Ulva Phylogeny And Time Of Origin Based On Transcriptome Data

Chlorophyta is an important part of green plants and plays an essential role in the global ecosystem.Ulvophyceae is one of the main classes of Chlorophyta,and its morphological and ecological diversity makes it a very special group in Chlorophyta.The ulvophytes are dominated by marine macroalgae(green algae)and play the role of the main primary producers in the coastal ecosystem,but their proliferation will lead to ‘green tide' and then result in the coastal water pollution.In addition,as some ulvophycean groups have potential value in biofuel,aquaculture,as well as food and medicine production industry,there are broad ecological and economic prospects for Ulvophyceae.However,the phylogenetic relationship of Ulvophyceae has been controversial for a long time,and most of analyses focus on few nuclear genes or chloroplast genes,and the genome and transcriptome data are still insufficient.Without a reliable phylogenetic framework,the divergence time of Ulvophyceae and even Chlorophyta cannot be well solved.In this study,we carried out the most extensive nuclear gene sampling,used multispecies coalescent model and concatenation model to reconstruct the phylogenetic relationship.Our topologies were consistent in most analyses with robust supports.Ulvophyceae was recovered as a paraphyletic group including Bryopsidales and the remaining Ulvophyceae.The gene tree heterogeneity and coalescent simulation suggested that the topological conflict could be well explained by incomplete lineage sorting and multispecies coalescent model fit our dataset better.Molecular clock analysis was performed with 200clock-like genes and the coalescent tree.We selected more reliable fossil calibrations,containing the newly published one billion-year-old fossil,and assigned secondary calibrations for some nodes.The evolutionary timescale showed that Ulvophyceae originated from 1240.6 Ma(1368.6-1123.9 Ma)in the Mesoproterozoic Era,much earlier than previous studies.As photoautotrophic eukaryotes,the ulvophycean origin and early diversification had a profound influence on the oceanic redox structure and biochemical cycle of the earth in the Mesoproterozoic.