Abstract
Excretory organs such as nephridia and kidneys are seen as a nephrozoan novelty since they are neither present in Xenacoelomorpha (Xenoturbella + Acoelomorpha) nor outside Bilateria. However, the evolutionary origin of excretory organs and the ancient mode of excretion before their existence remain unclear. In this study we investigated the excretion in an acoel and a nemertodermatid, both members of the Xenacoelomorpha, the sister group to all remaining Bilateria (fish, frogs, humans). We examined the expression of excretion-related genes associated with the different compartments of nephridia (ultrafiltration-affiliated genes, rootletin, solute carrier transporters, aquaporins) and genes involved in ammonia excretion (Rhesus, Na+/K+ ATPase, v-ATPase and Carbonic Anhydrase). We show that in acoelomorphs the ultrafiltration-affiliated genes have a broad expression while most of the other excretion-related genes are expressed in defined domains neighboring the reproductive and digestive systems. These domains are spatially unrelated to nephridial compartments of other Bilateria. Our experimental approach indicates that Na+/K+ ATPase, v-ATPase and Rhesus proteins are involved in the active ammonia excretion mechanism in acoels, contrary to the previously hypothesized excretion by passive diffusion. To test if we find similar mechanisms outside Bilateria, we extended our study to the cnidarian Nematostella vectensis and show that also in this group the ammonia excretion-related genes are expressed in gastrodermal domains. Together, our results suggest that in acoelomorphs and cnidarians excretion takes place through digestive-tissues and that this mode of excretion might have been the major excretory mechanism present before the specialized excretory organs of animals evolved.
