Stach, T.*; Turbeville, J. M.: Reconstruction of tunicate phylogeny using molecular and morphological data

The phylogeny of the Tunicata and ascidian families within the Tunicata was reconstructed using molecular and morphological evidence. Mitochondrial cytochrome oxidase I (COI) and 18S rDNA sequences of 15 tunicate species were obtained and aligned with gene sequences of cephalochordates and craniates. Morphological characters of larval and adult stages were compiled from the literature and analyzed cladistically. Implications of separate and simultaneous analyses of molecular and morphological data were examined. Aplousobranch ascidians were included in a molecular phylogenetic analysis for the first time. COI data and the combined analysis of COI data and morphological characters do not support monophyly of the Aplousobranchiata, whereas morphological characters suggest monophyly. When a single aplousobranch species is included, Appendicularia (= Larvacea) nest within the Ascidiacea in the analysis of the preliminary 18S rDNA data and the combined molecular (18S rDNA) and morphological analysis. In the morphological analysis Agnesiidae and Octacnemidae (= Hypobythiidae) nest within the Phlebobranchiata (Enterogona). Morphological evidence suggests that the Sorberacea (= Aspiraculata), traditionally thought to represent a separate tunicate class, are aberrant pleurogonid deep-sea ascidians, constituting the sister group of the Molgulidae. No molecular data exist for Agnesiidae, Octacnemidae, and Sorberacea. Morphological character evolution and implications for the reconstruction of the chordate groundplan will be discussed. The preliminary results show that more molecular and morphological data are needed to reliably resolve tunicate and ascidian phylogeny and acquisition of these data is currently underway.