Mechanisms of asymmetrical body formation during flatfish metamorphosis and prevention of malformed juveniles in seed production process

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For the mechanisms of asymmetrical body formation of flatfish development, we proposed a working hypothesis "timing hypothesis". From several lines of evidences, thyroid hormone induces blind side characteristics (white body color and migration of eye of the own side), and the tissue responsiveness to thyroid hormone exists differentially between right and left side of the body. Thyroid hormone has to be secreted during a limited time window when the responsiveness only presents on one side, in order to develop into normal juveniles. This project aimed to confirm the validity and generality of the hypothesis more in detail and for more species.In vivo appearance of all pigment cells during Japanese flounder development revealed the presence of three types of appearance manor as follows, 1) symmetrical larval type (larval type melanophore and xanthophore, and leucophore), 2) asymmetrical adult type (adult type melanophore and xanthophore), 3) symmetrical adult type (iridophore). For detailed study in future, in vitro culture for pigment cell ontogeny was attempted. At the end of this project term, we have established the culture condition in which all pigment cells (except for larval xanthophore) appear.Slime flounder (Microstomus achne) has extremely long, and starry flounder (Platichthys stellatus) has relatively short, pelagic life stages among flatfishes in Japan. For both species, thyroid hormone (T4, thyroxine) was added to the rearing tanks at various timings, and composition of normal, pseudoalbino, and ambicolorate, was examined after the completion of metamorphosis. Timing of T4 addition significantly altered the ratio, with a manor that can be explained by the timing hypothesis. In addition, the highest ratio of normal juveniles was observed in the tanks where the larvae metamorphosed at a similar timing to the sea. These results indicate the generality of the hypothesis to both species.