Use Bayesian Molecular Clock To Estimate The Origin And Evolution Time Of Green Plants

Green plants(Viridiplantae)are an ancient group of photosynthetic eukaryotes and play a central role in global ecosystems.They are represented by more than 500,000 species in two main subdivisions,Chlorophyta and Streptophyta,and have a nearly global distribution.Chlorophyta includes ecologically,morphologically,and cytologically diverse green algae living in a wide range of habitats.Streptophyta contains embryophytic land plants and a small but diverse group of freshwater green algae(Zygnematophyceae,Coleochaetophyceae,Charophyceae,Klebsormidiophyceae,Chlorokybophyceae and Mesostigmatophyceae).Inferring a robust evolutionary timescale for green plants is critical for testing hypotheses about the interaction and coevolution between green plants and Earth's climate system.Here,we infer the evolutionary timescale of green plants by analysing 81 protein-coding genes from 99 chloroplast genomes,using a core set of 21 fossil calibrations.We explore the impacts of the tree topology,clock models,clock-partitioning schemes,rate priors,and fossil calibrations on our inferred ages,thereby accounting for a wide range of factors that potentially affect molecular dating.We find that the choice of clock model affects date estimation and that the independent-rates model provides a better fit to the data than the autocorrelated-rates model.Varying the rate prior and tree topology had little impact on age estimates,with far greater differences observed among calibration choices and clock-partitioning schemes.Our analyses yield date estimates ranging from the Paleoproterozoic to Mesoproterozoic for crown-group green plants,and from the Ediacaran to Middle Ordovician for crown-group land plants.Our analyses emphasize the importance of considering various sources of uncertainty when using Bayesian phylogenetic methods for molecular dating.Our estimate of the evolutionary timescale of green plants implies the formation of three Snowball Earth events(Sturtian,Marinoan and Gaskiers glaciation)are related to the rise of DMS-producing green algae between the late Mesoproterozoic and early Neoproterozoic,and the Gaskiers glaciation at the end of Neoproterozoic is the trigger for the colonization of the land by plants.