Abstract
Flatfish metamorphosis is the most dramatic post-natal developmental event in teleosts. Thyroid hormones (TH), thyroxine (T4) and 3,3'-5'-triiodothyronine (T3) are the necessary and sufficient factors that induce and regulate flatfish metamorphosis. Most of the cellular and molecular action of TH is directed through the binding of T3 to thyroid nuclear receptors bound to promoters with consequent changes in the expression of target genes. The conversion of T4 to T3 and nuclear availability of T3 depends on the expression and activity of a family of 3 selenocysteine deiodinases that activate T4 into T3 or degrade T4 and T3. We have investigated the role of deiodinases in skin and muscle metamorphic changes in halibut. We show that, both at the whole body level and at the cellular level in muscle and skin of the Atlantic halibut (Hippoglossus hippoglossus) during metamorphosis, the coordination between activating (D2) and deactivating (D3) deiodinases expression is strongly correlated with the developmental TH-driven changes. The expression pattern of D2 and D3 in cells of both skin and muscle indicate that TH are necessary for the maintenance of larval metamorphic development and juvenile cell types in these tissues. No break in symmetry occurs in the expression of deiodinases and in metamorphic developmental changes occurring both in trunk skin and muscle. The findings that two of the major tissues in both larvae and juveniles maintain their symmetry throughout metamorphosis suggest that the asymmetric changes occurring during flatfish metamorphosis are restricted to the eye and head region.