Effects of Thyroid Hormone Receptor α Gene Knockout on Hindlimb and Intestinal Development in Pre‐Metamorphic <i>Xenopus tropicalis</i> Larvae

During anuran metamorphosis, rapid and extensive morphological transformations occur throughout the body, and these changes are triggered by the thyroid hormone. The thyroid hormone receptor (TR) is a nuclear receptor that is present in all vertebrates. TR binds specific DNA sequences to repress target genes in the absence of ligand and to activate them when ligand-bound. This dual regulatory function has been proposed to underlie the rapid pace of anuran metamorphosis. One TR subtype, TRα, suppresses hindlimb (HL) development in pre-metamorphic Xenopus tropicalis. However, the genes repressed by TRα remain unidentified, and it is unclear whether TRα-mediated developmental suppression occurs in specific organs. This study aimed to identify HL genes regulated by TRα during developmental suppression and to determine whether this suppression is widespread in the pre-metamorphic tadpole. We analyzed temporal changes in morphology and gene expression in the HL buds and intestines of TRα-knockout (KO) pre-metamorphic X. tropicalis tadpoles. HL buds appeared earlier in KO tadpoles than in the wild type. Whole-mount in situ hybridization showed that the interval from fertilization to initial expression of shh was shorter in KO HLs. However, the expression pattern in HL buds with comparable morphology was essentially identical between genotypes. On the other hand, there was no significant acceleration in the growth of the intestine or body (from snout to vent) of the KO tadpoles. Our findings suggest that unliganded TRα delays the onset of HL development prior to metamorphosis.