Gen Comp Endocrinol 2004 Feb 01;1353:345-57. doi: 10.1016/j.ygcen.2003.10.012.

Molecular cloning and developmental expression patterns of thyroid hormone receptors and T3 target genes in the turbot (Scophtalmus maximus) during post-embryonic development.

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Thyroid hormones (TH) are pleiotropic factors important for many developmental and physiological functions in vertebrates and particularly in amphibian metamorphosis. Their effects are mediated by two specific receptors (TRalpha and TRbeta), which are ligand-dependent transcription factors, members of the nuclear hormone receptor superfamily. Besides their pivotal role in amphibian metamorphosis, TH are also critical for fish metamorphosis. As this later role of TH is less studied, we analyzed their action in the turbot (Scophtalmus maximus), a metamorphosing flat fish. We describe the isolation of sequences for the turbot orthologs of a number of Xenopus genes, which are induced during amphibian metamorphosis. Developmental expression of these genes during turbot metamorphosis was studied by several methods and the expression patterns of these genes compared with those in Xenopus and flounder. We find that the period between the onset and the end of eye migration (day 22 to day 30 post-hatching) most likely corresponds to the metamorphic climax with either high TRalpha or high TH levels. Our results show that in contrast to amphibians, it is TRalpha and not TRbeta mRNA that is up-regulated during metamorphosis. Our results highlight the notion that TH regulates, through a rise of TR expression, a genetic cascade during turbot metamorphosis. The fact that TH regulates metamorphosis in amphibian and teleost fishes suggests that TH-regulated metamorphosis is a post-embryonic process conserved in most vertebrates.

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Species referenced: Xenopus
Genes referenced: thra thrb