DAVIDSON, B.J.*; SWALLA, B.J.: Heads or Tails? Larval vs. Post-larval Development in the Urochordates

The vast majority of research on tunicate development focuses on the embryogenesis of axial structures (notochord and dorsal nerve cord) within the larval tail. However, the majority of tunicate development takes place after the tail is resorbed at settlement. After settlement, rudiments within the larval head differentiate to form the juvenile gut, with chordate pharyngeal slits and endostyle, as well as mesodermal organs such as the body wall musculature, heart and blood cells. Very little is known about the developmental signals used to coordinate the differentiation of these post-larval structures. We have been conducting an in-depth analysis of tunicate post-larval development, looking at cell migrations, cell cycle patterns and gene expression. This has led to some fascinating insights into (1) the nature of metamorphic competence, (2) the timing of organ differentiation during metamorphosis, (3) an immune response which occurs during metamorphosis and (4) the developmental signals which are coordinating this dramatic life-history transformation. A better understanding of post-larval development in the tunicates will provide a valuable resource for work on the evolution and development of key chordate structures such as the pharynx, endostyle, heart and blood cells.