18S and ITS2 rRNA gene sequence-structure phylogeny of the Phaeophyceae (SAR, Stramenopiles) with special reference to Laminariales.

The phylogeny of brown algae (Phaeophyceae) has undergone extensive changes in the recent past due to regular new scientific insights. We used nuclear 18S rDNA with an extensive dataset, aiming to increase the accuracy and robustness of the reconstructed phylogenetic trees using a simultaneous sequence-structure approach. Individual secondary structures were generated for all 18S rDNA sequences. The sequence-structure information was encoded and used for an automated simultaneous sequence-structure alignment. Neighbor-joining and profile neighbor-joining trees were calculated based on 186 phaeophycean sequence-structure pairs. Additionally, sequence-structure neighbor-joining, maximum parsimony and maximum likelihood trees were reconstructed on a representative subset. Using a similar approach, ITS2 rDNA sequence-structure information was used to reconstruct a neighbor-joining tree including 604 sequence-structure pairs of the Laminariales. Our study results are in significant agreement with previous single marker 18S and ITS2 rDNA analyses. Moreover, the 18S results are in wide agreement with recent multi-marker analyses. The bootstrap support was significantly higher for our sequence-structure analysis in comparison to sequence-only analyses in this study and the available literature. This study supports the simultaneous inclusion of sequence-structure data at least for 18S to obtain more accurate and robust phylogenetic trees compared to sequence-only analyses.