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Proc Natl Acad Sci U S A. 2017 Sep 12;114(37):E7737-E7745. doi: 10.1073/pnas.1620089114. Epub 2017 Aug 14.

Early photosynthetic eukaryotes inhabited low-salinity habitats.

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School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, United Kingdom;
Division of Plant Science, University of Dundee at the James Hutton Institute, Dundee DD2 5DA, United Kingdom.
Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Ultimo, NSW 2007, Australia.
School of Biological Sciences, University of Bristol, Bristol BS8 1TH, United Kingdom.
School of Earth Sciences, University of Bristol, Bristol BS8 1TH, United Kingdom.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.


The early evolutionary history of the chloroplast lineage remains an open question. It is widely accepted that the endosymbiosis that established the chloroplast lineage in eukaryotes can be traced back to a single event, in which a cyanobacterium was incorporated into a protistan host. It is still unclear, however, which Cyanobacteria are most closely related to the chloroplast, when the plastid lineage first evolved, and in what habitats this endosymbiotic event occurred. We present phylogenomic and molecular clock analyses, including data from cyanobacterial and chloroplast genomes using a Bayesian approach, with the aim of estimating the age for the primary endosymbiotic event, the ages of crown groups for photosynthetic eukaryotes, and the independent incorporation of a cyanobacterial endosymbiont by Paulinella Our analyses include both broad taxon sampling (119 taxa) and 18 fossil calibrations across all Cyanobacteria and photosynthetic eukaryotes. Phylogenomic analyses support the hypothesis that the chloroplast lineage diverged from its closet relative Gloeomargarita, a basal cyanobacterial lineage, ∼2.1 billion y ago (Bya). Our analyses suggest that the Archaeplastida, consisting of glaucophytes, red algae, green algae, and land plants, share a common ancestor that lived ∼1.9 Bya. Whereas crown group Rhodophyta evolved in the Mesoproterozoic Era (1,600-1,000 Mya), crown groups Chlorophyta and Streptophyta began to radiate early in the Neoproterozoic (1,000-542 Mya). Stochastic mapping analyses indicate that the first endosymbiotic event occurred in low-salinity environments. Both red and green algae colonized marine environments early in their histories, with prasinophyte green phytoplankton diversifying 850-650 Mya.


Cyanobacteria; chloroplast; photosynthetic eukaryotes; phylogenomics; relaxed molecular clock

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