A Molecular Timeline for the Origin of Photosynthetic Eukaryotes

Hwan Su Yoon, Jeremiah D. Hackett, Claudia Ciniglia, Gabriele Pinto, Debashish Bhattacharya
2004 Molecular biology and evolution  
The appearance of photosynthetic eukaryotes (algae and plants) dramatically altered the Earth's ecosystem, making possible all vertebrate life on land, including humans. Dating algal origin is, however, frustrated by a meager fossil record. We generated a plastid multi-gene phylogeny with Bayesian inference and then used maximum likelihood molecular clock methods to estimate algal divergence times. The plastid tree was used as a surrogate for algal host evolution because of recent phylogenetic
more » ... vidence supporting the vertical ancestry of the plastid in the red, green, and glaucophyte algae. Nodes in the plastid tree were constrained with six reliable fossil dates and a maximum age of 3,500 MYA based on the earliest known eubacterial fossil. Our analyses support an ancient (late Paleoproterozoic) origin of photosynthetic eukaryotes with the primary endosymbiosis that gave rise to the first alga having occurred after the split of the Plantae (i.e., red, green, and glaucophyte algae plus land plants) from the opisthokonts sometime before 1,558 MYA. The split of the red and green algae is calculated to have occurred about 1,500 MYA, and the putative single red algal secondary endosymbiosis that gave rise to the plastid in the cryptophyte, haptophyte, and stramenopile algae (chromists) occurred about 1,300 MYA. These dates, which are consistent with fossil evidence for putative marine algae (i.e., acritarchs) from the early Mesoproterozoic (1,500 MYA) and with a major eukaryotic diversification in the very late Mesoproterozoic and Neoproterozoic, provide a molecular timeline for understanding algal evolution. Materials and Methods Taxon Sampling and Sequencing Forty-six species were used to infer the plastid phylogeny including 32 red algae including the chromists, 12 green algae and land plants, the glaucophyte Cyanophora paradoxa, and a cyanobacterium (Nostoc sp. PCC7120) as the outgroup (for strain identifications and GenBank accession numbers, see table 1 in the Supplementary Material online). A total of 42 new plastid
doi:10.1093/molbev/msh075 pmid:14963099 fatcat:jegtunni4bcmlcheygt76qzepu