Genetic Basis of Paedomorphosis in the Axolotl, Ambystoma mexicanum: A Test of the Single-Gene Hypothesis

A single gene is thought to control paedomorphosis in the axolotl, Ambystoma mexicanum. However, recent studies of related ambystomatids that show a facultative life-history response indicate a polygenic basis affected by local environment. Expression of life-history pathway was examined among a backcross family of hybrid ambystomatids. Ambystoma mexicanum was crossed to an obligately metamorphic species, A. tigrinum tigrinum and two F1 hybrids from this cross were backcrossed to the paedomorphic A. mexicanum lineage. First- and second-generation offspring were reared in isolation under controlled environmental conditions. All first-generation offspring metamorphosed, but for second-generation off spring, non-Mendelian ratios of metamorphs and paedomorphs were observed within environmental treatments. In contrast, expected Mendelian ratios were observed within treatment groups for color phenotypes that are known to be controlled by single-gene loci. These data do not support the hypothesis of a single-gene difference underlying life-history pathway differences between paedomorphlc versus metamorphic species of ambystomatids.