Comparative ontogeny of skin glands in Rhinella and Incilius toads

A key component of amphibian antipredator strategies is the chemical defenses that make them toxic and/or distasteful, like the cardiotoxic bufadienolides synthesized by the true toads and stored in granular skin glands. The morphology of adult toad glands is well-described, but the ontogenetic timing and distribution of glands during larval stages are poorly understood. A comparative understanding of granular gland development is needed to elucidate the mechanisms underlying the diversification of toad chemical defenses and their ecological roles across lineages and ontogeny. Herein, we analyzed gland morphology before and after metamorphosis of Rhinella horribilis, Incilius melanochlorus, and I. luetkenii. We hypothesized that granular gland development would begin earlier and progress faster in relatively more toxic Rhinella species. Our results showed that the timeline of skin development, the relative dimensions and quantity of structures, and the appearance of protein and mucin content in granular and mucous glands did not vary among Rhinella and Incilius. Furthermore, epidermal mucus and/or giant cells in larval toads may act as sources of chemical defenses prior to gland development. Our findings suggest that gland development is well-conserved among these genera; it is possible that reported differences in toxin profiles are not due to divergent morphogenesis during larval or metamorphic stages, but rather to differences in molecular function or structural differentiation in juveniles or adults. Therefore, complementary studies using integrative techniques such as immunohistochemistry and comparative transcriptomics are needed to uncover the mechanisms responsible for the diversity of chemical defenses found in bufonid toads across development.