Evolutionary Conservation of Thyroid Hormone Receptor and Deiodinase Expression Dynamics in ovo in a Direct-Developing Frog, Eleutherodactylus coqui

Abstract

Direct development is a reproductive mode in amphibians that has evolved independently from the ancestral biphasic life history in at least a dozen anuran lineages. Most direct-developing frogs, including the Puerto Rican coquí, <i>Eleutherodactylus coqui</i>, lack a free-living aquatic larva and instead hatch from terrestrial eggs as miniature adults. Their embryonic development includes the transient formation of many larval-specific features and the formation of adult-specific features that typically form postembryonically-during metamorphosis-in indirect-developing frogs. We found that pre-hatching developmental patterns of thyroid hormone receptors alpha (<i>thra</i>) and beta (<i>thrb</i>) and deiodinases type II (<i>dio2</i>) and type III (<i>dio3</i>) mRNAs in <i>E. coqui</i> limb and tail are conserved relative to those seen during metamorphosis in indirect-developing frogs. Additionally, <i>thra, thrb</i>, and <i>dio2</i> mRNAs are expressed in the limb before formation of the embryonic thyroid gland. Liquid-chromatography mass-spectrometry revealed that maternally derived thyroid hormone is present throughout early embryogenesis, including stages of digit formation that occur prior to the increase in embryonically produced thyroid hormone. <i>Eleutherodactylus coqui</i> embryos take up much less 3,5,3'-triiodothyronine (T₃) from the environment compared with <i>X. tropicalis</i> tadpoles. However, <i>E. coqui</i> tissue explants mount robust and direct gene expression responses to exogenous T₃ similar to those seen in metamorphosing species. The presence of key components of the thyroid axis in the limb and the ability of limb tissue to respond to T₃ suggest that thyroid hormone-mediated limb development may begin prior to thyroid gland formation. Thyroid hormone-dependent limb development and tail resorption characteristic of metamorphosis in indirect-developing anurans are evolutionarily conserved, but they occur instead <i>in ovo</i> in <i>E. coqui</i>.