Meeting Abstract
Direct development is a reproductive mode in anurans that has evolved independently at least a dozen times. Direct-developing frogs, including the common coquí, Eleutherodactylus coqui, hatch from terrestrial eggs as miniature adults. While their embryonic development resembles metamorphosis in several respects, characters develop in a different sequence compared to metamorphosing frogs. In metamorphosing frogs, for example, limb growth and tail resorption both occur following thyroid gland formation. In contrast, limbs in direct-developing frogs develop before the thyroid. This suggests that limb development is either thyroid hormone (TH) independent or mediated by maternally derived TH provisioned in the egg before fertilization. Changes in thyroid hormone provisioning, metabolism, or action may thus underlie the evolution of direct development. Specifically, maternally derived TH and changes in temporal or spatial expression of the nuclear thyroid receptor (TR) α, TRβ, deiodinase type II, or deiodinase type III in the target tissue could facilitate tail resorption and early development of limbs. Expression of TR in E. coqui, suggests that tail resorption is mediated by TH. Similarly, TR expression dynamics in the limb approximate those in the developing limb of the metamorphosing frog Xenopus laevis, which is dependent on TH. Liquid-chromatography mass-spectrometry indicates that maternally derived TH is present in E. coqui at the onset of limb development. These data suggest that the E. coqui limb is at least TH competent and that thyroid-mediated embryonic development may begin much earlier than previously thought. Maternally provisioned TH may confer developmental flexibility that facilitates the evolution of direct development.
