Alternative Development Pathways in Ascidians and Aspects of Chordate Evolution

The tunicate ascidians possess a swimming larva with a chordate body plan that metamorphoses into a sessile oozooid. By blastogenesis, many ascidians form colonies with numerous blastozooids, similar to the oozooid. We rear and study the ascidian Botryllus schlosseri with the aim to compare embryonic and blastogenetic developmental pathways and thank the key phylogenetic position of the ascidians, to analyze evolutionary aspects of invertebrate–vertebrate transient forms. Attention is on organ structure, and cellular and molecular mechanisms of their formation. With previous structural and molecular research on neural complex development in embryos and buds, we suggested that the common chordate ancestor possessed vertebrate placodal/neural crest-like cells, whose embryonic genetic pathways were co-opted for blastogenesis. We now refer to development of muscles that in ascidians presents three forms: striated in the larval tail and heart, and smooth in the postmetamorphic sessile phase. The larval tail and cardiac muscles have the myofilaments arranged like vertebrate striated muscle, whereas the smooth musculature has intermediate characters between smooth and striated muscle of vertebrates. In blastozooids, the cardiac and smooth muscles derive from mesenchymal cells. Their organization, differentiation, and gene expression have been analyzed beginning from the early bud to adult and regression stage. We have characterized two transcripts, a muscle-type actin (BsMA2) and a troponin T (BsTnT) and detected their expression by in situ hybridization (ISH), in parallel with phalloidin staining experiments. Phylogenetic analyses showed a close relationship between urochordates and vertebrates muscle genes.