Current Biology
Volume 27, Issue 3, 6 February 2017, Pages 386-391
Journal home page for Current Biology

Report
An Early-Branching Freshwater Cyanobacterium at the Origin of Plastids

https://doi.org/10.1016/j.cub.2016.11.056Get rights and content
Under an Elsevier user license
open archive

Highlights

  • Plastids branch early in the cyanobacterial tree

  • Gloeomargarita-like cyanobacteria are the closest modern relatives of plastids

  • The first eukaryotic algae most likely evolved in freshwater environments

Summary

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, 2, 3, 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, 6, 7, 8, 9, 10, 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.

Keywords

plastids
chloroplasts
cyanobacteria
evolution
molecular phylogeny
phylogenomics

Cited by (0)

4

Lead Contact