KUSAKABE, T*; KUSAKABE, R; NAKASHIMA, Y; HORIE, T; NAKAGAWA, M; TSUDA, M; Himeji Institute of Technology; Himeji Institute of Technology; Himeji Institute of Technology; Himeji Institute of Technology; Himeji Institute of Technology; Himeji Institute of Technology: Development, function, and molecules of photoreceptive organs in ascidians: insights into the origin and evolution of vertebrate eyes

Vertebrates have evolved two types of eyes; the lateral eyes (paired eyes) and the median eyes (pineal or parietal eyes). The urochordate ascidian larva has an eye-spot (ocellus) in its brain. Putative photoreceptor organs have also been reported in adult ascidians. Understanding evolutionary relationships between the ascidian photoreceptors and the vertebrate eyes is a key to uncover the origin and evolution of the vertebrate eyes. In the ascidian Ciona intestinalis, we have characterized and examined expression patterns of homologues of genes involved in function or development of the vertebrate eyes. The results suggest that ascidians have photoreceptor systems more similar to those of vertebrates than to those of other invertebrates. The larval ocellus expresses a vertebrate-type opsin gene and the surrounding brain cells express visual cycle genes similar to those found in the retinal pigment epithelium of vertebrates. A number of genes related to eye function and development are also expressed in part of the primordial pharynx and atrial primordia, suggesting that adult photoreceptors develop in these regions, possibly oral and atrial siphons. Based on comparisons of the developmental origins, gene expression patterns, and functions of eyes between vertebrates and ascidians, we propose a hypothesis that the larval ocellus and the adult anterior photoreceptors (of the oral siphon) are homologous to the vertebrate median eye and lateral eyes, respectively. The last common ancestor of urochordates and vertebrates may have possessed distinct precursors of the lateral eyes and the median eye of vertebrates.