Thyroid hormone receptors in perennibranchiate amphibians

Thyroid hormone has long been known to induce metamorphosis in amphibians. The understanding of the molecular steps controlling the completion of metamorphosis has nevertheless been hampered by the complexity of this event. The comparison of organisms in which metamorphosis does or does not occur, may provide clues into the molecular cascade that control it. Up to now the available data suggest that perennibranchiate amphibians retain their larval characters mainly because their tissues do not respond to thyroid hormones. In such a context the recent identification of a thyroid hormone receptor alpha in the perennibranchiate Proteus anguinus is provocative (Ho Huynh et al., Int. J. Dev. Biol. 40:537-543, 1996). In the present paper, we provide evidences that this recently described sequence is in fact a sequence from Xenopus laevis. Indeed, we identified the authentic thyroid hormone receptors of both alpha and beta types in two perennibranchiate species Necturus maculosus and Proteus anguinus. The various controls required to ascertain the authenticity of a developmental gene cloned by PCR or RT-PCR analysis are presented. The results reported in the present paper are relevant with phylogenetical analysis. This induces our team to conclude that the Proteus TR alpha sequence reported by Ho Huynh et al. (1996) reflects a contamination of the RT-PCR by Xenopus laevis material.