Phylogenetic hypotheses of gorgoniid octocorals according to ITS2 and their predicted RNA secondary structures

Gorgoniid octocorals taxonomy (Cnidaria; Octocorallia; Gorgoniidae) includes diagnostic characters not well defined at the generic level, and based on the family diagnosis some species could be classified in either Gorgoniidae or Plexauridae. In this study, we used sequences from the Internal Transcribed Spacer 2 (ITS2) and their predicted RNA secondary structure to both correct the alignment and reconstruct phylogenies using molecular morphometrics for 24 octocorals mostly from the Atlantic. ITS2 exhibited the six-helicoidal ring-model structure found in eukaryotes, and provided 38 parsimony-informative characters. The proposed phylogenies, though differing between sequence- and structure-base results, provided consistent support for several clades. Genera considered part of the polyphyletic genus Leptogorgia, such as Filigorgia, were distantly related to the former in all phylogenetic hypotheses. Main differences among the hypotheses consisted in the placement of Muriceopsis (previously considered from the Plexauridae family) and Filigorgia. Excluding Muriceopsis and an undescribed octocoral from Tobago, Plexaurella and Pterogorgia grouped together as a sister branch of Pinnigorgia spp. but long-branch attraction was evident for the grouping of Plexaurella nutans (another plexaurid) and Pterogorgia citrina. Unexpected results were the divergence between Caribbean genera, Gorgonia and Pseudopterogorgia, which were placed basal respect to Pacifigorgia and Leptogorgia (=Lophogorgia). ITS2 provided support to corroborate observations based on sclerite morphology: species with "capstan sclerites" (e.g., Pacifigorgia and Leptogorgia) were characterized by a long helix IV with one internal loop and a helix V with four internal loops; "scaphoid sclerites" had a predominantly long helix V if compared to helix IV; "asymmetric spiny sclerites" (Muriceopsis, Pinnigorgia and the undescribed octocoral) exhibited one or two lateral bulges in the V helix. Remarkably, Muriceopsis and Pinnigorgia were supported by a complete Compensatory Base Change (CBC) (A-U to G-C) in helix V. Filigorgia with simple "spindles" had a short helix IV and a large central ring. DNA sequences from the nuclear ITS2 region, including information from predicted RNA secondary structure, despite their reduced length, provided numerous characters and phylogenetic information among Gorgoniidae genera and species.