Hormones influence the development of every organ in the body and may underlie the evolution of biodiversity. In particular, thyroid hormone (TH) plays critical roles in brain and skeletal development, yet the role TH plays in development varies widely across vertebrate species. Indeed, evolutionary changes in TH physiology have been implicated in the enormous diversity found in frog and salamander life history. How such an important hormone can change its developmental role in evolution is not known. To more fully understand the role of TH in evolution, this project will examine the role of altered expression of TH receptors (TRs) underlying the evolution of accelerated metamorphosis in desert frogs. Frog metamorphosis is dependent on TH, and desert frog species have shorter larval periods compared to their non-desert relatives. Based on our preliminary data, it is hypothesized that desert species will have a more robust response to TH compared to non-desert species underlying their shorter larval period. Successful completion of this project will functionally test the effect of an evolutionary increase in TR expression on the rate of metamorphosis. The rationale for this research is that knowledge of how altered TR expression affects metamorphosis between frog species will advance our understanding of the developmental and evolutionary roles of this powerful hormone system. Integration of this research project with education will be accomplished by involving undergraduate and graduate students, not only in the research project, but also by enabling presentations of successful research at conferences. The current project enhances infrastructure for research and education through international collaborations and by providing educational material for undergraduate teaching. Results of the proposed research will be disseminated widely by publications in peer-reviewed journals and presentations at professional meetings, and all material used in the project will be deposited in research collections available for the scientific community. Finally, the research is expected to benefit society as a whole, primarily through bringing awareness to the general public of the underpinnings of evolution and biodiversity.
." The specific outcomes of the researsh were 1) desert frog species with short larval period durations have higher levels of thyroid hormone receptor expression and 2) consistent with this finding, experimentally increased levels of thyroid hormone receptor caused an increased rate of morphological changed induced by thyroid hormone. To put these findings in context, thyroid hormone is essential for proper timing and morphological development across vertebrates, yet there are many examples of substantial amounts of evolutionary change in the roles of thyroid hormone in development across species. Discovering how thyroid hormone can be so influential to vertebrate development and yet be so labile in evolution is critical for understanding the developmental basis of vertebrate evolution. As mediators of the thyroid hormone signal, thyroid hormone receptors represent a likely target for evolutionary change in hormone-dependent development. The funded research is significant because such a mechanism (altered TR expression levels) explains, at least in part, larval period diversity among frog species where larval period duration ranges from eight days to three years. Further, understanding the role of thyroid hormone receptor in frog development and evolution is expected to provide insights into how this hormonal system may underlie diversity in other vertebrate groups, because all vertebrates share many hormone-dependent developmental processes that influence phenotypic differences between species. In short, knowledge of how thyroid hormone physiology underlies metamorphosis in frogs will help elucidate questions in vertebrate evolution. The broader impacts of this grant relate to STEM discipline education for graduate and undergraduate students and knowledge dissemination. Three graduate students have been employed and research supported by this grant, two of whom have graduated. In addition, three undergraduate students have received research training through this grant. Three articles published in scientific journals and four manuscripts submitted or in preparation have been supported by this research funding. All published work resulting from this grant has graduate student co-authors and some have undergraduate co-authors. The published articles are available at the University of Cincinnati web site.
