Phylogenetic analysis of phenotypic characters and evolutionary transformations in Appendicularia (= Larvacea) and their consequences for the reconstruction of the groundplan of Chordata

• Applicant: Dr. Thomas Stach
• Subject Area: Systematics and Morphology (Zoology)
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 451041039

Chordata comprises three monophyletic animal taxa: tunicates (= Tunicata), lancelets (= Cephalochordata), and vertebrates (= Craniota). With approximately 2700 species, tunicates show a bewildering diversity of life-history strategies and body plans; many species live attached to the substrate as sessile organisms, others float freely in the plankton, and some display spectacular polymorphisms between generations. Recent phylogenomic and morphology-based phylogenies concur in positioning Appendicularia (= Larvacea) as sister group to the remaining tunicates. This puts Appendicularia with approximately 70 species in a key position to elucidate tunicate evolution, because primitive traits can be expected to occur preferentially in appendicularians. Modern morphological studies are almost exclusively restricted to one species – Oikopleura dioica – impeding a reliable assessment of the disparity within this phylogenetically important group of animals. For the members within the deep-sea genus Bathochordaeus not even light microscopical modern investigations exist. The recent phylogenomic analyses comprise but a single appendicularian species, older molecular phylogenies based on 18S rDNA-sequences included up to three species of a single family. In the proposed project specimens of 13 appendicularian species of all three families will be morphologically analyzed in detail. A major goal is the conceptualization of a data matrix for cladistic analysis based on digital anatomies and immunohistological analyses combined with confocal laser scanning microscopy. Wherever possible, these analyses will be complemented by transmission electron microscopic investigation. In this way, the following goals will be achieved:(1) The morphological disparity of this key taxon will be sampled and documented according to modern standards.(2) Primary homology hypotheses of a maximum number of characters will be formulated.(3) For the first time, a morphology based phylogenetic hypothesis based on a transparent cladistic analysis will be suggested.(4) Hypotheses of the functional morphology of organ systems can be proposed based on the ultrastructural analyses.(5) Evolutionary transformations will be reproducibly explained in all organ systems.(6) A cladistic reconstruction of the ground pattern in the last common ancestor of Appendicularia will be achieved.(7) Implications for evolutionary scenarios of higher taxa Tunicata, Olfactores (= Tunicata + Craniota), and Chordata grounded in these cladistic analyses will be discussed.

• DFG Programme: Research Grants