FIG. 1.

Transgenic expression of dnTR inhibits T₃-induced morphological changes. Wild-type tadpoles (wt) or transgenic tadpoles expressing the dnTR under control of the CMV promoter (Tg) were treated at stage 54 with 0 or 5 nM T₃ for 3 days and then examined for changes in external morphology. Transgene expression was confirmed by observation of fluorescence in the nares from the GFP moiety at the N terminus of the dnTR fusion protein. Note that regression of gills (brackets) and proliferation of Meckel's cartilage (arrows) were induced after 3 days in wild-type tadpoles (compare panels A and B), whereas such changes did not occur in the T₃-treated transgenic animals (C). Similarly, hind limb growth and morphogenesis were induced by T₃ treatment in the wild type (compare panels D and E) but not in transgenic animals (F). In the absence of T₃, wild-type and transgenic animals are expected to be the same because endogenous TR (TRα is expressed during premetamorphosis) and dnTR function similarly without ligand, and they were indeed found to be the same during premetamorphosis (not shown). Thus, transgenic animals without T₃ treatment were not used in this and other experiments. All experiments in Fig. 1 to 6 were repeated at least three times with similar results. Bars: 2 mm (A to C) and 1 mm (D to F).