Introducing a Dominant Gene

Photo: A male Hifin Redwag Swordtail.

I’m beginning a blog series on introducing new genes (technically alleles, but I’ll bow to common usage) into a xiphophorine population. In this blog I’ll discuss the mechanics of introducing a single dominant gene into a xiphophorine population. By the way, this doesn’t only apply to xiphophorines or even fish; it can be applied to any animal or plant.

First, let’s define a xiphophorine. A xiphophorine is any fish in the genus Xiphophorus. This genus has 28 accepted species at this time. Three of these species, X. hellerii (green swordtail), X. maculatus (platy or the old “moon”), and X. variatus (variatus platy) and their hybrids, constitute the hobby’s colorful swordtails, platies, and variatus. The other 25 species aren’t common in the hobby, but many are available from clubs like the American Livebearer Association (www.livebearers.org). We raise some of these too. In the context of this blog when I say xiphophorine, I’m speaking about the commercially available swordtails, platies, and variatus.

That settled, let’s get to work. Let’s imagine you have a breeding colony of green swordtails and you’ve acquired a single male hifin redwag maculatus type platy. Furthermore, you want hifin redwag swordtails, but can’t find any to buy. You have with these fish the materials you need to create hifin redwag swordtail. The male pictured above is what the fish might look like four or five generations into your breeding program. You’d want a brighter, non-iridescent red body color and darker black fins. But, his shape, swordtail, and hifin aren’t bad.

Below is pictured your male hifin redwag platy. He has excellent red color, clear and without iridescence. His fins are decent black. His hifin is not good, but that can be corrected in later generations through judicious selection.

Next are your females. I cheated a bit here. Instead of green swordtails, I let you use blushing swordtails. They already have some red genes and this gives you a jump-start on red body color. The photo below is one of your females.

So with these starting fish what do you do? How do you end up with the fish at the top? Well, first of all, swordtails, platies, and variatus hybridize fairly easily, so that’s not a problem. There is, however, a characteristic of these fish that can cause a problem in breeding them; that is the females can store sperm from earlier matings. This can make it difficult to determine paternity of the offspring. Many breeders, especially those raising fancy guppies, get around this by raising virgin females and mate them to a single male. Raising virgin females isn’t an easy task, but as I’ll explain it’s not necessary to do that in this case, so I’ll reserve that discussion for a future article.

You may have surmised from the beginning paragraph that the hifin gene in xiphophorines is dominant and you would be correct. I’m assuming here that you know basic genetics and understand what a dominant gene is. That said, the hifin gene in xiphophorines is somewhat tricky and requires a little explanation. First, it is a homozygous lethal gene. When the same gene (technically an allele) is inherited from both parents the offspring is called homozygous for that gene. If two different genes (alleles) are inherited, then the offspring is heterozygous. “Homo” means same and “hetero” means different. When a fish inherits two copies of hifin it dies before birth. The practical aspect of this is you can’t develop a true-breeding population of hifins. Since each hifin fish is heterozygous and carries a non-hifin gene, two hifins mated together will produce both hifins and non-hifins in the approximate ratio of 2:1 hifins to non-hifins. If the hifin gene weren’t a homozygous lethal you’d expect a ratio of 3:1 hifins to non-hifins.

Okay so you have some green swordtail females and a male hifin redwag platy. The green swords have the shape you want. That shape being a swordtail body, which is much more elongated than the stocky bodied platy. But the color is wrong since you want a bright, clear red body with jet black fins. The male platy displays the color pattern and sports the hifin dorsal you want. But he has no long sword and has that stocky body. So what do you do?

First step is to make the male platy happy by giving him a group of green swordtail females. It doesn’t matter that the females have mated before since it’ll be easy to pick out the hybrids, the fish fathered by the platy. Female swordtails like males with swords, which the platy doesn’t have, but they’ll eventually succumb to the male’s advances and will mate with him and produce his offspring.

First, the female swords will each drop a batch of green swordtails from earlier matings. It’ll be the next batch of fry in which you’ll get some hybrids. The hybrids will be dull red and will have either black fins or have a pair of black stripes on the top and bottom of the caudal fin. They’ll be easy to pick out from among any green swordtails. Secondly, approximately half the hybrids should have hifins. With practice the hifins can be determined at birth. If you can’t, as the fish grow the hifins will become apparent. It’s the hifin fish you want to keep. The rest are Oscar food.

Segregate out the hifins to rear up. If you feed these fish well and keep them warm they should reach sexual maturity in three to four months. As they mature you should pick the hifin male with the most red on the body (there probably won’t be much) and some black in the fins. The black indicates he has the genes needed for the wag pattern. If you are lucky and reddest fish has black fins, you are ahead of the game. You don’t really need to know anything about the genes for the wagtail pattern, just pick the fish that looks most like a redwag platy but has a hifin. Retain all the hifin females so you can raise lots of fish in the next generation. In genetics large numbers are your friend. It would have been useful to have separated the sexes before reproduction commenced so you’d have virgin females, but it’s not necessary.

You’ve picked the hifin male that best resembles a hifin redwag swordtail. The resemblance isn’t that close; he has a shorter body than you like and if he has a sword it shorter than wanted. He probably doesn’t have solid black fins.  His body color is a dingy red. But, he has a hifin. You now give this lucky male a harem of his hifin sisters and half-sisters (all the hifins share as father the single hifin redwag platy male you used). If you didn’t raise virgins some females will have mated with males other than your selected one. In that event, he should father the next batches of fry from the selected females.  As before, from these matings you retain only the hifin offspring, discarding the others as you see fit; Oscars are always hungry. What you’ve started here is inbreeding to regain the redwag color pattern and a swordtail body and long sword. This next generation should produce at a least a few fish that are closer to redwag swordtails since you are selecting for swordtail genes while retaining the platy hifin and redwag color pattern genes.

Again, you’ll pick the hifin male closest to being a good redwag swordtail; as red as possible, with a wag pattern if possible, having a swordtail body shape, and a sword as long as possible. This new male is mated to his hifin sisters and half-sisters.

By continuing this process, it might take a few generations, but you will eventually produce nice hifin redwag swordtails.

Good fishkeeping!