Dissertation Research: Evaluating the role of thyroid hormone in embryonic limb development in direct-developing frogs

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This study will clarify the role of thyroid hormone in the development of amphibian limbs. Hormones are substances produced in the body that influences the way the body grows or develops. Thyroid hormone plays an important role in the transition of tadpoles into frogs, especially in limb development. Nearly a third of frog species bypass the tadpole stage and develop directly into frogs and the role of thyroid hormone in the development of these species is not known. This project will test two alternative hypotheses about the role of thyroid hormone in this process. In association with the Harvard Museum of Natural History, this project will also develop several educational activities on amphibian biodiversity for K-12 students, teachers, and parents. Some of these activities can be offered at little cost, which will allow many students to experience wildlife and witness the dramatic transformation of metamorphosis for themselves.Comparing how thyroid hormone regulates gene activity during limb development between direct-developing and metamorphosing frogs can tell us how fundamental endocrine networks evolve and generate different life cycles. Biphasic frogs (which undergo metamorphosis) require thyroid hormone (TH) for metamorphosis and limb development. This study evaluates the role of TH in limb development in the direct-developing frog Eleutherodactylus coqui for comparison to its role in three biphasic frog taxa (Xenopus laevis, Bombina orientalis, and Dendrobates tinctorius). Next-generation sequencing will be used to characterize and compare the TH-induced molecular phenotype in the developing limb of all four species. This study will provide a basis for further studies of amphibian life history evolution and identify interspecific variation in an otherwise highly conserved molecular mechanism of metamorphosis. Characterizing how one group achieves direct development will provide a comparison for investigating the repeated origins of this life history in other amphibians, and address questions at the intersection of endocrinology, evolution, and development.