Hobbit Posted January 24, 2022 Share Posted January 24, 2022 (edited) 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. 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. ☹️ 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. X = 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 Edited December 30, 2022 by Hobbit Whoops, made mistakes. 5 1 Link to comment Share on other sites More sharing options...
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