Ci‐opsin1, a vertebrate‐type opsin gene, expressed in the larval ocellus of the ascidian Ciona intestinalis

A novel gene encoding visual pigment, Ci‐opsin1, was identified in a primitive chordate, the ascidian, Ciona intestinalis. Molecular phylogenetic analysis and the exon–intron organization suggest that Ci‐opsin1 is closely related to the retinal and pineal opsins of vertebrates. During embryogenesis, Ci‐opsin1 transcripts were first detected in part of the brain of mid tailbud embryos; its expression was confined to photoreceptor cells of the ocellus (eye spot) in the larval brain as development proceeded. These results suggest a common descent of the ascidian ocellus and the vertebrate eyes. The ocellus of ascidian larvae may represent an ancestral state of the vertebrate eye.