Genome-scale Identification And Phylogenetic Analysis Of Critical Gene Families Involved In The Fucoidans Biosynthesis In Saccharina Japonica

Saccharina japonica(kelp)is one of the important commercial brown alga,which has significant ecological and economic values.S.japonica belongs to the phylum of stramenopiles,which raised from the secondary endosymbiosis event.The distinctive evolution history and biological characteristics of this kelp have attracted great interests from phycologists.Fucoidans are the major plant cell polysaccharides in S.japonica,with valuable antiviral,anticoagulant,antioxidant,immunomodulating,and antitumor bioactivities.Therefore,the analysis on the key gene families in the synthesis pathway of fucoidans has important theoretical and practical significance.Based on the whole genome and transcriptome sequence data,we performed genome-wide comparative analysis of fucoidans biosynthetic pathway on S.japonica against the whole organism kingdom using bioinformatics approaches.In total one(phosphomannomutase,PMM)gene,one GDP-mannose 4,6-dehydratase(GM46D)gene,one(GDP-L-fucose synthase,GFS)gene,one L-fucokinase(FK)gene,four mannose-6-phosphate isomerase(MPI)genes,seven xylosyltransferases(XylT)genes,eight fucosyltransferases(FK)genes,eighteen sulfatases(SULF)genes,sixty nine sulfotransferases(ST)genes were annotated.The ST and SULF gene family analysis.The position and content of sulfate group of the fucoidans have an important influence on its bioactivities.We found that the STs and SULFs showed expansion in brown algae when compared to diatoms,red algae and green algae.The gene duplication of STs and SULFs in brown algae has been completed in its ancestors.In addition,more STs and SULFs are found in S.japonica.Compared to the three species of Ectocarpales,STs(PF03567)and SULF(PF00884)genes expansion occurred in the kelp S.japonica through tandem duplication and segmental duplication.The sequences of STs and SULFs are conserved in both animals and plants,indicating that these enzymes are ancestral origin.The transcriptome results indicated that more than half of the genes were highly expressed in the S.japonica sporophytes compared to gametophytes,consistent with the high polysaccharide content in the sporophytes.The XylTs gene family analysis.XylTs play key roles in the biosynthesis of many different polysaccharides.These enzymes transfer D-xylose from UDP-xylose to substrate acceptors.In S.japonica,seven XylTs were identified as UDP-D-xylose: Lfucose-?-1,3-D-XylT(FucXylT)type that specifically transferred D-xylose to fucose.We propose that FucXylTs were originated from the last eukaryotic common ancestor,rather than being plant specific,as they are distributed in green plants(plants and green algae),Choanoflagellatea and Echinodermata.Compared with Ectocarpus siliculosus and Cladosiphon okamuranus,FucXylT gene expansion occurred in the S.japonica through tandem duplication and segmental duplication.Considering the evidence from many aspects,we hypothesize that the FucXylTs likely participated in fucoidans biosynthesis in brown algae.