Evolutionary Conservation of Endocrine-Mediated Development in the Direct-Developing Frog, Eleutherodactylus coqui

Abstract

Direct development, a life history mode wherein the free-living larval stage is bypassed, has independently evolved multiple times in amphibians. Direct-developing frogs, such as the Puerto Rican coquí (Eleutherdactylus coqui), hatch from terrestrial eggs as miniature adults. Although they lack both an aquatic tadpole and post-hatching metamorphosis, E. coqui undergoes morphological changes in the egg that mimic metamorphosis: adult features, such as limbs, form and grow while larval-specific features, such as gills and tail, resorb. In metamorphosing frogs, the timing of these changes is dependent on endocrine signaling. In particular, two hormones, thyroid hormone (TH) and corticosterone (CORT), have been shown to be important in regulating developmental timing in amphibians. In this dissertation, I investigate the degree of evolutionary conservation of TH and CORT signaling components in the direct-developing frog, Eleutherdactylus coqui. In Chapter 1, I find that dynamics of whole body TH content and thyroid hormone receptor mRNAs throughout development are conserved relative to metamorphosing frogs. Additionally, I measured TH in unfertilized oocytes and early E. coqui embryos. Thyroid hormones detected prior to development of the embryonic thyroid gland are likely maternal in origin. Altogether, these data suggest that limb development and tail resorption are mediated by conserved components of the hypothalamus-pituitary-thyroid axis, and that maternal TH could facilitate limb development prior to embryonic thyroid gland formation. In Chapter 2, I find that whole body CORT content increases at hatching and that CORT and TH treatment induces changes in TH-response gene expression in the tail. These data suggest that CORT and TH together promote rapid tail resorption after hatching through increased TH signaling. In Chapter 3, I use an RNA-seq approach to compare limb development modules and TH-signaling genes in the hind limb of E. coqui and the metamorphosing frog Xenopus tropicalis. One third of all orthologous genes and one third of TH-signaling genes have the same expression pattern throughout development in the hind limbs of both species. Altogether, these data suggest that several key features of hormonal control of tail resorption and limb development are evolutionarily conserved in the direct-developing frog E. coqui.