AUTHOR=Laslo Mara , Denver Robert J. , Hanken James 

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

JOURNAL=Frontiers in Endocrinology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2019.00307

DOI=10.3389/fendo.2019.00307

ISSN=1664-2392

ABSTRACT=<p>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, <italic>Eleutherodactylus coqui</italic>, 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 (<italic>thra</italic>) and beta (<italic>thrb</italic>) and deiodinases type II (<italic>dio2</italic>) and type III (<italic>dio3</italic>) mRNAs in <italic>E. coqui</italic> limb and tail are conserved relative to those seen during metamorphosis in indirect-developing frogs. Additionally, <italic>thra, thrb</italic>, and <italic>dio2</italic> 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. <italic>Eleutherodactylus coqui</italic> embryos take up much less 3,5,3′-triiodothyronine (T<sub>3</sub>) from the environment compared with <italic>X. tropicalis</italic> tadpoles. However, <italic>E. coqui</italic> tissue explants mount robust and direct gene expression responses to exogenous T<sub>3</sub> 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 T<sub>3</sub> 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 <italic>in ovo</italic> in <italic>E. coqui</italic>.</p>