INFANTE, C.R.; Harvard University: Deiodinases and the control of metamorphosis in the carnivorous larvae of the anuran Lepidobatrachus laevis

Anuran metamorphosis is one of the most extreme processes in vertebrate development. Typically it transforms a microphagous, herbivorous, aquatic larva into a macrophagous, terrestrial adult. This process is dependent on thyroid hormone (TH), which initiates – via nuclear receptors – a cascade of gene expression in responding tissues. It has been hypothesized that changes in the thyroid hormone-regulated metamorphic program mediate the evolutionary diversification of larval and adult morphology. This could be accomplished via temporal changes in tissue sensitivity and responsiveness to thyroid hormone on a tissue or organ-specific level. Recent studies point to the type II and type III deiodinase enzymes as potential players in the tissue-specific control of the metamorphic response. To investigate the role of these enzymes in the evolution of larval morphology, I focus on the extreme jaw morphology exhibited by the megalophagous carnivorous larvae of the South American frog Lepidobatrachus laevis. These tadpoles possess massive jaws and an enlarged head that allow them to consume prey, including other tadpoles, whole. These features, as well as a functional stomach and adult-like gut, mean the larvae of L. laevis attain adult characteristics while still in the larval state. At metamorphosis these structures are less extensively remodeling relative to the typical anuran tadpole. Of interest is the role of the local manipulation of active TH by the deiodinases on the metamorphic response in these �adult� tissues. To address this question I use an analysis of gene expression and enzyme activity during metamorphosis in whole tadpoles as well as on jaws grown in tissue culture.