The Genome Of Prasinoderma Coloniale Revealed New Insights Into The Evolution Of Viridiplantae

Green algae,consisting of the Chlorophyta and streptophyte algae(Charaphyta),comprises the largest group of primary producers.They produce the similar kind of carbohydrates during photosynthesis as do the Embryophyta(land plants).They constitute a paraphyletic group as they likely have a common ancestor with the Embryophyta.In order to explore the evolutionary relationship between the Embryophyta and green algae,we used a single-cell picoeukaryotic marine green alga Prasinoderma coloniale as our research object.After careful assembly,annotation,and phylogenetic analysis based on the whole genome sequencing(WGS)data,the 1000 plants(one KP or 1KP)transcriptome data,the 109 species ribosomal DNA(rDNA)data,we identified that the P.coloniale(28.8Mb)belong to a novel phylum within green plants(Viridiplantae),the Prasinodermophyta,that diverged before the split of Chlorophyta and Streptophyta.Based on comparative genomics,structural features of the genome and gene family comparisons revealed the intermediate position of the P.coloniale genome between the extremely compact early Chlorophyta and early Streptophyta.The exons,introns and intergenic regions of early Streptophyta were larger than P.coloniale,when the early Chlorophyta were smaller than P.coloniale.In addition,through bioinformatics methods,we have also found the unique biological characteristics of P.coloniale different from other green algae.For example,although P.coloniale has thick cell walls,it does not have any cell wall main components synthetase(eg,cellulose synthase,CesA)comparing with Embryophyta.Further analysis found that P.coloniale has a complete peptidoglycan synthesis pathway,however,the peptidoglycan is the main component of the bacterial cell wall.The carbonconcentrating mechanisms(CCM)of P.coloniale is a C4 CCM in both cytosol,mitochondria and chloroplast.P.coloniale also has a unique carbon-fixing key enzyme represented by PEPCK and delta CA.The light-harvesting complex(LHC)of P.coloniale also showed a large number of copies compared to the early Streptophyta and early Chlorophyta.A large number of horizontal gene transfer events has occurred in the P.coloniale genome,most of which are associated with important biological pathways.Based on all the results,we found that P.coloniale exhibits different characteristics from other Viridiplantae(especially early Streptophyta and early Chlorophyta)in the composition of the genome and in the biological pathway,and these characteristics are likely to be the result of the interaction with the environment.Meanwhile,the genome of early-diverging P.coloniale shed light on the dawn of Viridiplantae evolution.