Meeting Abstract

35.5  Jan. 5  Peripheral nervous system development in the ascidian Ciona intestinalis ZELLER, Robert W.; San Diego State University rzeller@sciences.sdsu.edu

Current hypotheses postulate that the ancestral bilaterian nervous originated as a basiepidermal nerve net that was interspersed with sensory neurons. During bilaterian evolution, this nerve net was independently centralized in both protostomes and in the chordate lineage. In the ascidian embryo, the larval peripheral nervous system (PNS) consists of a series of ciliated cells, called epidermal sensory neurons (ESNs), which are located only along the dorsal and ventral midlines of the larval tail and on the dorsal and anterior trunk. We have discovered that a key transcriptional factor important for sensory neuron development in other organisms, a Pou IV class gene � CiPouIV, is specifically expressed in the Ciona PNS sensory neurons. This gene is a target of Notch-Delta mediated lateral inhibition. When Notch-Delta signaling is disrupted throughout the epidermis in transgenic embryos, the domain of sensory neurons expands, but only along the dorsal and ventral midlines of the embryo. In addition, expansion of CiPouIV expression is also observed in these ectopic sensory neurons, however ectopic sensory neurons are never formed laterally, only along the embryonic midlines. When CiPouIV is ectopically expressed in all of the epidermis, ectopic sensory neurons are formed throughout the epidermis, both in the embryonic midlines and in lateral territories. We observe that nearly any epidermal cell that expressed CiPouIV may be transformed into a PNS sensory neuron. Our observations suggest that cell-signaling pathways have restricted the developmental fate of the potential PNS neurons, but that bypassing Notch-Delta signaling may abrogate this restriction. The resulting ectopic sensory neurons may represent what might have been present in the expansive nerve-net of a common tunicate-vertebrate ancestor.