Final Activity Report Summary - GASTRULATION GENES (Function-based expression screen for genes involved in gastrulation of the basal metazoan Nematostella vectensis) Gastrulation is one of the key developmental processes in eumetazoans. It is the time when germ layers are formed and definitive axes of the embryo are specified. In the triploblastic Bilateria, to which the great majority of animal phyla belong, three germ layers, ectoderm, endoderm and mesoderm, are formed. However, the diploblastic Cnidaria, one of the most ancient offshoots in animal evolution, also form ectoderm and endoderm during gastrulation. Hence, Cnidaria offer the opportunity to address the question of the evolutionary origin of the gastrulation processes and of the mesoderm, both on the level of morphogenetic movements as well as specification and further differentiation of germ layers. As a new model organism for the study of embryogenesis among the Cnidaria, the sea anemone Nematostella vectensis has recently been established (Hand and Uhlinger, 1992; Fritzenwanker and Technau, 2003) and its genome has been sequenced (Putnam et al., 2007). The goal of this project is to search for factors that are functionally involved in Nematostella gastrulation and endoderm differentiation. In this project we have focused on the role of the homologue of one of the key regulators of the mesoderm and nerve differentiation in triploblasts - the MADS box transcription factor mef2. We could show that like in higher animals Nvmef2 is giving rise to several alternative splice variants, which are differentially expressed in time and space. By performing a functional knockdown by means of the antisense morpholino oligonucleotides we could demonstrate that Nvmef2 is required for proper neuronal development and for the endoderm differentiation after gastrulation. While the process of gastrulation itself was not affected, there were severe defects in the maintenance and differentiation of the endodermal layer. With the use of the antisense morpholino oligonucleotides directed against particular exons in the endodermally expressed splice variants we were able to determine the part of the protein, which is critically important for the proper development of the endoderm in Nematostella.