Axolotl’s Neoteny

Axolotls exhibit a property called neoteny, meaning that they reach sexual maturity without undergoing metamorphosis. Many species within the Axolotl’s genus are either entirely neotenic or have neotenic populations. In the axolotl, metamorphic failure is caused by a lack of thyroid stimulating hormone, which is used to induce the thyroid to produce thyroxine in transforming salamanders. The genes responsible for neoteny in laboratory animals may have been identified, however they are not linked in wild populations, suggesting artificial selection is the cause of complete neoteny in laboratory and pet axolotls.

Unlike some other neotenic salamanders (Sirens and Necturus), Axolotls can be induced to metamorphose by an injection of iodine (used in the production of thyroid hormones) or by shots of thyroxine hormone. Another method for inducing transformation, though one that is very rarely successful, involves removing an axolotl in good condition to a shallow tank in a vivarium and slowly reducing the water level so that the axolotl has difficulty submerging. It will then, over a period of weeks, slowly metamorphose into an adult salamander. During transformation, the air in the vivarium must remain moist, and the maturing axolotl sprayed with a fine mist of pure water. The odds of the animal being able to metamorphose via this method are extremely small, and most attempts at inducing metamorphosis lead to death. This is likely due to the strong genetic basis for neoteny in laboratory and pet axolotls, which means that few captive animals have the ability to metamorphose on their own. Spontaneous metamorphosis has been known to occur very rarely, but attempts to do so artificially are best left to trained scientists. Artificial metamorphosis also dramatically shortens the axolotl’s lifespan, if they survive the process. A neotenic axolotl will live an average of 10–15 years (though an individual in Paris is credited with achieving 25 years), while a metamorphosed specimen will scarcely live past the age of five. The adult form resembles a terrestrial Mexican Tiger Salamander, but has several differences, such as longer toes, which support its status as a separate species.

Use as a model organism

Six adult axolotls (including a leucistic specimen) were shipped from Mexico City to the “Jardin des Plantes” in Paris in 1863. Unaware of their neoteny, Auguste Duméril was surprised when, instead of the axolotl, he found in the vivarium a new species, similar to the salamander. This discovery was the starting point of research about neoteny. It is not certain that Mexican Tiger salamanders were not included in the original shipment. The surviving neotenes are thought to be the ancestors of the majority of pet and laboratory axolotls in existence.

Vilem Laufberger of Germany used thyroid hormone injections to induce an axolotl to grow into a terrestrial adult salamander. The experiment was repeated by the Englishman Julian Huxley, who was unaware the experiment had already been done, using ground thyroid hormones. Since then, experiments have been done often with injections of iodine or various thyroid hormones used to induce metamorphosis.

Today, the axolotl is still used in research as a model organism and large numbers are bred in captivity. Axolotls are especially easy to breed compared to other salamanders in their family, which are almost never captive bred due to the demands of terrestrial life. One attractive feature for research is the large and easily manipulated embryo, which allows viewing of the full development of a vertebrate. Axolotls are used in heart defect studies due to the presence of a mutant gene that causes heart failure in embryos. Since the embryos survive almost to hatching with no heart function, the defect is very observable. The presence of several color morphs has also been extensively studied.

The feature of the salamander that attracts most attention is its healing ability: the axolotl does not heal by scarring and is capable of the regeneration of entire lost appendages in a period of months, and, in certain cases, more vital structures. Some have indeed been found restoring the less vital parts of their brains. They can also readily accept transplants from other individuals, including eyes and parts of the brain — restoring these alien organs to full functionality. In some cases, axolotls have been known to repair a damaged limb as well as regenerating an additional one, ending up with an extra appendage that makes them attractive to pet owners as a novelty. In metamorphosed individuals, however, the ability to regenerate is greatly diminished. The axolotl is therefore used as a model for the development of limbs in vertebrates.