Growing up you believe the world is meant to be the way it is. The older I get the more I realise that this is not the case. The world is full of injustices and atrocities that governments and the voice of faith expect us to accept, though with each passing year they grow fewer and fewer, at least one would hope. I have created this blog as a space for me to rant about all things science, politics, philosophy and religion, before it’s too late and the vessel of new atheism propelled by a growing surge in secularism solves all of the world’s problems for good.

Saturday, 30 April 2011

The salamander that got stuck...

Axolotl (Ambystoma mexicanum)

I can remember sitting in year 11 biology, way back when, learning about metamorphosis in amphibians. We had a tank in the classroom that had some rather sizable axolotls in it, and I can remember having my mind blown when I was informed that they were effectively stuck half way through metamorphosis. They had limbs like an adult frog or salamander; however, they still had gills much like tadpoles did. So it seemed to me like they were caught in limbo. 

Someone had said that the reason they still had gills was because they lived in an environment, a tank with water filled to the top, from which they couldn’t escape in order to metamorphose. It seemed to make sense. If there were no air to breathe, why would you want to acquire lungs? My teacher suggested that they could undergo metamorphosis if you slowly reduced the amount of water in the tank, though I was left wanting when I asked how exactly the axolotl controlled this pausal state, she didn’t know.

Tonight I found myself asking the same question again, so I looked the answer up!

Metamorphosis in amphibians is controlled by the hormones thyroxin, which is a thyroid hormone that induces metamorphosis, and prolactin, which counteracts the effects of thyroxin. Specific events trigger the release of thyroxin, which starts off the process of metamorphosis. The organs that become redundant, such as the gills, tail (in frogs) or ridges for teeth, are resorbed by the body via apoptosis (controlled cell death).   

Life cycles of frogs and salamanders

Toy Dog
When an animal’s development is slowed or delayed, or paused completely as is the case with axolotls, this process is called neoteny or “juvenilization”. Other animals that endure a neotenous state are toy dogs. Yep, those irritating, small and yappy rat look-alikes appear that way due to artificial selection on our part that has selected for the inactivation of the genes that would continue the dog’s development into an adult. Thus they resemble infant wolves. Other examples include flightless birds and even us humans. Neotenous traits we express are things such as sparse body hair, enlarged heads (think baby primates) and lactose tolerance in adults. In all cases, we still have the genes within us, the "wolf man" is an example of a human with a full coat of hair, and there are plenty of people out there with lactose intolerance, who can no longer digest foods made from or with lactose (milk).

Tiger Salamander
Anyway, back to the axolotls. These guys are a form of neotenic mole salamander and part of the Tiger Salamander complex. What’s interesting is that wild axolotls tend to go through metamorphosis like normal salamanders, though they can have some neotenic populations. It was only after artificial selection that laboratory or pet axolotls expressed permanent neoteny. So in other words, we chose only those individuals who didn’t produce thyroxin, which would’ve caused them to undergo metamophosis, to reproduce and removed the rest. Homozygosity of a single recessive gene was found to be the cause of neoteny in axolotls in a study by Tomkins.

However, axolotls can be induced to undergo metamorphosis if injected with iodine, which is used for producing thyroid hormones, or by injecting the specific hormone itself, thyroxin, into the animal (a study that looked at this). Wikipedia also mentions the ability to induce metamorphosis by reducing the level of water in a tank slowly over time. However, most times the animals will die and those that survive will have a reduced lifespan of 5 years compared to the usual 10-15 years. I might add though that this had no references, but as independent anecdotal evidence supports the idea somewhat. 

Another interesting side note is that numerous salamander species are reported to have an aquatic phase in spring and summer and a terrestrial phase in autumn and winter. 

For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
It was also discovered that this prolactin-thyroxin interaction (synergism) is how salamanders have such amazing tissue regenerating abilities. They are able to regenerate limbs and even eyes that have been amputated/removed. I could go on forever about every aspect of this extraordinary creatures biology, but I won't.

Conclusions: Axolotls were selected to become neotenic, so if you want a terrestrial salamander don't get an axolotl and torture it to become one through water deprivation or injections with his only reward being the prospect of a severely shortened life. Buy a different species...

1 comment:

  1. The reason humans are neotenous, of course, is because immature animals tend to have proportionately larger heads, so neoteny is a quick and dirty way of increasing brain size with only a minor change in gene. Bottlenose dolphins are also neotenous, and the fact that our physical form has been arrested in adolescence, relative to other apes or toothed whales, explains why both humans and dolphins are so highly-sexed. It's also why domestic cats and dogs - who tend to have been bred for relatively "baby" faces, soft coats, infantile dependency etc - come on heat far more often than their wild counterparts.