Abstract
Excretion is a fundamental process for animal survival and led to the evolution of specialized excretory organs that facilitate the removal of metabolic waste products. Excretory organs such as nephridia and kidneys are seen as a nephrozoan novelty since they have neither been found in Xenacoelomorpha (Xenoturbella + Acoelomorpha) nor outside Bilateria. Xenacoelomorphs are thus informative for the reconstruction of ancient bilaterian excretory mechanisms and the evolution of excretory organs. In this study we investigated excretion in the acoel Isodiametra pulchra, characterized by a syncytial gut, and the nemertodermatid Meara stichopi, with an epithelial gut, that both belong to the Acoelomorpha. We examined the expression of excretion-related genes that are associated with the different functional and anatomical sections of nephridia (ultrafiltration-affiliated genes, rootletin, solute carrier transporters (SLCs), aquaporins) and genes involved in ammonia excretion (Rhesus, Na+/K+ ATPase, v-ATPase (a and b), Carbonic Anhydrase A 2 and 4). We found that in both animals most of these genes are expressed in defined domains that are in proximity to the reproductive and digestive systems. Our functional analyses of the ammonia-excretion components in acoels suggest that Na+/K+ ATPase, v-ATPase and Rhesus proteins are involved in the active excretion mechanism, 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 most of the candidate genes are also expressed in gastrodermal domains correspondingly. Our results show that in acoelomorphs and cnidarians excretion-related genes are expressed in domains neighboring or part of the digestive system independently from an epithelial or syncytial organization of the gut and spatially unrelated to comparable nephridial compartments. Furthermore, the rather broad expression of ultrafiltration-affiliated genes indicates their non-ultrafiltration role and suggests a later cooperative recruitment in the assembly of the nephridial ultrafiltration apparatus. Our results also show that the role of Na+/K+ ATPase, v-ATPase and Rhesus proteins in the ammonia excretion mechanism predates the Xenacoelomorpha/Nephrozoa split. Our findings suggest that excretion took place through broader digestion-tract associated domains before the specialized excretory organs of bilaterians have been evolved.