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Axolotl Genetics, Part 3: Melanism and Axanthicism

In part 2 of this article, we went over albinism, a recessive genetic mutation which affects one type of pigment cells (melanophores, responsible for the dark pigment eumelanin). As you can easily guess, there are also mutations which affect the other two types of pigment cells: iridophores, which produce shiny white crystallized purines, and xanthophores, which produce yellow pteridines. In this section, we will focus on these two mutations, which are a bit more complex than albinism.

Melanism

Melanism is a recessive mutation similar to albinism, but instead of affecting melanophores, the mutation acts on iridophores. All axolotls receive either the M or m allele from each parent, which means their genotype for the melanism trait is either:

  • M/M (homozygous dominant)
  • M/m (heterozygous)
  • m/m (homozygous recessive)

Homozygous dominant and heterozygous axolotls develop normal iridophores, which means they are able to produce crystallized purines (the shiny white pigment). Homozygous recessive axolotls are called melanoids. Since they have no iridophores, they are unable to produce cystallized purines.

This mutation also has a spillover effect: the lack of iridophores triggers the conversion of some xanthophores into melanophores. This is why melanoid axolotls show more eumelanin (black) than any other color morph, and almost no pteridines (yellow). This gives them a grey appearance, which can border on blueish under the right wavelengths.

Due to the reduced number of pteridines, which are important to immune function, melanoid axolotl larvae have a slightly lower survival rate than wild-type or albino axolotls. This is why melanoid axolotls they tend to be a bit more expensive and slightly less common on the market than other color morphs.

The lower amount of pteridines makes melanoid axolotls appear almost blue under certain wavelengths. Here is X, one of my melanoids, under blue and white LED lights.

 

This melanoid axolotl has just a hint of yellow pigment on his face, which gives him an almost wild-type look. We can tell that he is a melanoid because his eye ring lacks the metallic shine of wild-type axolotls. Photo by Patricia’s Gill Babies.

This axolotl is both melanoid and albino, which means it has a lower than normal amount of xanthophores (caused by melanism) in addition to a complete absence of melanophores (caused by albinism). This explains why so few yellow pigments are visible on its skin. Photo by Samantha Nicole.

Axanthicism

As you can imagine, axanthicism acts on xanthophores, the pigment cells responsible for producing pteridines. But the name of the trait, which means “lack of xanthophores”, is actually misleading. As it turns out, axanthic axolotls do have a certain amount of xanthophores, but those xanthophores are unable to produce pteridines due to a genetic mutation, which is believed to have originated from a virus.

Even though they can’t produce pteridines, the mutant xanthophores are able to store some yellow pigments from the axolotl’s diet (chiefly riboflavin, also known as vitamin B2). This helps compensate a bit for the lack of pteridines, but since they are slowly accumulated over time, axanthic larvae still have a low survival rate compared to other color morphs. This, along with the strict import laws currently in place, explains why axanthic axolotls are nearly impossible to find on the Canadian market.

In addition to causing a complete lack of pteridines, the axanthic mutation prevents iridophores from differenciating during development. As a result, axanthic axolotls often look a lot like melanoids. One way to tell them apart is to look at them under a blueish light. The complete absence of yellow pigments at birth tends to give axanthic a purple hue, whereas melanoids are more of a blueish grey. The purple effect tends to fade over time due to the accumulation of other yellow pigments, but some axolotls (such as Sarah, below) do manage to retain it through adulthood.

To make matters more confusing, axolotls can be both axanthic and melanoid. If an axanthic axolotl is especially dark, chances are it is also melanoid, but there is no way to be certain unless the genotype of both parents is known. If an axanthic axolotl accumulates a lot of yellow pigment over the years, then it probably isn’t a melanoid, as melanism further reduces the overall number of xanthophores.

Sarah, showing the purple-grey color characteristic of axanthic axolotls. Photo by Leslee Anne Vanden Top (Axolotl Heaven).

 

Pale axanthic axolotls such as this one are sometimes called “lavender”. Despite looking similar to light melanoids, lavender axolotls are unlikely to possess the melanism trait. Photo by Leslee Anne Vanden Top (Axolotl Heaven).

This axanthic axolotl is extremely dark, so chances are it is also homozygous dominant for melanism. Photo by Leslee Anne Vanden Top (Axolotl Heaven).

 

<- Axolotl Genetics, Part 2: Mendelian Inheritance and Albinism | Axolotl Genetics, Part 4: Leucism, Copper and GFP [Coming Soon!]

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Axolotl Genetics, Part 2: Mendelian Inheritance and Albinism

There are six known genetic traits that affect an axolotl’s pigmentation:

  • Albinism
  • Melanism
  • Axanthicism
  • Leucism
  • The Copper trait
  • The GFP trait

All six of these traits follow a Mendelian pattern of inheritance, which is good news, because it’s a very simple pattern to explain and understand. Trust me! Keep reading and you’ll be an expert on the topic in less than 5 minutes.

Mendelian inheritance: the Ikea metaphor

DNA is a pretty amazing thing: the complete set of instructions for the construction of one particular living organism. It’s often portrayed as one huge chain, but DNA is actually broken into individual segments called chromosomes. Think of each chromosome as one assembly instruction booklet, like the ones that come with Ikea furniture. Obviously it takes a lot of instructions to build a whole living being, so we need a whole pile of booklets.

The instructions inside the booklets also have to be fool-proof, because the ones reading them and performing the assembly are proteins, which pretty much work like mindless drones. This is fine, except that when the information in one booklet is messed up or missing, the proteins can’t pick up the phone and call Ikea for help.

I got this from a meme somewhere. Let me use it, Ikea, it’s for a good cause!

 

Luckily, each instruction booklet comes in two copies: one that was obtained from the animal’s mother, and one from its father. So even if there is missing information in one of the booklets, the protein-drone just needs to look at the other copy. With any luck, the correct information will be there.

This is the basic principle behind Mendelian inheritance.

Let’s say I want to build a chair and I have two instruction booklets in my possession. Version 1 (which I got from my mom) shows detailed, step-by-step assembly instructions. Version 2 (which I got from my dad) has a bunch of mistakes in it, and it’s very confusing. If I follow version 1, I’ll end up with a chair. If I follow version 2, I might end up with some weird contemporary art sculpture that may or may not crumble when I sit on it. Obviously, I would rather follow version 1, right? I might call my dad up afterwards and tell him “Hey Dad, just so you know, the instructions you gave me made no sense! It’s okay though, I used a different set of instructions and I managed to build the chair in the end.”

But what if both of my parents had given me the faulty version 2? Since I’m not a mindless drone, chances are I would have gone “uhh, I don’t think this is right.” But if I were a mindless drone, I probably would take the fact that both sets of instructions are saying the same thing as a sign that the information is correct, and I would have built the weird contemporary art sculpture. And who knows, maybe the sculpture would have turned out even better than some boring old chair!

Don’t you dare question my art.

 

When I say that a particular genetic trait follows a pattern of Mendelian inheritance, what I mean is that the assembly instructions for that particular trait come in two different versions, and given the opportunity, the assembly protein-drone will always prefer one version over the other. The version that is always preferred is called the dominant allele. The one that’s used only if no other instructions are available is called the recessive allele.

Mendelian inheritance: the albinism trait

If a chromosome is like an instruction booklet, the section of the booklet that contains instructions for one particular trait is called a gene. Just like the booklet in our previous example, the albino gene comes in two versions: allele A and allele a. Dominant alleles are always represented by capital letters, whereas recessive alleles are always lowercase.

Just like humans, axolotls receive two versions of each chromosome — one from their mother and one from their father. Every axolotl either ends up with one of these pairs:

  • A/A (two identical copies of the dominant allele)
  • A/a (one copy of each allele)
  • a/a (two identical copies of the recessive allele)

Axolotls who end up with two copies of the dominant allele are said to be homozygous dominant. The ones with two copies of the recessive allele are called homozygous recessive. If they have one copy of each, we call them heterozygous (from homo = same, and hetero = different).

So what makes allele A the dominant version of the gene? It contains a set of instructions for the construction of melanophores, the pigment cells that produce the dark pigment eumelanin. In allele a, those instructions are either erroneous or missing due to a genetic mutation that randomly occured at some point during the evolution of the species. We call this mutation albinism.

Albinism works in a fairly straightforward manner: when an axolotl is homoyzgous for the recessive (mutant) allele a, it is unable to produce eumelanin (the brown/black pigment) because it simply does not have any melanophores. All other axolotls have melanophores and are able to produce eumelanin (with the possible exception of copper axolotls, which we will discuss later).

Even though albinism is a recessive mutation, it doesn’t mean that allele a is worse than allele A, or that albino axolotls are inferior in any way. Some mutations can yield positive results! Look at how cute these albino axolotls are:

A sparkly white albino axolotl. Photo by Leslee Anne Vanden Top.

 

A very fluffy melanoid golden albino axolotl. Photo by Ashlee Juanita Turner.

 

Pixel, one of my golden albino axolotls, holding onto a leaf during a water change.

 

A super shiny golden albino baby. Photo by Samantha Nicole.

 

Tangelo and Kumquat, two of my golden albino babies. Tangelo (left) has a high level of pteridines, whereas Kumquat (top) has lower pteridines and higher iridophores.

 

Of the six mendelian traits that affect pigmentation, albinism is the most straightforward, because it only acts on one type of pigment cell. The other traits are slightly more complex, but the principle behind them is the same: as long the right sets of instructions are present, all pigment cells will be created and behave normally. But if they’re not, the assembly drones will follow whatever instructions they can find, and turn those functional chairs into pieces of art!

<- Axolotl Genetics, Part 1: Color Pigments | Axolotl Genetics, Part 3: Melanism and Axanthicism -> [Coming soon!]

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Axolotl Genetics, Part 1: Color Pigments

An axolotl’s coloring is the result of genetics, and to a lesser degree, environment and diet. Let’s go over the different color pigments involved, and you’ll understand what I mean.

The three natural color pigments are:

  • Eumelanin (brown, black)
  • Crystalized purines (iridescent white)
  • Pteridines (yellow, orange)

There is also a fourth pigment that is present in some transgenic axolotls:

  • Green fluorescent proteins (bright yellow, glowing neon green under a UV light)

We’ll get back to this one later — let’s focus on the three natural pigments first. These are naturally present in the majority of axolotls. Besides looking pretty and helping with camouflage, they also come with health benefits: eumelanin helps protect the skin against UV radiation, and pteridines play an important role in the axolotl’s immune system.

You can see all three pigments expressed in the picture below:

Two of my light wild-type axolotls, showing all three natural pigments: eumelanin, pteridines and crystallized purines.

Axolotls that possess all three pigments are called wild-type. Even though they all have the same pigments, there can be a lot of variation in wild-type appearance. For instance, the axolotls shown above have a lot of yellow pteridines, which gives them an overall olive tint. They also have white spots on their tails. If I had taken the photo with the flash on, you would have seen that those white spots are shimmery, because they are made of crystallized purines.

The axolotl in the photo below is a much darker wild-type:

Katla, one of my dark wild-type axolotls, showing a predominance of eumelanin.

 

In this photo, we can see a lot of eumelanin. The other pigments are also present, but not very noticeable. You can see a little bit of crystallized purines in the eye ring and the tip of the gill stalks. Pteridines are almost completely invisible under the dark eumelanin.

Let me show you one more, very different wild-type look:

A “starburst” wild-type axolotl (front). Photo by Patricia’s Gill Babies

 

Isn’t this boy gorgeous? Here, eumelanin forms the base skin color, but the pteridines and crystallized purines being layered on top of each other create a gold flake effect.

In addition to the variety among wild-types, there are a lot of different color types, or “morphs”, besides wild-type. Over the course of their history, axolotls have undergone several genetic mutations which affect their pigmentation — some of which are natural, some of which are the result of human intervention.

Here are the six main genetic traits that affect axolotl pigmentation:

  • Albinism (affects eumelanin)
  • Melanism (affects crystallized purines)
  • Axanthicism (affects pteridines and crystallized purines)
  • Leucism (affects eumelanin, pteridines and crystallized purines)
  • Copper trait (affects eumelanin and/or pteridines)
  • GFP trait (affects green fluorescent proteins)

We’ll talk more about these traits in the next section of the article. For, now I just want you to keep in mind that there are several genetic traits that can essentially switch pigment production on and off, or affect how pigments are distributed around the body.

Let’s take a closer look at what each pigment looks like individually.

Eumelanin

Eumelanin is the pigment responible for shades of brown and black. It is produced by pigment cells called melanophores. To give you a better idea of what the pigment looks like on its own, here is what an axolotl looks like when it shows only eumelanin:

My melanoid axolotl, Z, showing only the pigment eumelanin.

 

Fun fact: the amount of eumelanin produced by an axolotl depends on two things: genetics, and environment. Axolotls whose parents were especially dark tend to exhibit similarly dark features. Axolotls who grow up in dark environments also tend to exhibit darker features than ones kept in lighter environments.

The absence of eumelanin, due to an inability to produce melanophores, is called albinism. Here is what an axolotl looks like when you completely remove eumelanin, keeping only the other two pigments:

A golden albino axolotl, showing pteridines and crystallized purines, but no eumelanin. Photo by Patricia’s Gill Babies

 

Pretty neat, right?

Crystallized purines

Crystallized purines are iridescent white pigments, which means they shimmer in a sort of rainbow effect. Combined with pteridines, they can also create a shiny golden color, as we’ve seen above. Crystallized purines are produced by pigment cells called iridophores. Here is what iridophores look like on their own:

One of my “starlight” white albinos, showing crystallized purines concentrated on the gill stalks and eye ring.

 

The inability to produce iridophores is called melanism. Notice how the shiny white pigments are missing in the picture below:

A melanoid white albino axolotl, showing a lack of crystallized purines. Photo by Patricia’s Gill Babies

 

Melanism is a little bit more complex than albinism. We’ll talk about it more in part 3 of this article.

Pteridines

Pteridines are responsible for yellow and orange coloration. They are produced by pigment cells called xantophores. This is what pteridines look like when you remove the other two pigments:

A melanoid golden albino axolotl (juvenile), showing only pteridines. Photo by Samantha Nicole.

 

The inability to produce pteridines is called axanthicism. Axanthic axolotls are exceedingly rare, if not impossible to find in the Canadian pet trade. This is partly due to strict import laws, and partly due to the effect axanthicism has on axolotl health. Since pteridines play a role in immune function, axanthic axolotls have a lower survival rate than other axolotls.

In the absence of pteridines, axanthic axolotls take on a purple-grey look:

Sarah the axanthic axolotl, showing a lack of pteridines. Photo by Leslee Vanden Top

 

Do you notice some odd things about this picture? Axanthicism is a much more complex mutation than albinism and melanism. We’ll talk more about it when we get to the next section.

Green fluorescent proteins (GFP)

In the course of their use as animal research models [more on this soon!], some axolotls got a pretty cool addition to their genomes: the GFP trait. Originally found in a species of jellyfish, this trait causes nearly every cell in the axolotl’s body to produce a bright yellow protein which glows neon green under a UV light. Why is this cool? First, it’s been very helpful to researchers working on limb regeneration and organ transplants. Second, it looks very pretty! And third, the trait can be passed down from generation to generation. But my favorite thing about it is that, since the effect isn’t limited to pigment cells, it isn’t affected by leucism. You’ll see what I mean when we get to the next part!

A GFP leucistic axolotl under UV light. Photo by Carey Lynn Cooper.

 

Now that you have a good idea of what the individual pigments do, let’s take a look at the genetics behind them!

Axolotl Genetics, Part 2: Mendelian Inheritance and Albinism ->