Mechanisms of eye relocation during flatfish metamorphosis, - asymmetrical morphogenesis by thyroid hormone

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The external asymmetry of flatfish is most significant in the animal kingdom. One eye of the symmetrical larva migrates to the opposite side, and the side with both eyes (ocular side) becomes dark by the expression of the adult-type melanophores(aMLP) during metamorphosis, particularly under the control of thyroid hormones(THs).The pseudomesial bar(Pb), a unique bone that is present in flatfish only on the side without eyes (blind side), simultaneously developed with the progress of eye migration during metamorphosis. Since Pb required TH at a specific time, the involvement of TH was indicated in the formation of the blind side. From in vivo and in vitro experiments, it was suggested that TH also plays a role in the coloration of the blind side by suppressing the spontaneous expression of aMLP.To answer the question why TH can exert its effect only on the future blind side, the larvae of spotted halibut were artificially metamorphosed by TH treatment at various timings. The occurrence of "ambicoloration" (ocular-side characteristic on both the sides) was increased by metamorphosis under the action of TH that was either too early or too late. On the other hand, "pseudoalbino" (blind-side characteristic on both sides) was observed under the action of TH at a specific time. Based on the results, TH responsiveness to induce blind-side characteristic was estimated. The responsive period was longer for the left side (blind side in the normal juveniles) than for the right side, and this difference was believed to be the central mechanism for the metamorphic asymmetry.