The phagotrophic origin of eukaryotes and phylogenetic classification of protozoa
The phagotrophic origin of eukaryotes and phylogenetic classification of protozoa
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DOI：
10.1099/00207713-52-2-297
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发表时间：
2002-03-01
影响因子：
2.8
通讯作者：
Cavalier-Smith, T
中科院分区：
文献类型：
--
作者：
Cavalier-Smith, T
真核生物和古细菌形成了新村分支，并且是姐妹篇，正如仅在古细菌中片段化的基因和许多序列树所明确显示的那样。这种姐妹关系驳斥了所有关于真核生物起源于古细菌和α-变形菌合并的理论，这些理论也解释了真核生物和放线菌特别共有的许多特征。我修改了真核生物起源的吞噬理论，认为大多数真核细胞的特性（吞噬，内膜系统，包括过氧化物酶体，细胞骨架，细胞核，有丝分裂和性别）的基本上是自生的起源部分重叠，并协同从α-变形杆菌线粒体的共生起源。这些根本性的创新发生在一个新穆拉共同祖先的衍生物中，它本身在真核生物和古细菌分化之前就已经进化了，通过对其真细菌祖先的剧烈改变，一种能够制造甾醇的放线菌posibacterium，通过用Minked糖蛋白取代murein肽聚糖和许多其他共享的新穆拉新奇事物。刚性的neomuran壁转化为一个灵活的表面涂层和相关的起源的吞噬功能的内膜系统，细胞骨架，核组织和分裂和性生活周期的演变是有用的。纤毛的进化不是通过共生而是通过细胞骨架的自体特化。我认为，祖先的真核生物是单纤毛与一个单一的中心粒（unikont）和一个简单的微管中心体锥，在有氧阿米巴动物鞭毛虫Phalansterium。我推断真核生物树的根在分歧opisthokonts（动物，Choanozoa，真菌）与一个单一的后纤毛和所有其他真核生物，指定为“anterokonts”，因为祖先存在的前纤毛。前鞭毛类包括变形虫，它可能是祖先单鞭毛类，和一个巨大的祖先二纤毛分支，称为'bikonts'。明显冲突的rRNA和蛋白质树可以相互和解，如果允许长枝扭曲，一些机械解释，这种超微结构的解释。Bikonts包括两个群体：皮质鞭毛虫，有一个年轻的前纤毛，没有中心体锥，祖先是一个半刚性的细胞皮层，在后成熟中心粒的两侧有一个微管带;和Rhizaria [一个新的亚界，包括Cercozoa（现在包括Ascetosporea classis nov.），Retaria phylum nov.，Heliozoa and Apusozoa phylum nov.]，有一个中心体锥或辐射状微管和两个微管根，表面柔软，经常有网状足。皮质鞭毛虫包括光核生物（Plantae和chromalveolates，祖先都有皮质肺泡）和掘孔动物（一种新的原生动物亚界，包括Loukozoa，Discicristata和Archezoa，祖先有三个微管根）。所有基础真核辐射都是线粒体需氧菌;氢化酶体是从线粒体进化而来的，在它们的基因组消失后，它们的双被膜仍然存在，这是膜遗传的一个突出例子。我讨论了这里所认识的13个原生动物门之间的关系，并通过更新Lankester 1878年将原生动物分为皮质动物亚界来修订高等原生动物的分类（挖洞目，泡孔目;具有突出的皮层微管和祖先定位的细胞口-副基目可能是第二次内化的细胞骨架）和Gymnomyxa [亚界Sarcomastigota（Choanozoa，Amoebozoa）和Rhizaria;两者祖先都具有辐射单线态微管的非皮层细胞骨架和相对柔软的细胞表面，具有扩散的进食]。由于真核生物的根几乎可以肯定位于Gymnomyxa中，可能在Sarcomastigota中，Corticata是衍生的。在具有皮层泡的皮层鞭毛虫宿主中叶绿体的单一共生起源之后，这种祖先植物迅速辐射成蓝绿藻、绿色植物和红藻。次级共生体随后将质体横向转移到不同的宿主中，制造出更复杂的细胞嵌合体--可能只有三次：从红色的藻类到铬藻的皮质鞭毛虫祖先（铬藻加泡藻），从绿色藻类到次级单纤毛尾虫，形成绿蜘蛛，并独立地到双纤毛挖掘，产生光合裸藻。第三共生涉及真核藻类共生体取代peridinin含质体在两个或三个甲藻谱系，但没有产生主要的新群体。真核生物的起源和清晰的主要分支可能发生在大约8.5亿年前的低温期，比分子“时钟”的无根据向后推断或早期大型微生物化石或更古老的甾烷的“真核生物”的可疑解释所建议的要近得多。叶绿体的起源和共生的红细胞进入皮质鞭毛虫创造chromalveolates可能都发生在大爆炸后，瓦兰热的雪球地球融化约5.8亿年前，从而刺激了随后的寒武纪爆发的动物和原生生物的形式，同时，难以分辨的后鞭毛体和前鞭毛体辐射。
Eukaryotes and archaebacteria form the clade neomura and are sisters, as shown decisively by genes fragmented only in archaebacteria and by many sequence trees. This sisterhood refutes all theories that eukaryotes originated by merging an archaebacterium and an alpha-proteobacterium, which also fall to account for numerous features shared specifically by eukaryotes and actinobacteria. I revise the phagotrophy theory of eukaryote origins by arguing that the essentially autogenous origins of most eukaryotic cell properties (phagotrophy, endomembrane system including peroxisomes, cytoskeleton, nucleus, mitosis and sex) partially overlapped and were synergistic with the symbiogenetic origin of mitochondria from an alpha-proteobacterium. These radical innovations occurred in a derivative of the neomuran common ancestor, which itself had evolved immediately prior to the divergence of eukaryotes and archaebacteria by drastic alterations to its eubacterial ancestor, an actinobacterial posibacterium able to make sterols, by replacing murein peptidoglycan by Minked glycoproteins and a multitude of other shared neomuran novelties. The conversion of the rigid neomuran wall into a flexible surface coat and the associated origin of phagotrophy were instrumental in the evolution of the endomembrane system, cytoskeleton, nuclear organization and division and sexual life-cycles. Cilia evolved not by symbiogenesis but by autogenous specialization of the cytoskeleton. I argue that the ancestral eukaryote was unicilliate with a single centriole (unikont) and a simple centrosomal cone of microtubules, as in the aerobic amoebozoan zooflagellate Phalansterium. I infer the root of the eukaryote tree at the divergence between opisthokonts (animals, Choanozoa, fungi) with a single posterior cilium and all other eukaryotes, designated 'anterokonts' because of the ancestral presence of an anterior cilium. Anterokonts comprise the Amoebozoa, which may be ancestrally unikont, and a vast ancestrally biciliate clade, named 'bikonts'. The apparently conflicting rRNA and protein trees can be reconciled with each other and this ultrastructural interpretation if long-branch distortions, some mechanistically explicable, are allowed for. Bikonts comprise two groups: corticoflagellates, with a younger anterior cilium, no centrosomal cone and ancestrally a semi-rigid cell cortex with a microtubular band on either side of the posterior mature centriole; and Rhizaria [a new infrakingdom comprising Cercozoa (now including Ascetosporea classis nov.), Retaria phylum nov., Heliozoa and Apusozoa phylum nov.], having a centrosomal cone or radiating microtubules and two microtubular roots and a soft surface, frequently with reticulopodia. Corticoflagellates comprise photokaryotes (Plantae and chromalveolates, both ancestrally with cortical alveoli) and Excavata (a new protozoan infrakingdom comprising Loukozoa, Discicristata and Archezoa, ancestrally with three microtubular roots). All basal eukaryotic radiations were of mitochondrial aerobes; hydrogenosomes evolved polyphyletically from mitochondria long afterwards, the persistence of their double envelope long after their genomes disappeared being a striking instance of membrane heredity.I discuss the relationship between the 13 protozoan phyla recognized here and revise higher protozoan classification by updating as subkingdoms Lankester's 1878 division of Protozoa into Corticata (Excavata, Alveolata; with prominent cortical microtubules and ancestrally localized cytostome - the Parabasalia probably secondarily internalized the cytoskeleton) and Gymnomyxa [infrakingdoms Sarcomastigota (Choanozoa, Amoebozoa) and Rhizaria; both ancestrally with a non-cortical cytoskeleton of radiating singlet microtubules and a relatively soft cell surface with diffused feeding]. As the eukaryote root almost certainly lies within Gymnomyxa, probably among the Sarcomastigota, Corticata are derived. Following the single symbiogenetic origin of chloroplasts in a corticoflagellate host with cortical alveoli, this ancestral plant radiated rapidly into glaucophytes, green plants and red algae. Secondary symbiogeneses subsequently transferred plastids laterally into different hosts, making yet more complex cell chimaeras - probably only thrice: from a red alga to the corticoflagellate ancestor of chromalveolates (Chromista plus Alveolata), from green algae to a secondarily uniciliate cercozoan to form chlorarachneans and independently to a biciliate excavate to yield photosynthetic euglenoids. Tertiary symbiogenesis involving eukaryotic algal symbionts replaced peridinin-containing plastids in two or three dinoflagellatelineages, but yielded no major novel groups. The origin and well-resolved primary bifurcation of eukaryotes probably occurred in the Cryogenian Period, about 850 million years ago, much more recently than suggested by unwarranted backward extrapolations of molecular 'clocks' or dubious interpretations as 'eukaryotic, of earlier large microbial fossils or still more ancient steranes. The origin of chloroplasts and the symbiogenetic incorporation of a red alga into a corticoflagellate to create chromalveolates may both have occurred in a big bang after the Varangerian snowball Earth melted about 580 million years ago, thereby stimulating the ensuing Cambrian explosion of animals and protists in the form of simultaneous, poorly resolved opisthokont and anterokont radiations.