The cnidarian blastoporal organizer

In modern developmental biology, the organizer is defined as "an embryonic region capable of inducing a secondary axis when transplanted to an ectopic site" (see, for example, Stewart and Gerhart, 1990).The blastopore- associated organizer is a complex signaling center that is responsible to maintain the BMP and Wnt signaling gradients regulating the establishment of primary body axes (Garcia-Fernandez et al., 2007). The question is whether we can trace back the blastoporal organiser governing axial patterning to the more distant metazoan phyla. The basal metazoan phylum Cnidaria, which traditionally is considered to have one body axis ("Radiata") is considered to be the evolutionary sister group to the bilaterally symmetric organisms (Ryan and Baxevanis, 2007). Therefore, the Cnidaria are pivotal to understand the evolution of bilaterality of animals. We have recently revealed by transplantation experiments that the blastopore lip - but not tissue from other parts of the embryo - is able to act as an organizer and to induce the formation of a secondary body axis (Kraus et al., 2007a). It has also been recently shown that in the anthozoan cnidarian Nematostella vectensis the cells in the blastopore region express organizerspecific genes related to both signaling pathways mentioned above (Kusserow, et al., 2005; Lee et al., 2006; Rentzsch et al., 2006). Strikingly, genes belonging to the BMP signaling pathway or BMP antagonists are expressed at the blastopore asymmetrically with respect to the oral-aboral body axis of the embryo, indicating a second body axis perpendicular to the oral-aboral body axis - at least on the molecular level. The functions of these asymmetrically expressed genes in body axis formation are, however, unclear. To address the role and evolutionary origin of the blastoporal organizer, we need to study the activity of cnidarian organizer in greater detail. The proposed goals of our research project are: 1) To disect the organizer activity leading to the establishment of the embryonic body plan during early development of Nematostella vectensis; 2) To determine the molecules mediating this activity. Detailed investigation of an organizer activity, which includes the study of regional specificity of induction, time frames of inductive activity and molecular basis of inductive activity will be performed for the first time for the basal metazoan representative. It will involve the experimental use of transgenic lines which have been recently successfully established for the first time in the Technau lab. We plan to perform a number of transplantation experiments as well as experiments on the ectopic expression of organizer-associated genes. The results of these experiments will be analyzed by the combination of histology, electron microscopy and molecular biology methods. In fact, we can consider transplantation experiments as functional analyses that allow us to understand the molecular basis of Nematostella organizer activity. This unique combination of tools and object of study makes our research proposal innovative and very promising for the evolutionary developmental biology.