Figure 4.

A hypothetical scenario suggesting the ancient regulatory relationship between Otx and ur-opsin predating the cnidarian–bilaterian split. The ur-opsin has been already regulated by the Otx gene and probably by Pax (not excluding other transcription factors involved). After the duplication and diversification of r- and c-opsin, the regulation by Otx has been preserved in both lineages, whereas Pax-dependent regulation has been lost in c-opsin lineage. Retinal homeobox Rx might have been recruited for regulation of c-opsin. Alternatively, Rx has regulated the ur-opsin and this role has been lost in r-opsin lineage. With increasing complexity of animal body plans, all the transcription factors have consequently acquired additional roles in eye development.