| Despite being deeply embedded in a clade of bilaterian animals, echinoderms posses pentameral symmetry. This pentameral symmetry is illustrated by their central nervous system consisting of a central nerve ring and five radial nerve cords. During the evolution of echinoderms this pentameral nervous system was transformed from a bilaterian one and represents one of the most diverged nervous systems of the Metazoa. A wealth of data exists on neurogenesis in model systems and it has been shown in highly diverged phyla that, despite dramatic differences in morphology, gene expression is remarkably conserved. The identification of genes that are expressed in the developing echinoderm nervous system will provide insights into the molecular workings of this highly derived nervous system and allow us to make comparisons to better studied animals.; We performed a random sequencing screen to identify genes that are expressed in the adult nervous system of Heliocidaris erythrogramma. Temporal and spatial expression of these genes was analyzed by RNA blots and whole mount in situ hybridization, respectively. The results indicated that there are no ancestral remnants of bilateral symmetry in the developing nervous system of the sea urchin, but there is much more transcriptional diversity than previously recognized. Further analysis of the expression of one of the genes isolated from this screen, 14-3-3, combined with immunolocalization of 14-3-3 proteins using an anti-14-3-3 antibody, allowed us to trace the morphology of the developing adult sea urchin nervous system. This information allowed us to identify developing neural structures and suggest a model for the growth of the radial nerve cords. Our screen also identified four presumptive neuropeptides, Henpep-1, Henpep-2, Henpep-3 , and Henpep-4. We identified a putative open reading frame in Henpep-2 that is structurally similar to neuropeptides, but has a novel sequence. Whole mount in situ analysis of these genes revealed a complex nervous system in the reduced larva of H. erythrogramma , suggesting that the nervous system has some novel functions associated with the change in developmental mode.; Lastly, we discuss the controversy of whether the ancestral bilaterian was an indirect developer or direct developer. The argument has been made that the dipleurula larva of deuterostomes (echinoderms and hemichordates) and the trochophore larva of lophotrochozoans are homologous, and that the last bilaterian ancestor was a related larva-like animal. We present ideas and data to show that larvae evolved secondarily in each of these groups by co-opting genes and gene modules from the adult body plan, and suggest that the ancestral bilaterian was a direct developer. |
