Ultrastructural and immunohistochemical studies of the ontogeny of the nervous system and the mesoderm in Enteropneusta and Pterobranchia in order to elucidate deuterostome evolution.

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Morphological and molecular data support different phylogenetic hypotheses of deuterostome taxa and therefore infer contrasting evolutionary character evolution. Many of the supposedly morphological “key characters” have not been adequately studied in proper ontogenetic or structural detail. In the present project, we analyze the ontogeny of morphological characters that have been considered important for deuterostome evolution utilizing computer assisted 3D-reconstruction based on serial transmission electron micrographs in combination with confocal laser scanning microscopy of immunolabeled specimens. We have so far documented the neurulation of the collar cord in an enteropneust and revealed that the collar cord is much more complex than was hitherto believed. E.g., areas of neuronal somata and neuropil are clearly organized in concise patterns, a intimate relation between the asymmetric proboscis pore and the collar cord exists, and we discovered a distinct gland in the mid-dorsal line that develops from a glandular ring around the entire collar. Our database allows the reconstruction of peripheral nerves in the branchial region along with coelomic cavities, blood vessels, skeletal elements, and musculature. Using serially sectioned specimens fixed for electron microscopy, we could for the first time observe the ontogenetic primordium of the pericard, corresponding to the right protocoel of other deuterostomes. Based on our detailed anatomical studies, we could find no further support for the often-quoted primary homology hypothesis between the post-anal tail of enteropneusts and the chordate tail. In the successfully collected two species of pterobranchs, we just started our morphological analyses and found similarities in the dorsal ganglion to the organization of chordate central nervous systems based on confocal laser scanning microscopical investigation of the serotonergic nervous system. Transmission electron microscopy of developmental stages of pterobranchs that were also successfully sampled, are scheduled for analysis later this year, as are studies of adult structures. In summary, this project has already yielded important results, has successfully completed acquisition of biological samples, and has therefore the potential to fulfill all goals aimed for in the original project, especially after Sabrina Kaul a PhD candidate ideally trained for this project joined the project.