Transcriptome-wide evolutionary analysis on essential brown algae (Phaeophyceae) in China

Brown algae (Chromista, Ochrophyta, Phaeophyceae) are a large group of multicellular algae that play important roles in the ocean’s ecosystem and biodiversity. However, poor molecular bases for studying their phylogenetic evolutions and novel metabolic characteristics have hampered progress in the field. In this study, we sequenced the de novo transcriptome of 18 major species of brown algae in China, covering six orders and seven families, using the high-throughput sequencing platform Illumina HiSeq 2000. From the transcriptome data of these 18 species and publicly available genome data of Ectocarpus siliculosus and Phaeodactylum tricornutum, we identified 108 nuclear-generated orthologous genes and clarified the phylogenetic relationships among these brown algae based on a multigene method. These brown algae could be separated into two clades: Clade Ishigeales-Dictyotales and Clade Ectocarpales-Laminariales-Desmarestiale-Fucales. The former was at the base of the phylogenetic tree, indicating its early divergence, while the latter was divided into two branches, with Order Fucales diverging from Orders Ectocarpales, Laminariales, and Desmarestiale. In our analysis of taxonomy-contentious species, Sargassum fusiforme and Saccharina sculpera were found to be closely related to genera Sargassum and Saccharina, respectively, while Petalonia fascia showed possible relation to genus Scytosiphon. The study provided molecular evidence for the phylogenetic taxonomy of brown algae.