Colour
The colour of axolotls is dependent upon pigment cells called chromatophores.
These cells are melanophores (containing eumelanin, a black-brown pigment),
xanthophores (containing carotenoids and pteridines, yellow and reddish
pigments) and iridophores (containing crystalised purines, which impart a shiny
iridescence).
Each cell in an axolotl contains 14 pairs of chromosomes.
Every characteristic of the animals is coded for by genes on pairs of
chromosomes. The genes for the pigment cells are inherited independently of one
another, and there is no known linkage to any other genes. So, each pigment type
is coded for by two different genes, one on each of a pair of chromosomes. These
contrasting genes that code for the same characteristic are known as alleles. A
pair of alleles is written like this: X/x. A capital letter means that gene is a
dominant gene, as opposed to the small letter, which means that gene is smaller.
For example, the allele that controls albinism could be found in an axolotl
in one of the following combinations: A/a, A/A, or a/a. If the animal was A/a,
because a is recessive and A is dominant, the animal's phenotype wouldn't be
albino, but it would still carry the gene for albinism (since it has an "a").
Since it carries both "A" and "a", it is known as "heterozygous". If the animal
had the A/A combination, its phenotype wouldn't be albino, and it wouldn't carry
the gene for albinism (both genes being the same, it is called "homozygous" for
"A"). If it were homozygous for "a" (i.e. a/a), the animal's phenotype would be
albino. Since "a" is recessive, both alleles need to be "a" in order for
albinism to be expressed in the phenotype. Albinism results in a lack of
eumelanin (the dark pigment). In axolotls, it also results in an increased
number of xanthophores (yellow pigment cells).
In the same way that a/a results in a lack of eumelanin, m/m (melanoid)
results in a lack of iridophores. Melanoid animals are very dark, with no reflective
pigment cells at all. M/m or M/M would result in normal iridophore development.
Animals homozygous for "ax" (i.e. ax/ax) are axanthic, meaning they have no
visible xanthophores or iridophores. These animals are almost as dark as
melanoids. Animals homozygous for both the albino gene and the axanthic gene
appear to be slightly off-white (yellowish). The following table summarises the
colour genes.
These cells are melanophores (containing eumelanin, a black-brown pigment),
xanthophores (containing carotenoids and pteridines, yellow and reddish
pigments) and iridophores (containing crystalised purines, which impart a shiny
iridescence).
Each cell in an axolotl contains 14 pairs of chromosomes.
Every characteristic of the animals is coded for by genes on pairs of
chromosomes. The genes for the pigment cells are inherited independently of one
another, and there is no known linkage to any other genes. So, each pigment type
is coded for by two different genes, one on each of a pair of chromosomes. These
contrasting genes that code for the same characteristic are known as alleles. A
pair of alleles is written like this: X/x. A capital letter means that gene is a
dominant gene, as opposed to the small letter, which means that gene is smaller.
For example, the allele that controls albinism could be found in an axolotl
in one of the following combinations: A/a, A/A, or a/a. If the animal was A/a,
because a is recessive and A is dominant, the animal's phenotype wouldn't be
albino, but it would still carry the gene for albinism (since it has an "a").
Since it carries both "A" and "a", it is known as "heterozygous". If the animal
had the A/A combination, its phenotype wouldn't be albino, and it wouldn't carry
the gene for albinism (both genes being the same, it is called "homozygous" for
"A"). If it were homozygous for "a" (i.e. a/a), the animal's phenotype would be
albino. Since "a" is recessive, both alleles need to be "a" in order for
albinism to be expressed in the phenotype. Albinism results in a lack of
eumelanin (the dark pigment). In axolotls, it also results in an increased
number of xanthophores (yellow pigment cells).
In the same way that a/a results in a lack of eumelanin, m/m (melanoid)
results in a lack of iridophores. Melanoid animals are very dark, with no reflective
pigment cells at all. M/m or M/M would result in normal iridophore development.
Animals homozygous for "ax" (i.e. ax/ax) are axanthic, meaning they have no
visible xanthophores or iridophores. These animals are almost as dark as
melanoids. Animals homozygous for both the albino gene and the axanthic gene
appear to be slightly off-white (yellowish). The following table summarises the
colour genes.
You may have noticed the "d" gene. This gene is a developmental mutant and not a
pigment mutant like the others. Animals homozygous or heterozygous for "D"
produce large numbers of yellow xanthophores. In combination with melanophores,
we get the wild type colouration (dark brown/olive-green). However, in animals
homozygous for "d", the normal pigment cells are produced, but they never
migrate off the neural crest of the embryonic animal, resulting in the white
phenotype. It is important to realise the this animal is not albino. This
phenotype is white, but has dark eyes. It is known as leucistic. Simple albinism
in axolotls leads to a yellow/golden animal, with red/pink eyes. In order to
produce a white albino, the animal must have the d/d genotype in combination
with the a/a genotype. Melanoid albinos (m/m with a/a) are also white animals
with pink/red eyes. This can make initial identification of a white albino's
phenotype difficult to determine for the novice.
pigment mutant like the others. Animals homozygous or heterozygous for "D"
produce large numbers of yellow xanthophores. In combination with melanophores,
we get the wild type colouration (dark brown/olive-green). However, in animals
homozygous for "d", the normal pigment cells are produced, but they never
migrate off the neural crest of the embryonic animal, resulting in the white
phenotype. It is important to realise the this animal is not albino. This
phenotype is white, but has dark eyes. It is known as leucistic. Simple albinism
in axolotls leads to a yellow/golden animal, with red/pink eyes. In order to
produce a white albino, the animal must have the d/d genotype in combination
with the a/a genotype. Melanoid albinos (m/m with a/a) are also white animals
with pink/red eyes. This can make initial identification of a white albino's
phenotype difficult to determine for the novice.