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Title: 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 (Eleutherodactylus 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, Eleutherodactylus coqui. In Chapter 1, I find that developmental dynamics of TH and thyroid hormone receptor mRNAs are conserved relative to metamorphosing frogs. Additionally, I measure maternal TH in unfertilized oocytes and early E. coqui embryos prior to development of the embryonic thyroid gland. Altogether, these data suggest that limb development and tail resorption are mediated by conserved components of the HPT axis, and that maternal TH could facilitate limb development prior to embryonic thyroid gland formation. In Chapter 2, I find that CORT secretion increases at hatching and that CORT and TH treatment induces TH signaling gene expression in the tail, suggesting 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 TH-signaling genes share the same expression pattern in the hind limbs of both species. Altogether, these data suggest that several key features of endocrine control of tail resorption and limb development are evolutionarily conserved in the direct-developing frog E. coqui.
Committee: Jim Hanken (Advisor), Robert Denver (U. Michigan), Cassandra Extavour, David Haig