orthodenticle/otx ortholog expression in the anterior brain and eyes of Sepia officinalis (Mollusca, Cephalopoda).

The origin of cerebral structures is a major issue in both developmental and evolutionary biology. Among Lophotrochozoans, cephalopods present both a derived nervous system and an original body plan, therefore they constitute a key model to study the evolution of nervous system and molecular processes that control the neural organization. We characterized a partial sequence of an ortholog of otx2 in Sepia officinalis embryos, a gene specific to the anterior nervous system and eye development. By in situ hybridization, we assessed the expression pattern of otx2 during S. officinalis organogenesis and we showed that otx is expressed (1) in the eyes, from early to late developmental stages as observed in other species (2) in the nervous system during late developmental stages. The otx ortholog does not appear to be required for the precocious emergence of the nervous ganglia in cephalopods and is later expressed only in the most anterior ganglia of the future brain. Finally, otx expression becomes restricted to localized part of the brain, where it could be involved in the functional specification of the central nervous system of S. officinalis. These results suggest a conserved involvement of otx in eye maturation and development of the anterior neural structures in S. officinalis.