellae硒簇虫和间日硒簇虫）以及多形勒氏簇虫的两种形态型的小亚基（SSU）核糖体RNA基因进行了测序。我们还使用扫描电子显微镜研究了图泽特氏勒氏簇虫、敏捷单囊簇虫、两种硒簇虫以及多形勒氏簇虫的两种形态型的滋养体的表面形态。间日硒簇虫和多形勒氏簇虫的SSU核糖体DNA序列具有其他簇虫序列特有的长分支长度。然而， terebellae硒簇虫的序列并没有异常分化，并且在系统发育分析中一直作为最早的“真正”簇虫之一出现。在包括 terebellae硒簇虫序列但排除比对中最长分支的分析中，对隐孢子虫属和簇虫之间姐妹关系的统计支持显著增强。真簇虫与新簇虫奥氏囊簇虫形成一个单系类群，这表明具有精细皮层褶皱和滑行运动的滋养体只进化了一次。两种硒簇虫的滋养体共有新颖的横向条纹，但在整体细胞形态和扭动行为上彼此不同。
