Meeting Abstract
Direct development is a novel reproductive mode that has evolved independently in at least ten anuran lineages. Direct-developing frogs, including the Puerto Rican coquí, Eleutherodactylus coqui, hatch from terrestrial eggs as miniature adults. While their embryonic development resembles metamorphosis in several respects, many traits develop in a different sequence compared to those in metamorphosing frogs. For example, limb development follows thyroid gland formation and occurs well after hatching in metamorphosing frogs. In contrast, limbs in direct-developing frogs begin to form early in embryogenesis and well before the thyroid, suggesting that their development is thyroid hormone (TH) independent. TH concentrations increase in the E. coqui embryo after thyroid gland formation, although maternal THs are present in the newly fertilized zygote. Thyroid hormone receptor (TR) and deiodinase expression dynamics and the increasing TH titer suggest that the last third of E. coqui limb development is TH-dependent. To determine if all TH-signaling genes share similar temporal expression patterns between these two contrasting life histories, we sequenced the hind-limb transcriptome of E. coqui and Xenopus tropicalis at three equivalent stages. While two TH-signaling genes, TRβ and dio2, share similar expression patterns in both species, a third gene, TRα, does not. Expression of TRα increases over time in X. tropicalis, whereas it decreases after thyroid gland formation in E. coqui. Our data are consistent with TH-mediated embryonic limb development in E. coqui, but the initial source of TH may be maternal rather than embryonic.
