Thyroid hormones (THs) have pleiotropic effects on vertebrate development, with amphibian metamorphosis as the most spectacular example. However, developmental functions of THs in non-vertebrate chordates are largely hypothetical and even TH endogenous production has been poorly investigated. In order to get better insight into the evolution of the thyroid hormone signaling pathway in chordates, we have taken advantage of the recent release of the amphioxus genome. We found amphioxus homologous sequences to most of the genes encoding proteins involved in thyroid hormone signaling in vertebrates, except the fast-evolving thyroglobulin: sodium iodide symporter, thyroid peroxidase, deiodinases, thyroid hormone receptor, TBG, and CTHBP. As only some genes encoding proteins involved in TH synthesis regulation were retrieved (TRH, TSH receptor, and CRH receptor but not their corresponding receptors and ligands), there may be another mode of upstream regulation of TH synthesis in amphioxus. In accord with the notion that two whole genome duplications took place at the base of the vertebrate tree, one amphioxus gene often corresponded to several vertebrate homologs. However, some amphioxus specific duplications occurred, suggesting that several steps of the TH pathway were independently elaborated in the cephalochordate and vertebrate lineages. The present results therefore indicate that amphioxus is capable of producing THs. As several genes of the TH signaling pathway were also found in the sea urchin genome, we propose that the thyroid hormone signaling pathway is of ancestral origin in chordates, if not in deuterostomes, with specific elaborations in each lineage, including amphioxus.

中文翻译：

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两栖动物基因组启发了甲状腺激素信号传导途径的进化。

甲状腺激素（THs）对脊椎动物的发育具有多效性，其中两栖类变态是最典型的例子。然而，THs在非脊椎动物脊索动物中的发育功能在很大程度上是假设的，甚至对TH的内源性生产也缺乏研究。为了更好地了解和弦中甲状腺激素信号传导途径的演变，我们利用了最近释放的两栖类基因组。我们发现脊椎动物中与甲状腺激素信号传导有关的大多数编码蛋白质的基因的双链同源序列，但快速发展的甲状腺球蛋白除外：碘化钠转运蛋白，甲状腺过氧化物酶，脱碘酶，甲状腺激素受体，TBG和CTHBP。由于仅检索到一些编码参与TH合成调控的蛋白质的基因（TRH，TSH受体，和CRH受体，但没有相应的受体和配体），则可能有另一种上游调节双歧杆菌TH合成的方式。与在脊椎动物树的基部发生两个完整的基因组重复的观点一致，一个双歧杆菌基因通常对应于几个脊椎动物同源物。但是，发生了一些两栖类的特定重复，这表明在头孢酸盐和脊椎动物谱系中TH通路的几个步骤是独立完成的。因此，目前的结果表明，双歧杆菌能够产生TH。由于在海胆基因组中还发现了TH信号通路的几个基因，因此我们认为甲状腺激素信号通路起源于脊索动物（如果不是氘代动物），在每个谱系（包括两栖动物）中都有特定的修饰。