Meeting Abstract
Tunicata, a diverse clade of over 3,000 species of marine, filter-feeding chordates, is the closest living relatives of vertebrates and thus is of great interest to researchers studying our own evolution and development. Despite their diversity and importance, relationships among major lineages of Tunicata are not completely understood. In order to improve understanding of tunicate phylogeny, we supplemented publicly available data with transcriptomes from seven additional species spanning the diversity of Tunicata and conducted phylogenomic analyses on data sets with up to 798 genes. We also conducted sensitivity analyses to examine the influence of reducing compositional heterogeneity and branch-length heterogeneity in our data. All analyses maximally supported a monophyletic Tunicata within Olfactores (Vertebrata + Tunicata). Within Tunicata, all analyses recovered the long-branched Larvacea sister to the rest of Tunicata and confirmed (with maximal support) that Thaliacea is nested within Ascidiacea. Stolidobranchia is the sister taxon to a clade in which Thaliacea is sister to Aplousobranchia + phlebobranch tunicates. Interestingly, in most maximum likelihood (ML) analyses, phlebobranchs were usually recovered paraphyletic with respect to Aplousobranchia. Support for this topology varied but was strong in some cases. However, BI analysis using the site heterogeneous CAT-GTR-Γ model recovered Phlebobranchia and Aplousobranchia reciprocally monophyletic with a posterior probability of 0.99, consistent with traditional morphology-based hypotheses. Examination of internode certainty also casts doubt on the ML result of phlebobranch paraphyly, which may be due to limited sampling for Aplousobranchia. Taken together, these results provide a higher-level phylogenetic framework for our closest living invertebrate relatives.
