Genie Control of Axolotl Metamorphosis

Abstract

SYNOPSIS. Neoteny in the Mexican axolotl, Ambystoma mexicanum, is caused by homozygosity for a single recessive gene. The dominant allele causing physical metamorphosis is found in the closely related species, Ambystoma tigrinum, with which it can hybridize. Despite the failure of axolotls to undergo physical metamorphosis, they do undergo a cryptic metamorphosis. A larval-to-adult hemoglobin form change, serum protein changes and other physiological events usually associated with amphibian metamorphosis occur during early larval life at ages comparable to the age at which Ambystoma tigrinum undergoes both the cryptic and external metamorphic events. Axolotl cryptic metamorphosis can be induced precociously by immersion of the larvae in low concentrations of thyroxine; physical metamorphosis can be induced with higher thyroxine concentrations. The site of action of the gene responsible for axolotl neoteny has not been identified. A change in the sensitivity of external metamorphic processes to thyroxine, or reduced hormonal stimulation by the pituitary or hypothalamus may be responsible. A comparison of these functions in Ambystoma tigrinum and the axolotl may identify the lesion.