Phylogeny of the Aplousobranchia (Tunicata: Ascidiacea)

Abstract

The phylogenetic relationships of genera and families of Aplousobranchia Lahille (Tunicata, Ascidiacea) is reconstructed based on morphological characters - the first comprehensive morphology-based phylogenetic analysis for the Aplousobranchia. Monophyly of Aplousobranchia and its families were tested with samples of 14 families. The final character matrix comprised 47 characters and 41 genera as terminal taxa. Nine equally most parsimonious trees (length 161, CI = 0.5031, RI = 0.7922) were found. Characters describing replication, colony system formation, and branchial walls were the more important in phylogenetic reconstruction. These characters were more useful than others more traditionally used in ascidian taxonomy, such as: body division, position of the heart, gonads and epicardium. Characters not frequently used in phylogenetic analysis, such as body wall muscles, muscles associated with transversal blood vessels and arrangement of the larval papillae, also have phylogenetic information. Results supported monophyly of the Aplousobranchia sensu Lahille, 1887 including only Polycitoridae, Polyclinidae, and Didemnidae. On the other hand, Aplousobranchia including also Cionidae and Diazonidae is not monophyletic since Perophora and Ecteinascidia were included as ingroups in the cladogram, Ciona (now closer to Ascidia) was no longer included in Aplousobranchia and the position of Rhopalaea and Diazona is not resolved. We propose a revised classification based on this phylogenetic analysis, in which Aplousobranchia, with three new families and an indeterminate taxon, now has 15 families.