THYROID-HORMONE RECEPTOR GENES OF NEOTENIC AMPHIBIANS

Since thyroid hormones play a pivotal role in amphibian metamorphosis we used PCR to amplify DNA fragments corresponding to a portion of the ligand-binding domain of the thyroid hormone receptor (TR) genes in s everal neotenic amphibians: the obligatory neotenic members of the fam ily Proteidea the mudpuppy Necturus maculosus and Proteus anguinus as well as two members of the facultative neotenic Ambystoma genus: the a xolotl Ambystoma mexicanum and the tiger salamander Ambystoma tigrinum . In addition, we looked for TR genes in the genome of an apode Typhlo nectes compressicaudus. TR genes were found in all these species inclu ding the obligatory neotenic ones. The PCR fragments obtained encompas s both the C and E domains and correspond to or and beta genes. Their sequences appear to be normal, suggesting that there is no acceleratio n of evolutionary rates in the TR genes of neotenic amphibians. This r esult is not surprising for Ambystomatidae, which are known to respond to T3 (3,3',5-triiodothyronine) but is not in agreement with biochemi cal and biological data showing that Proteidea cannot respond to thyro id hormones. Interestingly, by RT-PCR analysis we observed a high expr ession levels of TR alpha in gills, intestine, and muscles of Necturus as well as in the liver of Ambystoma mexicanum, whereas TR beta expre ssion was only detected in Ambystoma mexicanum but not in Necturus. Su ch a differential expression pattern of TR alpha and TR beta may expla in the neoteny in Proteidea. The cloning of thyroid-hormone-receptor g ene fragments from these species will allow the molecular study of the ir failure to undergo metamorphosis.