Li Jing Yan, Wu Chuan Fen
Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan.
Naturwissenschaften. 2005 Jul;92(7):305-9. doi: 10.1007/s00114-005-0635-z. Epub 2005 May 19.
The origin of eukaryotic flagella has long been a mystery. Here we review the possibility that flagella sprouted evolutionarily from the eukaryotic cell proper seems very unlikely because it is hard to imagine what function and benefit in natural selection the flagella would have provided to the cells when they first emerged as simple buds. Lynn Margulis' 1970 spirochete hypothesis, though popular still, has never been confirmed. Moreover, the absence of tubulin and axonemal dynein in the spirochetes and the incapability of the bacterial and eukaryotic membranes' making a continuum now suggest that the hypothesis is outdated. Tubulin genes were recently identified in a new bacteria division, verrucomicrobia, and microtubules have also been found in one of these species, epixenosomes, the defensive ectosymbionts. On the basis of these data, we propose a new symbiotic hypothesis: that the mid-ancestor of eukaryotic cells obtained epixenosomelike verrucomicrobia as defensive ectosymbionts and the ectosymbionts later became endosymbiotic. They still, however, protruded from the surface of their host to play their role. Later, many genes were lost or incorporated into the host genome. Finally, the genome, the bacterial membrane, and the endosymbiotic vesicle membrane were totally lost, and fingerlike protrusions with microtubules formed. As the cells grew larger, the defensive function of the protrusions eventually weakened and then vanished. Some of the protrusions took on a new role in cell movement, which led them to evolve into flagella. The key step in this process was that the dynein obtained from the host evolved into axonemal dyneins, attaching onto the microtubules and forming motile axonemes. Our hypothesis is unproven, but it offers a possible explanation that is consistent with current scientific thought. We hope that our ideas will stimulate additional studies on the origin of eukaryotic flagella and on investigations of verrucomicrobia. Whether such studies confirm, refine, or replace our hypothesis, they should nevertheless further our understanding of the origin of eukaryotic cells.
真核生物鞭毛的起源长期以来一直是个谜。在此，我们回顾一下这样一种可能性：鞭毛从真核细胞本身进化出芽似乎非常不可能，因为很难想象在自然选择中，当鞭毛最初作为简单的芽出现时，它们会为细胞提供什么功能和益处。林恩·马古利斯1970年提出的螺旋体假说尽管仍然流行，但从未得到证实。此外，螺旋体中缺乏微管蛋白和轴丝动力蛋白，并且细菌膜和真核细胞膜无法形成连续体，这表明该假说已过时。微管蛋白基因最近在一个新的细菌门类疣微菌门中被鉴定出来，并且在这些物种之一、防御性外共生体附生菌中也发现了微管。基于这些数据，我们提出一种新的共生假说：真核细胞的中间祖先获得了类似附生菌的疣微菌作为防御性外共生体，这些外共生体后来变成了内共生体。然而，它们仍然从宿主表面突出以发挥其作用。后来，许多基因丢失或整合到宿主基因组中。最后，基因组、细菌膜和内共生囊泡膜完全消失，形成了带有微管的指状突起。随着细胞变大，这些突起的防御功能最终减弱然后消失。一些突起在细胞运动中承担了新的角色，这导致它们进化成鞭毛。这个过程的关键步骤是从宿主获得的动力蛋白进化成轴丝动力蛋白，附着在微管上并形成能动的轴丝。我们的假说尚未得到证实，但它提供了一种与当前科学思想一致的可能解释。我们希望我们的想法将激发对真核生物鞭毛起源以及疣微菌研究的更多研究。无论这些研究是证实、完善还是取代我们的假说，它们都应该增进我们对真核细胞起源的理解。
