Effects of Thyroxine Hormone on the Reproductive Efficiency and Gene Expression of Thyroid Hormone Receptors in the Eggs of Caspian Trout (Salmo caspius)

Caspian trout (Salmo caspius, Kessler 1877) is an anadromous brown trout species of ecological and conservation significance. Thyroxine (T4) plays a crucial role in oocyte maturation, fertilization, and early development in teleosts; however, its specific effects on Caspian trout embryos remain insufficiently understood. This study investigated the influence of exogenous T4 immersion on fertilization, eyeing, and hatching rates (FR, ER, HR), endogenous T4 and T3 levels in eggs, and thyroid hormone receptor (THR) gene expression (Thrα and Thrβ). Fertilized eggs were exposed to four treatments for 60 minutes: D1 (control), D2 (0.25 mg l⁻¹), D3 (0.5 mg l⁻¹), and D4 (0.75 mg l⁻¹). Samples were collected from pre-fertilization through post-hatching to capture developmental changes. FR showed no significant differences among treatments, whereas ER and HR were significantly reduced only in the highest T4 concentration (D4, p< 0.05). Egg T4 levels were elevated in D2 during hatching, while T3 concentrations peaked in control eggs (D1), indicating dose-dependent modulation of yolk thyroid hormone dynamics. Two-way ANOVA revealed significant effects of time, T4 concentration, and their interaction on Thrβ expression (p< 0.001). In contrast, Thrα transcripts were undetectable throughout development, suggesting isoform-specific regulation or temporal suppression of this receptor during early ontogeny. Moderate T4 exposure improved hatching performance and stimulated Thrβ expression, whereas excessive supplementation impaired development, potentially due to metabolic overload, altered hormone conversion, or receptor downregulation. Collectively, these findings demonstrate that embryo-yolk thyroid hormone signaling in Caspian trout is sensitive to exogenous T4 levels and plays a critical role in supporting embryogenesis and morphogenesis. Understanding these hormone-mediated processes has important implications for hatchery practices and conservation strategies aimed at enhancing early life-stage survival in this threatened species.