Editing Axolotl (section)

==== Neoteny ====
{{Main|Neoteny}}
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{{Side box|metadata=no||text='''Role of iodine'''
In animals with functioning thyroid glands, iodine in the form of iodide is selectively gathered into the colloid of the thyroid. Inside the colloid, iodide is reduced to elemental iodine (I<sub>2</sub>), which reacts with the [[tyrosyl]] residues of [[thyroglobulin]]. Two iodinated tyrosyl residues are conjugated together. When they are cleaved from the thyroglobulin chain, thyroid hormone is obtained.<ref name=boron2012>Chapter 49, "Synthesis of Thyroid Hormones" in: {{cite book |author1=Walter F. Boron |author2=Emile L. Boulpaep |title=Medical Physiology |edition= 2nd|publisher=Elsevier/Saunders |year=2012 |isbn=9781437717532}}{{page needed|date=April 2016}}</ref>

[[Diiodotyrosine]], an analogue of the iodinated [[thyroglobulin]] precursor in thyroxine biosynthesis, causes metamorphosis in axolotls that have their thyroids removed.<ref>{{cite journal |last1=Swingle |first1=W. W. |title=Iodine and Amphibian Metamorphosis |journal=The Biological Bulletin |date=November 1923 |volume=45 |issue=5 |pages=229–253 |doi=10.2307/1536749|jstor=1536749 }}</ref> [[Lugol's solution]], which contains both iodide and I<sub>2</sub>, triggers metamorphosis when injected.<ref>{{cite journal |last1=Ingram |first1=W. R. |title=Metamorphosis of the Colorado Axolotl by Injection of Inorganic Iodine. |journal=Experimental Biology and Medicine |date=1 December 1928 |volume=26 |issue=3 |pages=191 |doi=10.3181/00379727-26-4212}}</ref> This is because diiodotyrosine and thyroxine is produced when I<sub>2</sub> reacts with proteins other than thyroglobulin. If given in a bath instead of injected, I<sub>2</sub> has no effect on axolotls.<ref>{{cite journal |last1=Dvoskin |first1=Samuel |title=The Thyroxine-Like Action of Elemental Iodine in the Rat and Chick1 |journal=Endocrinology |date=May 1947 |volume=40 |issue=5 |pages=334–352 |doi=10.1210/endo-40-5-334|pmid=20245954 }}</ref> [[Iodide]], which does not react with proteins, does not trigger metamorphosis. It does speed up the rate of metamorphosis, once it has been triggered by thyroid hormone extract.<ref>{{cite journal |last1=Krylov |first1=O. A. |title=The role of haloids (bromine and iodine) in the metamorphosis of amphibia |journal=Bulletin of Experimental Biology and Medicine |date=January 1961 |volume=50 |issue=1 |pages=724–727 |doi=10.1007/BF00796048}}</ref>
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Most amphibians begin their lives as [[aquatic animal]]s which are unable to live on dry land, often being dubbed as [[tadpole]]s. To reach [[adult]]hood, they go through a process called [[metamorphosis (biology)|metamorphosis]], in which they lose their gills and start living on land. The axolotl is unusual in that it has a lack of [[thyroid-stimulating hormone]], which is needed for the [[thyroid]] to produce [[thyroxine]] in order for the axolotl to go through metamorphosis; it keeps its gills and lives in water all its life, even after it becomes an adult and is [[sexual maturity|able to reproduce]]. Neoteny is the term for reaching sexual maturity without undergoing metamorphosis.<ref name="ley196802">{{Cite magazine |last=Ley
 |first=Willy |date=February 1968 |title=Epitaph for a Lonely Olm |department=For Your Information|url=https://archive.org/stream/Galaxy_v26n03_1968-02_modified#page/n37/mode/2up |magazine=Galaxy Science Fiction |pages=95–104}}</ref>

The genes responsible for neoteny in laboratory axolotls may have been identified; they are not linked to the genes of wild populations, suggesting [[artificial selection]] is the cause of complete neoteny in laboratory and pet axolotls.<ref name=":3">{{Cite journal|last=Malacinski |first=George M.|date=1978-05-01|title=The Mexican Axolotl, ''Ambystoma mexicanum'': Its Biology and Developmental Genetics, and Its Autonomous Cell-lethal Genes|journal=American Zoologist|volume=18|issue=2|pages=195–206|doi=10.1093/icb/18.2.195 |doi-access=free}}</ref> The genes responsible have been narrowed down to a small chromosomal region called ''met1'', which contains several candidate genes.<ref name=Crowner>{{cite journal |last1=Crowner |first1=Anne |last2=Khatri |first2=Shivam |last3=Blichmann |first3=Dana |last4=Voss |first4=S. Randal |title=Rediscovering the Axolotl as a Model for Thyroid Hormone Dependent Development |journal=Frontiers in Endocrinology |date=12 April 2019 |volume=10 |page=237 |doi=10.3389/fendo.2019.00237 |doi-access=free|pmid=31031711 |pmc=6473073 }}</ref>

Many other species within the axolotl's genus are also either entirely neotenic or have neotenic populations. [[Siren (amphibian)|Siren]]s, ''[[Necturus]]'' mudpuppies, and the troglobytic [[olm]] are other examples of neotenic salamanders, although unlike axolotls, they cannot be induced to metamorphose by an injection of iodine or thyroxine hormone.

Neoteny has been observed in all [[salamander]] families in which it seems to be a survival mechanism, in aquatic environments only of mountain and hill, with little food and, in particular, with little iodine. In this way, salamanders can reproduce and survive in the form of a smaller larval stage, which is aquatic and requires a lower quality and quantity of food compared to the big adult, which is terrestrial. If the salamander larvae ingest a sufficient amount of iodine, directly or indirectly through [[cannibalism]], they quickly begin metamorphosis and transform into bigger terrestrial adults, with higher dietary requirements, but an ability to disperse across dry land.<ref>{{cite web|last=Venturi|first= S.|year=2004 |url= https://sites.google.com/site/iodinestudies/morosini |archive-date=4 March 2017 |url-status = dead|archive-url=https://web.archive.org/web/20170304010444/https://sites.google.com/site/iodinestudies/morosini|access-date = 25 September 2020|title= Iodine and Evolution. DIMI-Marche}}</ref> In fact, in some high mountain lakes there live dwarf forms of [[salmonids]] that are caused by deficiencies in food and, in particular, iodine, which causes [[cretinism]] and [[dwarfism]] due to [[hypothyroidism]], as it does in humans.