Endocrine Mechanisms Underlying Phenotypic Evolution in Frogs

Evolutionary developmental biology has united the theory of developmental basis of phenotypic variation and theory of natural selection to explain phenotypic evolution. Developmental plasticity, a developmental response to environmental stimulus is a universal property of organisms and thus is hypothesized to play a main role in fostering phenotypic diversity. Selection for the adaptive phenotype is expected to alter the underlying pre-existing developmental pathways to initiate phenotypic evolution. Here I attempted to understand phenotypic evolution in frogs by examining the underlying developmental regulatory pathways in a phylogenetic context. Specifically, in the first part of thesis, I examined the role of plasticity in adaptive evolution of accelerated development in New World spadefoot toads in response to aridification process. Here I show that plastic acceleration of development in response to artificial desiccation is present in spadefoot toad ancestors (species resembling ancestral state) and is associated with increased expression of two main hormones regulating metamorphosis, thyroid (TH) and corticosterone (CORT). The evolution of shorter larval periods in New World species surviving the ephemeral environment is associated with constitutive up-regulation of TH and CORT levels (higher mean values) and reduced developmental plasticity. Our results provide the evidence that endocrine mechanisms underlying plasticity in ancestral state has been co-opted to facilitate adaptive divergence in larval periods in New World spadefoot toads. In the third chapter of the thesis, I examined the effect of successive stressors on the stress response ability of tadpoles. Specifically, I tested if experiencing nutritional stress during early development in tadpoles affects their ability to respond to a subsequent water reduction stress later during metamorphosis. I show that an initial stressor attenuates an animal's ability to mount an appropriate stress response to a subsequent stressor. Thus, experiencing stressors consecutively may be more harmful than expected based on studies on isolated stressors. CORT is the primary stress hormone in tadpoles and its actions on development are complex and dependent on the presence of TH, however, the molecular basis of CORT and TH cross-talk is still not well understood. Therefore, comprehensive knowledge of genes regulated by CORT and TH during metamorphosis would significantly contribute to our understanding of their metamorphic actions. My microarray study (fourth chapter) on hormone-treated tails identified genes regulated by CORT independently as well as expected and unexpected interactions with TH in tails of X. tropicalis. My data is the first to find CORT-response genes in tadpoles using microarray analysis and show novel interactions between TH and CORT on gene expression. In the last chapter of the thesis, I examined the role of hypothalamic regulation of development in the direct developing frog Eleutherodactylus coqui to gain insights into the evolution of direct developing strategy achieved by deleting the free swimming larval phase from ancestral biphasic developmental mode. My results show that the early post-embryonic development in both biphasic and direct developing frogs is regulated by the same hypothalamic factor, corticotropin releasing factor (CRF) and is conserved during evolution. My results further extended the idea that direct developing frogs though not undergoing as dramatic metamorphosis as tadpoles do, they still are dependent on the thyroid hormone and its regulation by CRF for their post-embryonic development and undergo cryptic metamorphosis inside eggs.