| Sargassum is one of the most important genera of the family Sargassaceae in brown algae and is used to produce carrageenan,mannitol,iodine,and other economic substances.The morphological characters are similar between species of the genus Sargassum,so the classification is much controversial and the evolutionary relationship is not clear enough.In this study,seven complete plastid genomes of Sargassum feldmannii,Sargassum graminifolium,Sargassum henslowianum,Sargassum ilicifolium var.conduit,Sargassum mcclurei,Sargassum muticum and Sargassum phyllosystum were assembled and annotated using next-generation sequencing,and the basic characteristics of the plastid genomes of the genus Sargassum were systematically studied,compared the differences between the plastid genomes of different species of Sargassum,reconstructed the phylogenetic relationship of the Phaeophyceae and Fucales,and calculated the divergence time.After alignment and calibration with the close related species,the plastid genomes of seven Sargassum species were 124258?124563 bp,and the structures were all tetrad structures with closed loops,with the large single copy region of 73500?73693 bp,the small single copy region of 39892?40012 bp,and the two inverted repeats of 10838-10872 bp.The number and type of coding genes were consistent,which consisted of 139protein-coding genes,28 t RNA genes and 6 r RNA genes,all of which had no intron insertions.In terms of base composition,the GC contents of the whole plastid genomes of seven Sargassum species ranged from 30.35%?30.49%,all of which were significantly lower than the AT content,which was consistent with the basic structural features of plastid genomes of other Sargassum species.The plastid genomes of Sargassum algae and most other brown algae seem to use a common genetic code,with the exception of the genes psb F,rpl3,and thi S,all protein coding genes use the codon ATG as their start codon,and 78% use the codon TAA,about 18% use TAG and 4% use TGA as stop codon.At the taxonomic level of Sargassum,collinearity analysis and sequence difference comparison were performed between the plastid genomes of five published Sargassum species and the plastid genomes of seven species in this study.The comparison of plastid genomes sequences showed that the differences of plastid genomes of Sargassum were mainly concentrated in the non-coding region,and the codon composition and RSCU values of different species were basically the same.The IR boundaries of different Sargassum species were compared and it was found that the genes of Sargassum species were identical near the IR boundaries with little difference in length and relative position.Collinearity analysis results showed that only Sargassum horneri had inversion of trn C and trn N genes,while the other homologous genes were consistent and had high collinearity,showing a conservative evolutionary pattern.Phylogenetic analysis and calculation of the divergence time of the species in Phaeophyceae showed that the species clustered together according to the group followed by the orders,revealing the phylogenetic relationship among different groups of brown algae.The species of Sargassum form two large branches that support the existing taxonomic system.The results of the divergence time calculated that the classification of Phaeophyceae appeared at 201.42 Mya,the internal differentiation branch of Fucales began to form before 92.52 Mya,and the species of Sargassum were divided into two subgroups before 14.33 Mya.In this study,we sequenced the plastid genomes of new Sargassum species,combined with the plastid genomes of congeneric species genomic comparative analysis and phylogenetic analysis were performed,such as structural features and codon preference,and calculated the divergence times.The results provided evidence for the speciation and plastid genome evolution events of brown algae,provided new plastid genome data for subsequent studies of brown alga phylogeny,and laid the foundation for genetic research and practical application of algae. |
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