ABSTRACT
The emergence of neurogenesis led to the acquisition of an effi cient neuromuscular transmission in eumetazoans, as shown by cnidarians that use evolutionarily-conserved neurophysiological principles to crumple, feed, swim. However, the cnidarian neuroanatomies are quite diverse and reconstructing the urcnidarian nervous system is not an easy task. Three types of characters shared by anthozoans and medusozoans appear plesiomorphic: (1) three cell types forming the cnidarian nervous system, neurosensory cells, ganglionic neurons and nematocytes (cnidocytes) that combine mechano-chemosensation and venom secretion; (2) a chemical conduction through nerve nets and nerve rings, those being considered as annular central nervous systems; (3) a larval apical sensory organ that initiates metamorphosis. Other characters receive a disputed origin: (1) the neural stem cell(s), that are multipotent interstitial stem cell in hydrozoans, but not identifi ed in other classes; (2) the electrical conduction
Department of Zoology and Animal Biology, Faculty of Science, University of Geneva, Sciences III, 30 quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland. E-mail: brigitte.galliot@unige.ch
through neurons and epithelial cells so far detected only in hydrozoans; (3) the embryonic origin of the nervous system; (4) the medusa sensory organs, ocelli or lens-eyes for light, statocysts for pressure, which are lacking in anthozoans. Interestingly numerous gene families that regulate neurogenesis in bilaterian are expressed during cnidarian neurogenesis, e.g. cnidarian eyes express Pax, Six and opsin, supporting a common origin for vision. However data establishing a clear picture of the cnidarian neurogenic circuitry are currently missing. In fact many “neurogenic” gene families likely arose and evolved in the absence of neurogenesis, as exemplifi ed by Porifera that express them but lack synaptic transmission. Therefore some eumetazoan-specifi c families, missing in Porifera as ParaHox/Hox-like and Otx-like genes, might have contributed to the emergence of neurogenesis.
