Phylogeny of Oedogoniales, Chaetophorales and Chaetopeltidales (Chlorophyceae): inferences from sequence-structure analysis of ITS2

Background and Aims The green algal class Chlorophyceae comprises five orders (Chlamydomonadales, Sphaeropleales, Chaetophorales, Chaetopeltidales and Oedogoniales). Attempts to resolve the relationships among these groups have met with limited success. Studies of single genes (18S rRNA, 26S rRNA, rbcL or atpB) have largely failed to unambiguously resolve the relative positions of Oedogoniales, Chaetophorales and Chaetopeltidales (the OCC taxa). In contrast, recent genomics analyses of plastid data from OCC exemplars provided a robust phylogenetic analysis that supports a monophyletic OCC alliance. Methods An ITS2 data set was assembled to independently test the OCC hypothesis and to evaluate the performance of these data in assessing green algal phylogeny at the ordinal or class level. Sequence-structure analysis designed for use with ITS2 data was employed for phylogenetic reconstruction. Key Results Results of this study yielded trees that were, in general, topologically congruent with the results from the genomic analyses, including support for the monophyly of the OCC alliance. Conclusions Not all nodes from the ITS2 analyses exhibited robust support, but our investigation demonstrates that sequence-structure analyses of ITS2 provide a taxon-rich means of testing phylogenetic hypotheses at high taxonomic levels. Thus, the ITS2 data, in the context of sequence-structure analysis, provide an economical supplement or alternative to the single-marker approaches used in green algal phylogeny.