Among Molluscs, Cephalopods are remarkable by their cognitive abilities. Their brain, resulting from the condensation of some ganglia, set up through a direct development, and their nervous system is not organized as a nerve cord along clear anterior-posterior and medio-lateral axes, unlike most studied organisms (Mus musculus, Drosophila melanogaster, and more recently Platynereis dumerilii). The expression study of genes known to be involved in the neural regionalization (Otx, Pax, Nk, Gsx) and differentiation processes (Elav, Musashi) has been conducted in Sepia officinalis. The results enable us : 1) to establish the dynamics of ganglionic maturation, showing in particular a late maturation of the lobes involved in learning and memory; 2) to provide a map of embryonic territories, based for the first time on molecular data, which gives a new insight into the cephalopod organogenesis; 3) to highlight new roles for these genes in the neural differentiation and specification in the development of cephalopod synapomorphies (arms, funnel, camerular eyes); 4 ) to show that despite the derived features of the embryogenesis and nervous system in cephalopods, some genes of neural specification present a regionalized expression, similar to that is observed in the median nerve axis of the model species. These results emphasize the importance of taking into account the evolutionary relationships between gene functions and the various organizations of nervous systems in a wide range of metazoan phyla.
Key words: Sepia officinalis, cephalopod, mollusc, neurogenesis, Evo-Devo, body axes.