9321572 Hanken This proposal outlines a series of descriptive and experimental studies that examine the hormonal mediation of embryonic development in direct-developing anurans. Precocious activity of the thyroid axis is frequently cited as the mechanism underlying direct development, but this has never been fully examined. To test this hypothesis, initial appearance and subsequent development of the thyroid axis in a direct-developing frog will be compared to the patterns observed in metamorphosing frogs. The role of thyroid hormones (TH) in mediating direct development will also be evaluated experimentally by manipulating endogenous hormone levels. Methods include histology (to document thyroid gland and median eminence formation), radioimmunoassay (to determine TH levels through embryonic development), immunocytochemistry (to examine the ontogeny of thyrotropic cells and TH receptor expression), and hormone manipulation (to experimentally examine the roles of TH in direct development). Analyses will focus on embryos of the direct developing frog Eleutherodactylus coqui (Leptodactylidae) derived from an established breeding colony. For comparative purposes, analyses will also include several metamorphosing species, including Leptodactylus albilabris (Leptodactylidae), Bombina orientalis (Discoglossidae), and Xenopus laevis (Pipidae). Results will contribute to our understanding of the endocrine mediation of direct development in amphibians. They also will provide new information on the mechanistic basis underlying life-history evolution. In addition, this study will examine maternal provisioning of hormones in early embryogenesis and development, a process relevant to understanding early development in all classes of vertebrates. ***
