Cavalier-Smith T, Chao E E
Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, Vancouver, Canada.
J Mol Evol. 1996 Dec;43(6):551-62. doi: 10.1007/BF02202103.
We have sequenced the small ribosomal subunit RNA gene of the diplozoan Trepomonas agilis. This provides the first molecular information on a free-living archezoan. We have performed a phylogenetic analysis by maximum likelihood, parsimony, and distance methods for all available nearly complete archezoan small subunit ribosomal RNA genes and for representatives of all major groups of more advanced eukaryotes (metakaryotes). These show Diplozoa as the earliest-diverging eukaryotic lineage, closely followed by microsporidia. Trepomonas proves to be much more closely related to Hexamita, and, to a lesser degree, to Spironucleus, than to Giardia. The close relationship between the free-living Trepomonas on our trees and the parasites Hexamita inflata and Spironucleus refutes the idea that the early divergence of the amitochondrial Archezoa is an artefact caused by parasitism. The deep molecular divergence between the three phagotrophic genera with two cytostomes (Hexamita, Trepomonas, Spironucleus) and the saprotrophic Giardia that lacks cytostomes is in keeping with the classical evidence for a fundamental difference in the symmetry of the cytoskeleton between the two groups. We accordingly separate the two groups as two orders: Distomatida for those with two cytostomes/cytopharynxes and Giardiida ord. nov. for Giardia and Octomitus that lack these, and divide each order into two families. We suggest that this fundamental divergence in manner of feeding and in the symmetry of the cytoskeleton evolved in a free-living diplozoan very early indeed in the evolution of the eukaryotic cell, possibly very soon after the origin of the diplokaryotic state (having two nuclei linked together firmly by the cytoskeleton) and before the evolution of parasitism by distomatids and giardiids, which may have colonized animal guts independently. We discuss the possible relationship between the two archezoan phyla (Metamonada and Microsporidia) and the nature of the first eukaryotic cell in the light of our results and other recent molecular data.
我们已对双滴虫类敏捷三滴虫（Trepomonas agilis）的小核糖体亚基RNA基因进行了测序。这提供了关于自由生活古原生动物的首个分子信息。我们通过最大似然法、简约法和距离法，对所有可用的近乎完整的古原生动物小亚基核糖体RNA基因以及所有更高级真核生物（后核生物）主要类群的代表进行了系统发育分析。这些分析表明双滴虫类是最早分化的真核生物谱系，紧随其后的是微孢子虫。事实证明，三滴虫与六鞭毛虫的关系比与贾第虫更为密切，在较小程度上与螺旋体核虫也有一定关系。我们构建的系统发育树显示，自由生活的三滴虫与寄生虫膨胀六鞭毛虫和螺旋体核虫之间的密切关系，驳斥了无线粒体古原生动物的早期分化是由寄生作用导致的人为现象这一观点。具有两个胞口的三个吞噬营养属（六鞭毛虫、三滴虫、螺旋体核虫）与缺乏胞口的腐生营养贾第虫之间存在深刻的分子差异，这与两组细胞骨架对称性存在根本差异的经典证据相符。因此，我们将这两组分为两个目：具有两个胞口/胞咽的归入双口目（Distomatida），而缺乏这些结构的贾第虫和八鞭虫则归入新建立的贾第目（Giardiida ord. nov.），并将每个目再分为两个科。我们认为，这种摄食方式和细胞骨架对称性的根本差异，实际上在真核细胞进化的早期，在自由生活的双滴虫类中就已出现，可能就在双核状态（通过细胞骨架将两个细胞核紧密连接在一起）起源后不久，且在双口类和贾第类寄生虫进化出寄生习性之前，它们可能是独立地定殖于动物肠道的。我们根据我们的研究结果和其他近期分子数据，讨论了两个古原生动物门（双滴虫门和微孢子虫门）之间可能存在的关系以及首个真核细胞 的性质。
