A CORRELATION OF THYROID-HORMONE RECEPTOR GENE-EXPRESSION WITH AMPHIBIAN METAMORPHOSIS

The expression of the thyroid hormone (TH) receptor genes α (TRa) and β(TRβ) in Xenopus laevis begins after the embryo hatches. The TRα mRNA increases throughout the premetamorphosis stage of tadpole development, is maximal by prometamorphosis, and falls after climax of metamorphosis to a lower level in frogs. The TRβ mRNA is barely detectable during premetamorphosis. In synchrony with the onset of endogenous TH synthesis by the thyroid gland (prometamorphosis), the level of TRβ mRNA rises in parallel with endogenous TH, reaching a peak at the climax of metamorphosis (stage 61) and drops to ∼10% of its peak level after metamorphosis. As suggested by this correlation, exogenous TH up-regulates TRβ mRNA as much as 20-fold during premetamorphosis, whereas TH up-regulates TRa mRNA by ∼ 2-fold during the same period. Up-regulation of TRβ mRNA is the earliest response to exogenous TH by competent tadpoles yet detected.