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

We used two different experimental approaches to test the hypothesis that thyroid hormone receptor (TR) variation is associated with alternate life cycles modes in ambystomatid salamanders. In the first experiment, the inheritance of TRα and TRβ genotypes was determined for metamorphic and non metamorphic offspring from backcrosses between Ambystoma mexicanum (an obligate metamorphic-failure species) and metamorphic F₁ hybrids (A. mexicanum x A. tigrinum tigrinum). The segregation of TR genotype was independent 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 TRα and TRβ was cloned and sequenced for DNA samples from 14 different ambystomatid salamander populations, including obligate metamorphic, facultative metamorphic, and obligate metamorphic-failure taxa. Nucleotide sequence variation was found for both TRα and TRβ, with several nonsynonomous substitutions that presumably code for nonconservative amino acid replacements. However, no general relationship was found between TR allelic variation and life cycle mode among populations or species. These data do not implicate TRs as candidate loci involved in the current maintenance or past evolution of alternate life cycle modes in members of the tiger salamander complex.