Candidate gene analysis of thyroid hormone receptors in metamorphosing vs.nonmetamorphosing salamanders

We used two different experimental approaches to test the hypothesis that t hyroid hormone receptor (TR) variation is associated with alternate life cy cles modes in ambystomatid salamanders. In the first experiment, the inheri tance of TR alpha and TR beta genotypes was determined for metamorphic and non metamorphic offspring from backcrosses between Ambystoma mexicanum (an obligate metamorphic-failure species) and metamorphic F-1 hybrids (A. mexic anum x A. tigrinum tigrinum). The segregation of TR genotype was independen t of the expression of life cycle mode phenotype, and neither TR locus was linked to DNA markers that flank a major-effect locus for life cycle mode. In the second experiment, a portion of the ligand-binding domain of TR alph a and TR beta was cloned and sequenced for DNA samples from 14 different am bystomatid salamander populations, including obligate metamorphic, facultat ive metamorphic, and obligate metamorphic-failure taxa. Nucleotide sequence variation was found for both TR alpha and TR beta, with several nonsynonom ous substitutions that presumably code for nonconservative amino acid repla cements. However, no general relationship was found between TR allelic vari ation and life cycle mode among populations or species. These data do not i mplicate TRs as candidate loci involved in the current maintenance or past evolution of alternate life cycle modes in members of the tiger salamander complex.