Abstract
The specification of the endostyle in non-vertebrate chordates and of the thyroid gland in vertebrates are fundamental steps in the evolution of the thyroid hormone (TH) signaling to coordinate development and body physiology in response to a range of environmental signals. The physiology and biology of TH signaling in vertebrates have been studied in the past, but a complete understanding of such a complex system is still lacking. Non-model species from non-vertebrate chordates may greatly improve our understanding of the evolution of this complex endocrine pathway. Adaptation of already existing proteins in order to perform new roles is a common feature observed during the course of evolution. Through sequence similarity approaches, we investigated the presence of bona fide thyroid peroxidase (TPO), iodothyronine deiodinase (DIO), and thyroid hormone receptors (THRs) in non-vertebrate and vertebrate chordates. Additionally, we determined both the conservation and divergence degrees of functional domains at the protein level. This study supports the hypothesis that non-vertebrate chordates have a functional thyroid hormone signaling system and provides additional information about its possible evolutionary adaptation.
Highlights
Via thyroid hormone (TH) signaling, organisms are able to regulate many physiological processes
The sequences from H. sapiens, D. rerio, P. marinus, and G. gallus were retained for further analyses
The presence of a thyroid hormone-producing endostyle in non-vertebrate chordates supports the hypothesis that the thyroid hormone signaling system is an ancient feature of all chordates
Summary
Via thyroid hormone (TH) signaling, organisms are able to regulate many physiological processes. In animals, this system is very important for key biological features such as development, metamorphosis, and cell and organism regeneration [1,2,3]. Thyroid hormone signaling has emerged as a biological system to convey environmental information to cells and maximize the exploitation of environmental resources. It may be considered a key regulator of development and metabolism that arose very early in life history [3]. Thyroid hormone signaling pathways have been deeply characterized mainly in vertebrates. Among the many elements involved in this signaling pathway, there are three key enzymes that have been identified in a few invertebrate taxa [4]
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