A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids

In modern oceans, eukaryotic phytoplankton is dominated by lineages with red algal-derived plastids such as diatoms, dinoflagellates, and coccolithophores. Despite the ecological importance of these groups and many others representing a huge diversity of forms and lifestyles, we still lack a comprehensive understanding of their evolution and how they obtained their plastids. New hypotheses have emerged to explain the acquisition of red algal-derived plastids by serial endosymbiosis, but the chronology of these putative independent plastid acquisitions remains untested. Here, we establish a timeframe for the origin of red algal-derived plastids under scenarios of serial endosymbiosis, using Bayesian molecular clock analyses applied on a phylogenomic dataset with broad sampling of eukaryote diversity. We find that the hypotheses of serial endosymbiosis are chronologically possible, as the stem lineages of all red plastid-containing groups overlap in time. This period in the Meso- and Neoproterozoic Eras set the stage for the later expansion to dominance of red algal-derived primary production in the contemporary oceans, which profoundly altered the global geochemical and ecological conditions of the Earth. There are several competing hypotheses for the acquisition of red algal-derived plastids by eukaryotic phytoplankton. Here, the authors use Bayesian molecular clock analyses to evaluate the chronological possibility of the proposed plastid origins and transmissions.

Automated summary

Researchers evaluated the chronological possibility of proposed plastid origins and transmissions in eukaryotic phytoplankton through Bayesian molecular clock analyses. The study established a timeframe for the origin of red algal-derived plastids under scenarios of serial endosymbiosis, revealing that the hypotheses of serial endosymbiosis are chronologically possible.