Unité d'Ecologie, Systématique et Evolution, CNRS UMR 8079, Université Paris-Sud/Paris-Saclay, AgroParisTech, 91400 Orsay, France.
Institut de Génétique et Microbiologie, CNRS UMR 8621, Université Paris-Sud/Paris-Saclay, 91405 Orsay, France.
Curr Biol. 2017 Feb 6;27(3):386-391. doi: 10.1016/j.cub.2016.11.056. Epub 2017 Jan 26.
Photosynthesis evolved in eukaryotes by the endosymbiosis of a cyanobacterium, the future plastid, within a heterotrophic host. This primary endosymbiosis occurred in the ancestor of Archaeplastida, a eukaryotic supergroup that includes glaucophytes, red algae, green algae, and land plants [1-4]. However, although the endosymbiotic origin of plastids from a single cyanobacterial ancestor is firmly established, the nature of that ancestor remains controversial: plastids have been proposed to derive from either early- or late-branching cyanobacterial lineages [5-11]. To solve this issue, we carried out phylogenomic and supernetwork analyses of the most comprehensive dataset analyzed so far including plastid-encoded proteins and nucleus-encoded proteins of plastid origin resulting from endosymbiotic gene transfer (EGT) of primary photosynthetic eukaryotes, as well as wide-ranging genome data from cyanobacteria, including novel lineages. Our analyses strongly support that plastids evolved from deep-branching cyanobacteria and that the present-day closest cultured relative of primary plastids is Gloeomargarita lithophora. This species belongs to a recently discovered cyanobacterial lineage widespread in freshwater microbialites and microbial mats [12, 13]. The ecological distribution of this lineage sheds new light on the environmental conditions where the emergence of photosynthetic eukaryotes occurred, most likely in a terrestrial-freshwater setting. The fact that glaucophytes, the first archaeplastid lineage to diverge, are exclusively found in freshwater ecosystems reinforces this hypothesis. Therefore, not only did plastids emerge early within cyanobacteria, but the first photosynthetic eukaryotes most likely evolved in terrestrial-freshwater settings, not in oceans as commonly thought.
光合作用是在真核生物中通过蓝细菌（未来的质体）在内共生体中被异养宿主吞噬而进化而来的。这种最初的内共生发生在古生菌的祖先中，古生菌是一个真核超群，包括蓝藻、红藻、绿藻和陆地植物[1-4]。然而，尽管质体的内共生起源于单一的蓝细菌祖先已被牢固确立，但该祖先的性质仍存在争议：质体可能来自早期或晚期分支的蓝细菌谱系[5-11]。为了解决这个问题，我们对迄今为止分析的最全面数据集进行了基因组和超级网络分析，该数据集包括来自早期光合作用真核生物内共生基因转移（EGT）的质体编码蛋白和质体起源的核编码蛋白，以及来自蓝细菌的广泛基因组数据，包括新的谱系。我们的分析强烈支持质体是由深分支的蓝细菌进化而来的，并且现今与原始质体最接近的培养相关物是绿球藻（Gloeomargarita lithophora）。该物种属于一个最近发现的蓝细菌谱系，广泛存在于淡水微生物岩和微生物垫中[12,13]。该谱系的生态分布为光合作用真核生物出现的环境条件提供了新的线索，最有可能是在陆地-淡水环境中。第一个分支的古生菌谱系——蓝藻，仅存在于淡水生态系统中，这一事实加强了这一假设。因此，不仅质体在蓝细菌中很早就出现了，而且第一个光合作用真核生物很可能是在陆地-淡水环境中进化的，而不是像通常认为的那样在海洋中进化的。
