Heterochronic Changes in Gonadal Differentiation in <i>Pleurodema borellii</i> Tadpoles Under Thyroid Gland Stimulation and Inhibition

Thyroid hormones (THs) are important regulators of somatic development in vertebrates. In anurans, they control critical processes such as early limb differentiation, tail resorption, and tissue and organ restructuring, enabling the transition from aquatic larvae to terrestrial adults. However, their role in gonadal development remains less understood, particularly as gonadal differentiation often occurs independently of larval growth and metamorphic remodeling. In this study, we analyzed the morphogenesis of gonads in Pleurodema borellii, revealing an asynchrony between ovarian and testicular differentiation during larval development. We then evaluated the effects of methimazole (a TH synthesis inhibitor) and thyroxine (T4, an endogenous TH agonist) on gonadal development under mesocosm conditions at different larval stages. Methimazole exposure affected testicular morphogenesis more profoundly during later stages, leading to a disorganized morphology, that is, with poorly defined seminiferous tubules, indistinct cysts of spermatogonia, and scattered germ and somatic cells. Early exposure to T4 affected testicular organization as well, while later exposure accelerated differentiation. In ovaries, the timing of T4 exposure significantly influenced lobulation and ovarian cavity formation, with early exposure exerting greater effects than late exposure. These findings suggest that gonadal differentiation appears to reflect an early sensitivity or response to initial TH signals during premetamorphic stages. Collectively, our results highlight the importance of THs in coordinating early gonadal development, reflecting the complexity of endocrine regulation in amphibian sexual differentiation, likely involving interactions with other hormonal axes such as corticosteroids and sex steroids.