Monday Organism – Axolotl/Ambystoma mexicanum

250px-axolotl

Ambystoma mexicanum is binomial for Axolotl, or “tiger salamander”. This amiable amphibian is typically found in Mexico (what is it with me and Mexican organisms?), and is widely used in scientific research since its embryos are rather transparent, making it ideal for researchers in developmental biology.

I found it a bit surprising that this creature is actually a common pet, since I’ve only stumbled upon it while reading books about biology. I have to admit, though, it’s awfully cute.

The Axolotl is a neotenic salamander, meaning: the Axolotl remains in its larval stage albeit reaching sexual maturity. Salamanders (and a great number of biological taxa) typically undergo structural and physiological metamorphosis before reaching sexual maturity, which results in post-embryonic development, often profoundly so. In some cases, metamorphosis may change the habitat of the organism, and its morphogenetic changes allow it to adapt to a different set of environmental pressures and to new tasks (the most obviously important one is sexual reproduction, an errand typically neglected by larvae).

In the case of the Axolotl, metamorphosis in the wild sometimes never occurs, although the individual larvae do reach sexual maturity. It should be noted that the neotenic individuals far outlive their metamorphosising peers – which could indicate that the neoteny in Axolotls is adaptive (I wouldn’t be so sure about that in such a hurry, though).

The domesticated Axolotl is emphatically neotenic, and spontaneous metamorphosis is extremely rare. However, it is possible to inject the Axolotl with thyroxine, the growth hormone, which will artificially induce it to metamorphose. It’s a giant kicker. You inject a growth hormone, and the larva simply grows into a species that “isn’t supposed to exist” – since the Axolotl’s neoteny has a strong genetic element.

It’s also possible to behaviorally induce metamorphosis by slowly “weaning” the larva out of the water – by lowering the water level in the axolotl’s tank while constantly spraying the air around it with water. It’s kind of a cruel experiment, but it seldom works, although often it simply ends in the poor bugger dying. My advice: no need to torment any more poor axolotl larvae.

Since all it takes for axolotls to metamorphose is thyroxine, this indicates that all the paraphernalia to metamorphose exist, but the ability to trigger metamorphosis has been hindered or stinted for some reason. Even though it’s hard to imagine why would neoteny be favorable (although the longer life-span is a very good candidate explanation) – hypothetically, at least, this has an important bearing on evolutionary biology:

Just as is the case in homeobox genes, a lot of evolution can occur thanks to a minute or trivial genetic variation or change in non-structrual genes. This to say: a great number of animals that are superficially very different are actually varieties generated by mutations occuring in “toolbox genes” (in the case of homeobox genes).

As for neoteny, the enormous superficial (and significant) differences between adult and larval Axolotls are a result of a simple degenrative mutation, which happened to be beneficial to the organism.

It just so happens that not all wild Axolotls are neotenic, and it is actually possible to directly measure the selective advantage for Axolotl’s neoteny.

Oh, and grab a pet Axolotl today, these poor fellers are endangered (again, cause of mean ol’ H. sapiens – since we’ve been screwing with its native habitat for yonks).

The world could use some cuteness.

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4 Responses to “Monday Organism – Axolotl/Ambystoma mexicanum”

  1. John Phillips, FCD Says:

    Thought I would leave you some love from a godless puppy eater in case PZ’s ‘Mary’s Monday Metazoan’ post lost you some clicks šŸ™‚

    P.S. cute salamander, love the exterior gills.

  2. Mark Says:

    Great Monday Organism post!

  3. Freidenker Says:

    Cheers!

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