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Platy color genetics and the development of melanistic tumors


Hobbit
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I’m starting this thread so we can collect our knowledge about platy color genetics, specifically as it relates to the development of black melanomas. This is a known issue that can occur when crossing different species of platys—in fact, cancer researchers often use platys as a model organism—but it seems like this is yet another instance where the scientific community isn’t sharing all its notes with us hobbyists. 😉 Just kidding. The papers are out there, but us hobbyists can’t always find or understand them. So I’m going to do my best to summarize what I’m learning. Anyone should feel free to add their knowledge!

First, an example of what I’m talking about. Here are some fry I’ve been raising that ended up developing melanomas. You can tell they’re melanomas because they stick out from the body of the fish. It’s particularly obvious on the caudal peduncle.

E13FE447-E58D-48E0-9723-6C9A07B7DD46.jpeg.8d6802510f327812d6b0acfb5b9f1e22.jpeg 

Melanomas in platys develop from crosses between different platy species. The one that’s relevant to our hobby is the cross between X. maculatus (the southern platyfish) and X helleri (the variatus platy). Many, if not most, platys in the hobby are the ancestors of a cross between these two species because that’s how we’ve gotten so many color variations.

To understand why these hybrid platys can develop tumors, we have to dive into some genetics. 🤓

A quick reminder of genetics basics: living things that come from two parents (i.e. sexual reproduction) get half their genes from mom, and half from dad. (I’m using genders for clarity of discussion.)

In order to make sure each offspring ends up with ALL the genes they need, each parent actually contributes a full set of genes. The offspring then end up with two copies of each gene: one from mom, and one from dad.

And when that offspring has babies, it will contribute one of each gene to its baby, while the other parent contributes one of each gene as well. And so on and so on, etc, etc.

Back to platys:

Southern platys are the species of platy that naturally have black spots. The genes for black spots are actually located on the sex chromosomes, but that’s not super relevant for this discussion. There are different genes for black spots on the dorsal fin, the body, the tail, etc. Close by is a gene that controls how far those black spots spread, called Xmrk-2. (Sounds like a droid name.)

Southern platys also have a gene on a totally different chromosome that used to be called Diff. (Now they think is actually CDNK2X, but I’m going to stick with Diff for this post, because two droid names is too confusing.) Diff is what we call a tumor suppressor gene. It’s a handy gene that keeps cells from growing out of control. And it seems to be directly responsible for keeping a southern platy’s black spots in check. 

Variatus platys, on the other hand, don’t naturally have any black spots. They don’t have the black spot genes, and they don’t have Xmrk-2. They do have Diff, but it’s not a very strong version of Diff. It’s a weak Diff. (Okay that’s a total oversimplification—actually it just doesn’t seem to be activated when it comes to black spots. I don’t mean to insult the strength of your Diff, variatus platys!)

So what happens when a Southern Platy mom gives a baby platy black spots (Xmrk-2) and a strong copy of Diff, and a Variatus Platy dad gives the baby no genes for black spots and a weak Diff? 

Well… the baby has black spots, and because there’s only one strong Diff in its genome, those black spots have a stronger pigmentation than they would otherwise. We’ll call these babies the hybrids.

No tumors yet, hypothetically. I think. These scientific papers aren’t focused on colors, they’re focused on cancer, so sometimes it’s hard to figure out what these fish actually look like. 😆

But now those hybrid babies have babies. And now things get interesting, because each hybrid parent could give each of their babies EITHER black spots (Xmrk-2) OR no black spots, and EITHER a strong Diff or a weak Diff. So with all those possible combinations, we could get a variety of different babies.

And, unfortunately, it’s possible that both parents could give their baby the weak Diff gene, so the baby ends up with two weak Diffs… as well as black spots (Xmrk-2).

And then there’s no strong Diff to control Xmrk-2. Those black spots can just grow and grow, and nothing stops them.

And what happens when a tissue doesn’t stop growing? Well, that’s cancer. ☹️

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In my fish room, I’ve been breeding some sunburst platys with some firetail tuxedo platys. My goal was to get a sunburst tuxedo platy.

image.jpeg.ae716f138bf4496da15b418309bccac2.jpeg X image.jpeg.78b35caa0579da79a6a785521d9d1722.jpeg

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Unfortunately, this means I have one parent, the sunburst platy, who has no black spots, and therefore likely two weak versions of Diff. And because some of my fry have tumors—babies that I think are the first generation from my parent fish—I suspect the tuxedo platy has only one strong Diff, because you need each parent to contribute a weak Diff in order to get tumors.

I’m still not exactly sure what I’m going to do about this. Finding parents who are guaranteed to have two strong Diffs would be extremely time consuming, since I’d have to wait for several generations of their fry to grow up (to what looks like at least 3 months old) in order to see if anyone was developing tumors, and then I’d have to breed those fry to see if any of them produced babies with tumors. Because, remember, the first generation hybrids won’t develop tumors as long as they have one strong Diff. So if only one of my broodstock had just one weak Diff, it wouldn’t show up until the hybrids had babies, i.e. the broodstock’s grandchildren.

Then there would be the guessing game of which parent the weak Diff came from.

One way around this would be to only breed platys without any black coloration at all. Or only breed tuxedo platys to other platys with black spots—and just assume the breeders have eliminated any fish with weak Diff from their strains a long time ago. Which is not a great thing to assume, since that’s so hard to do—and I have evidence that my current tuxedo platy is a Diff hybrid herself.

The good news is that I think a platy that has one weak and one strong Diff will have darker pigmentation than a platy with two strong Diffs. Going into this breeding project, I thought I wanted to select for the darkest tuxedo possible. Now I’m thinking I should probably select the other way!

Unfortunately, at this point, I’ll probably be stuck culling a handful of fish every generation.

I’ll try to pick fish for my broodstock that have light, well-controlled tuxedo patterns, and hope that’s a reflection that they have two strong Diffs.

References:

Xiphophorus Interspecies Hybrids as Genetic Models of Induced Neoplasia 

Genetic Linkage and Color Polymorphism in the Southern Platyfish ( Xiphophorus maculatus ): A Model System for Studies of Color Pattern Evolution

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Edited by Hobbit
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The platys pictured above were between 2 and 4 months old—unfortunately I don’t have exact ages because I don’t have room to keep each drop separate. 😄 I’m not sure if some platys may develop tumors later. I haven’t kept many platys past that age.

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This is great information. I assume that the same would apply to swordtails? Very neat. I breed swordtails and I'm planning to cross my black line to a red wag. Maybe I should rethink this. The wag has black fins. I will inspect him more closely for any black on his body. I just doubt that the gene is inclusive of fin color since that's a controlled area of pigmentation.

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On 1/25/2022 at 10:04 AM, Minanora said:

This is great information. I assume that the same would apply to swordtails? Very neat. I breed swordtails and I'm planning to cross my black line to a red wag. Maybe I should rethink this. The wag has black fins. I will inspect him more closely for any black on his body. I just doubt that the gene is inclusive of fin color since that's a controlled area of pigmentation.

Yeah is it ANY black pigmentation that carries this gene, even just on the fins? Or is it just for body coloration? I have a bunch of blue platies I'm trying to "clean up" - reduce black spots on the body and darken the black on their fins. 

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That’s a really good question, and I’m not sure of the answer. I’ve tried looking it up, but most references in the literature discussing specifics of platy color genetics are articles from the 60’s-80’s, which aren’t free online unless you have a university research login.

From what I can tell, it seems like what matters most is what type of cell produces the black pigment. They call these cells “melanophores.” So if the black pigment on fins is produced via melanophores, I think the fish would need the same regulatory gene to keep those cells from overgrowing.

I do have a line of blue platys going right now in which I’m breeding blue wags to blue spot-sided. In their offspring, I’ve only found one fry out of probably 200+ that’s developed a tumor. (The other blue platy fry in the pictures above was being culled for a different deformity. ☹️) I’m assuming that one fry got a random unlucky mutation rather than inheriting problem genes from its parents. Of course, I could have gotten lucky with the parent genetics, but from my very limited experience, hybrids of wag + spot-sided parents seem fine.

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On 1/26/2022 at 9:32 AM, Hobbit said:

That’s a really good question, and I’m not sure of the answer. I’ve tried looking it up, but most references in the literature discussing specifics of platy color genetics are articles from the 60’s-80’s, which aren’t free online unless you have a university research login.

From what I can tell, it seems like what matters most is what type of cell produces the black pigment. They call these cells “melanophores.” So if the black pigment on fins is produced via melanophores, I think the fish would need the same regulatory gene to keep those cells from overgrowing.

I do have a line of blue platys going right now in which I’m breeding blue wags to blue spot-sided. In their offspring, I’ve only found one fry out of probably 200+ that’s developed a tumor. (The other blue platy fry in the pictures above was being culled for a different deformity. ☹️) I’m assuming that one fry got a random unlucky mutation rather than inheriting problem genes from its parents. Of course, I could have gotten lucky with the parent genetics, but from my very limited experience, hybrids of wag + spot-sided parents seem fine.

Interesting. Thanks!

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  • 1 month later...

@H.K.Luterman I have more data now (meaning more baby platys) and it looks like I am getting tumors on the wag+spotted hybrids. 😞 One of the males in my broodstock is a first generation offspring who’s spotted—not wag—but may still have been a hybrid. It could be his + my blue wag mom’s offspring who are getting the tumors. The other guess is that I wasn’t getting many offspring from my blue wag mom before the most recent two batches, but I’m not sure how likely that is.

My current blue baby population is all from the blue wag mom and F1 spotted males. Many of them are showing signs of potentially developing tumors, but only time will tell for sure.

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On 3/11/2022 at 9:12 AM, Hobbit said:

...baby population is all from the blue wag mom and F1 spotted males. Many of them are showing signs of potentially developing tumors, but only time will tell for sure.

What are the signs that you are seeing? My blue platy moms' offspring that are mature came from unknown LFS dad and are mostly yellow tux and orange tux. I have lost three of these as young adults, weeks apart from each other,  with no previous signs of illness and bodies looked intact, but they all do have that black body in common. I wondered if they might have interior tumors that did not show up. 

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@PineSong oh no! That’s a bummer. Have you done any autopsies? I can’t confirm or deny if any of mine are developing internal tumors, but on the outside, I look for either really dark spots that seem larger+darker than the fish’s other pigmentation, or spots where they shouldn’t have spots. On the tuxedo platys, this has looked like spots on their head. Later I’ll find tumors on the caudal peduncle and sometimes the tail fin.

I’m still watching the blue spotted platys to see if any of the dark spots I’m seeing turn into tumors. In this generation I’m seeing some with dark black streaks on their tails, and I’m not sure yet if that will turn into anything problematic.

Side note: a yellow tux platy sounds really pretty.

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On 3/12/2022 at 12:49 AM, Hobbit said:

@PineSong oh no! That’s a bummer. Have you done any autopsies? I can’t confirm or deny if any of mine are developing internal tumors, but on the outside, I look for either really dark spots that seem larger+darker than the fish’s other pigmentation, or spots where they shouldn’t have spots. On the tuxedo platys, this has looked like spots on their head. Later I’ll find tumors on the caudal peduncle and sometimes the tail fin.

Side note: a yellow tux platy sounds really pretty.

I have not/cannot do autopsies, too squeamish.

I actually kept two of them in a container of water for hours after discovering them because I couldn't believe fish that had looked fine at the last feeding and who looked fine now other than being immobile were actually dead-- (not fat, not skinny, no missing/raised scales, no redness near gills, no weird body position). I was thinking "maybe she hit her head and is knocked out, maybe she's having a seizure, maybe she's the fainting goat of the fish world". But no.These guys were born in my tanks and physically looked healthy, were less than eight months old. 😞

The yellow platies are meh to me. Kind of a watered down yellow, not as snazzy next to the tux as the orange ones. I was disappointed when none of that batch turned out blue, but the orange ones are actually so sporty looking I'm not disappointed any more. 

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On 3/12/2022 at 6:48 AM, PineSong said:

I have not/cannot do autopsies, too squeamish.

I actually kept two of them in a container of water for hours after discovering them because I couldn't believe fish that had looked fine at the last feeding and who looked fine now other than being immobile were actually dead-- (not fat, not skinny, no missing/raised scales, no redness near gills, no weird body position). I was thinking "maybe she hit her head and is knocked out, maybe she's having a seizure, maybe she's the fainting goat of the fish world". But no.These guys were born in my tanks and physically looked healthy, were less than eight months old. 😞

The yellow platies are meh to me. Kind of a watered down yellow, not as snazzy next to the tux as the orange ones. I was disappointed when none of that batch turned out blue, but the orange ones are actually so sporty looking I'm not disappointed any more. 

That is really rough, I'm so sorry...

Apparently it's zebra danios that are the fainting goats of the fish world... And I managed to buy a male carrier and a female carrier...

Made a very exciting morning catching fish on the third generation, when a quarter of the fish turned belly up...🙄

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On 3/12/2022 at 11:08 PM, Torrey said:

That is really rough, I'm so sorry...

Apparently it's zebra danios that are the fainting goats of the fish world... And I managed to buy a male carrier and a female carrier...

Made a very exciting morning catching fish on the third generation, when a quarter of the fish turned belly up...🙄

I'd have needed defibrillated! Good to know and I wonder if other danios have the same trait--I do have long term plans to have long fin leopard danios and gold ring danios one day...

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I have had a few of my black swordtail babies be really really matte black. I have one from P1 brood 6 that is mottled and has a tumor on it's side. First tumor in the youth. However it could be a spinal deformity. It's too small to know for sure yet.

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I saw the same thing when I had swordtails with black on them, especially the ones who were predominantly or wholly black. I remembered reading an article way back then about tumors being a known result of the melanin-heavy genetics. I haven't had them since then.

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On 3/11/2022 at 8:12 AM, Hobbit said:

but I’m not sure how likely that is

Just saw this thread.  Wonderful summary of the genetics behind this.  We like to think of hybridization as having "hybrid vigor" all the time, but you can just as easily have a genetic mismatch which leads to disorders never seen in the original populations.  I say populations because you can get this sometimes within the same species from different geographic populations too.  Genetically, the best results are often obtained from keeping large enough populations within a specific area.  But that doesn't always line up with what a breeder wants to do, so...

You might consider contacting Karl Trochu of Miami Swordtails.  He gave a wonderful talk to the Minnesota Aquarium Society a few months back about breeding swords and dealing with this exact problem.  He seems like a knowledgeable and accessible guy.  And he may be able to give you advice on the breeding aspect that is missing from the scientific papers.

Just a thought.  Regardless, this was a great summary of the issue.  Thanks for posting it!

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  • 9 months later...

Hi Hobbit. I just wanted to say thank you for this post and everybody’s comments. I’ve been wondering for months why my panda platy fry were looking like that only around the black areas of their bodies. This thread has clarified a lot of questions for me. 
Thanks again 🐠

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