Meeting Abstract
Excretion is the process of eliminating metabolic waste products from the system of the organism and can occur through passive or active transport. Active transport often takes place by ultrafiltration though specialized organs, named nephridia. The homology of nephridia is unsolved: although they show a great variation in morphology and development across bilaterians, recent studies have shown that the same orthologous developmental and structural genes are expressed in the nephridia of planarians and vertebrates. The phylogenetic position of the Xenacoelomorpha (comprised of the groups of Xenoturbellida, Nemertodermatida and Acoela), as possible sister group of all remaining bilaterians, and their lack of excretory structures make them a key group to study for understanding the evolutionary origin of nephridia. We investigated the putative role of candidate excretory genes by characterizing their expression patterns in two representatives of Xenacoelomorpha, the nemertodermatid Meara stichopi and the acoel Isodiametra pulchra, that differ in the morphology of their digestive tract. We focused on genes that either have a conserved role in nephridia development (e.g. Sall) or structure (e.g. Neph) in vertebrates and planarians. We also studied genes related to ammonia excretion (e.g. Rh) in crustaceans, planarians, nematodes and vertebrates. Our results show that in nemertodermatids and acoels all excretion related genes are mainly expressed in components of the reproductive and digestive systems as well as in individual – yet uncharacterized – cells. Our results hint at the presence of a not yet described excretory mechanism in Xenacoelomorpha, which could function as an early active transport system.
