Effects and phenotypic consequences of transient thyrotoxicosis and hypothyroidism at different stages of zebrafish Danio rerio (Teleostei; Cyprinidae) skeleton development

Thyroid hormones (THs) are one of the main regulators of remodeling, homeostasis and development of skeletal tissues in teleosts, and the effects of hypo- and hyperthyroidism on skeleton are among the objectives of research in the fields of fishes development and evolution. However, in most experimental models used, the altered THs status is a constant characteristic of the developing organism, and the observed phenotypic outcomes are the cumulative consequences of multiple THs induced developmental changes. The effects of the transient fluctuations of THs content on the skeleton development have been studied much less. Here, we present experimental data on the developmental effects and phenotypic consequences of transient, pharmacologically induced thyrotoxicosis and hypothyroidism at different stages of zebrafish ossified skeleton patterning. In accordance with the results, skeleton structures differ in the timing and degree of THs sensitivity. Some of them displayed a notable shift in the developmental timing and rate, whereas other demonstrated a subtle or absence of reaction in respond to changes of THs content. The developmental stages also differ in THs sensitivity. A relatively short developmental period has been revealed, during which changes in THs level (mainly thyrotoxicosis) sharply increase the developmental instability and plasticity, leading to phenotypic consequences comparable to those in fish with permanently altered THs status. These findings allow us consider this period as a critical developmental window. Summary statementStudy of the developmental effects and phenotypic consequences of acute transient changes in thyroid hormones content have identified a critical developmental window for zebrafishs skeleton Ethics statementAll procedures with fish were carried out according to the guidelines and following the laws and ethics of the Russian Federation, and approved by the ethics committee of the Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences (Approval ID: N 95 issued on 27.05.2024).