Thyroid hormones and goitrogen-induced metamorphosis in the sea lamprey (Petromyzon marinus)

grantor: University of TorontoThe larval sea lamprey undergoes a true metamorphosis from a sedentary, filter-feeding larva to a free-swimming, parasitic juvenile that feeds on the blood and body fluids of teleost fishes. As is the case with amphibians and teleosts, thyroid hormones (TH) are believed to be involved in lamprey metamorphosis. However, the onset of lamprey metamorphosis is characterized by a sharp decline in serum TH titers rather than the typical increase observed in all other vertebrates known to undergo this type of postembryonic developmental event. Furthermore, the goitrogen potassium perchlorate (KClO4), which lowers serum TH titers and prevents metamorphosis in most vertebrates, can induce precocious metamorphosis in larval lampreys. The primary goals of this thesis were to elucidate the role of depressed serum TH titers in the goitrogen-induced metamorphosis of lampreys and to provide some insight into the mechanisms involved in this induction. I have shown that KClO 4-induced lamprey metamorphosis can be blocked by using exogenous TH to prevent the decline in serum TH titers associated with induced metamorphosis. The ability to induce metamorphosis was not unique to KClO4. The goitrogens sodium perchlorate, potassium thiocyanate and methimazole also induced metamorphosis in larval sea lampreys, and the incidence of metamorphosis was positively correlated to the magnitude of the decline in serum TH concentrations. These data are consistent with the proposed ideas that either a decline in serum TH concentrations is essential for the onset of metamorphosis in lampreys, or elevated serum TH concentrations are inhibitory to metamorphosis (i.e., high TH titers have an anti-metamorphic effect in lampreys). In addition, I have shown that KClO4 administered to larval lamprey endostyles ' in vitro' reduces iodide uptake and organification, as well as total endostylar thyroglobulin. The fact that the goitrogen KClO4 acts directly on the endostyle to inhibit thyroidal activity further supports the notion that goitrogens induce metamorphosis via their effects on TH synthesis. In summary, I have shown that goitrogens act directly on the larval lamprey endostyle to inhibit thyroidal activity, thus resulting in a decline in serum TH titers that is linked to and essential for the onset of metamorphosis in lampreys.Ph.D