Koonin Eugene V
National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA
Philos Trans R Soc Lond B Biol Sci. 2015 Sep 26;370(1678):20140333. doi: 10.1098/rstb.2014.0333.
The origin of eukaryotes is a fundamental, forbidding evolutionary puzzle. Comparative genomic analysis clearly shows that the last eukaryotic common ancestor (LECA) possessed most of the signature complex features of modern eukaryotic cells, in particular the mitochondria, the endomembrane system including the nucleus, an advanced cytoskeleton and the ubiquitin network. Numerous duplications of ancestral genes, e.g. DNA polymerases, RNA polymerases and proteasome subunits, also can be traced back to the LECA. Thus, the LECA was not a primitive organism and its emergence must have resulted from extensive evolution towards cellular complexity. However, the scenario of eukaryogenesis, and in particular the relationship between endosymbiosis and the origin of eukaryotes, is far from being clear. Four recent developments provide new clues to the likely routes of eukaryogenesis. First, evolutionary reconstructions suggest complex ancestors for most of the major groups of archaea, with the subsequent evolution dominated by gene loss. Second, homologues of signature eukaryotic proteins, such as actin and tubulin that form the core of the cytoskeleton or the ubiquitin system, have been detected in diverse archaea. The discovery of this 'dispersed eukaryome' implies that the archaeal ancestor of eukaryotes was a complex cell that might have been capable of a primitive form of phagocytosis and thus conducive to endosymbiont capture. Third, phylogenomic analyses converge on the origin of most eukaryotic genes of archaeal descent from within the archaeal evolutionary tree, specifically, the TACK superphylum. Fourth, evidence has been presented that the origin of the major archaeal phyla involved massive acquisition of bacterial genes. Taken together, these findings make the symbiogenetic scenario for the origin of eukaryotes considerably more plausible and the origin of the organizational complexity of eukaryotic cells more readily explainable than they appeared until recently.
真核生物的起源是一个基本且极具挑战性的进化难题。比较基因组分析清楚地表明，最后的真核生物共同祖先（LECA）拥有现代真核细胞的大多数标志性复杂特征，特别是线粒体、包括细胞核在内的内膜系统、先进的细胞骨架和泛素网络。许多祖先基因的复制，如DNA聚合酶、RNA聚合酶和蛋白酶体亚基，也可以追溯到LECA。因此，LECA不是一个原始生物，它的出现一定是细胞复杂性广泛进化的结果。然而，真核生物起源的过程，尤其是内共生与真核生物起源之间的关系，远未明确。最近的四项进展为真核生物起源的可能途径提供了新线索。第一，进化重建表明大多数主要古菌类群有复杂的祖先，随后的进化以基因丢失为主导。第二，在不同的古菌中检测到了标志性真核蛋白的同源物，如构成细胞骨架核心的肌动蛋白和微管蛋白或泛素系统。这种“分散的真核基因组”的发现意味着真核生物的古菌祖先可能是一个复杂细胞，它可能具备原始形式的吞噬作用，因此有利于捕获内共生体。第三，系统发育基因组分析聚焦于大多数源自古菌的真核基因在古菌进化树内的起源，具体来说，是TACK超群。第四，有证据表明，主要古菌门的起源涉及大量细菌基因的获取。综上所述，这些发现使得真核生物起源的共生起源假说比直到最近看起来更加合理，真核细胞组织复杂性的起源也更容易解释。
