Hybridization Harmful?

If someone were to be breeding hybrids for an aquarium, would those hybrids be in any way impared other than spawning? I know that with some animals hybridization can cause many health issues, but I'm not sure about fish.

You have to be more specific. There are a lot of different fish, and the hybrids between them can be very different.

Two of my favorite hybrids are the modern aquarium swordtail and platy, 'Xiphophorus hellerii' and 'Xiphophorus maculatus'. These fish are actually hybrids, and both contain hellerii and maculatus genes. In the wild, both fish attain a mild rosy hue. When combined, the genes from hellerii and maculatus mix together to form a bright red pigment. These offspring were then backcrossed to maculatus or to hellerii to regain either a shorter or longer body type.

Pictures.
Wild Xiphophorus maculatus: http://theaquariumwi...maculatus_2.jpg
Red 'Xiphophorus maculatus': http://tropicalfisha...latyFemale1.jpg
Wild Xiphophorus hellerii: http://www.fmueller....tted-male-2.jpg
Red 'Xiphophorus hellerii': http://nas.er.usgs.g...00722121713.jpg

Backcrossing, to give you more detail, is when you take a large batch of the first generation hybrid offspring (xiphophorus have about 20 per birth), keep only the ones that have the color you want, and breed them to a fish of the body type you want, again keeping only the offspring with the color you want. You keep doing this through the generations and progressively get closer and closer to the body type you want. The first generation hybrid offspring had an intermediate body size between maculatus (short) and hellerii (long). Eventually you can retain the original body type.

Here's a picture of an intermediate body length: http://e0.aqua-fish....il-4-female.jpg
I personally consider that body to be too short to be a good hellerii. (Harrumph) When I was breeding swordtails, I mixed my hellerii with X. alvarezi to get an even longer body and longer sword. They were gorgeous. Image: http://img.photobuck...imiru/008-2.jpg That black spot gene he had causes melanoma, though. Here's a picture of him with a chunk of his tail missing: http://img.photobuck...imiru/003-3.jpg That chunk grew and grew and eventually claimed his life. I sold off that strain and worked with a different gene for black spots that didn't cause melanoma.

So, there are pros and cons to hybridization. You just have to know what traits to select for. Avoid the cancer causing genes. *nods* Here's more info if you're interested: http://www.xiphophor....edu/about.html

Edited by EricaWieser, 11 September 2011 - 09:57 AM.

Yeah, like Erica touches on, a huge swath of the animals available in the tropical fish trade are multiple hybrids, which is why their fertility is often close to zero. The extreme in this is those horrible "parrot cichlids". Anyone who likes those should be assumed to have room temperature IQ until proven otherwise.

Yeah, like Erica touches on, a huge swath of the animals available in the tropical fish trade are multiple hybrids, which is why their fertility is often close to zero.

Hybridization does not necessarily reduce fertility. I have kept both wild Xiphophorus hellerii and the hybrid hellerii/maculatus 'helleri', and they both drop the same number of fry.

Yes, but how successful are those fry, for how many generations? Some of these strains are less gefarkt than others, of course.

Yes, but how successful are those fry, for how many generations? Some of these strains are less gefarkt than others, of course.

gefarkt? I'm sorry, I don't know what that means. And they're very successful; hybrid swordtails and platies are some of the most well loved and ubiquitous of aquarium fish.

Yes, successful in the aquarium... They can survive in such a controlled environment even with diminished fecundity or fitness.

Yes, successful in the aquarium... They can survive in such a controlled environment even with diminished fecundity or fitness.

*headdesk* There's no convincing you, huh? Let me tell you the story of my 55 gallon tank.

A while ago, I had a 55 gallon tank. In it, I kept lovely swordtails. One day I saw wild Xiphophorus montezumae, the swordtail species with the longest sword, for sale on aquabid. I ordered them and they arrived, healthy (at the time). At that time I had about a dozen pet store swordtails and two dozen montezumae. Slowly, over time, my poor fish keeping skills (I was new to the hobby) killed off every single wild X. montezumae... and not a single pet store swordtail. They're hardy. They're wonderful for new fishkeepers.

So then, thinking it was just the montezumae being lame (maybe they didn't like my water parameters), I bought some F1 Neolamprologus multifasciatus. And one day I had a nitrogen spike, it went up to maaaaybe 40 ppm, and every single N. lamprologus died. Not a single swordtail died.

Years and years of having to deal with aquarium conditions (which are not pristine! If those fish aren't nitrogen resistant, they're gone) have bred a stronger, more hardy fish than the wild fish. There would never be a nitrate concentration of 40 ppm in the wild, so why would wild fish have to have a nitrogen resistance?

My point is, hybrid fish do not automatically have decreased fecundity or fitness. It all depends on what your hybrid is (gag, blood parrots) and what genes you take from the parents (note my melanoma example). Each hybrid is different and the hybridization step itself does not necessarily 'weaken' the offspring.

Edited by EricaWieser, 12 September 2011 - 08:59 AM.

No, I'm not convinced with the swordtails. I know there is hybrid vigor in some species. Look at pupfishes in W. Texas, Sheepshead minnows and their hybrids are stronger swimmers and attract mates more successfully than the endemic species. This is why hybrid swarms are such a problem out there. Fitness goes far beyond ability to stay alive in a home aquarium. When looking at fitness of a species, you have to look at its ability to procure food (usually in competition with others), ability to reproduce competitively (including attracting mates, and having a large enough number survive to successfully breed later on), and not getting killed before passing on genetics. There are other factors, but the basic ones are getting your genes out there successfully for eternity. Sure the colorful sword crosses are successful in the aquarium. A lot of it is the same way a domestic toy dog is successful. They have traits that encourage people to take care of them. In the wild, they prove to be less-than-fit, and end up as a more-fit animal's meal.

What I'm getting at is that they might be more fit in an aquarium due to their attractive colors (which encourage people to successfully encourage their reproduction) and ability to cope with bad water, but that doesn't mean that they're more fit than an wild type in the grand scheme of things. The ability to cope with aquarium conditions probably comes from generations being bred in aquaria, not being hybrids.

It would be interesting to see a comparison of hybrid vs "purebred" fecundity over generations in swordtails. I'm sure somebody's done this, and I might do a lit search later on today... I'll share whatever I find.

To the original question: Yes, hybrid offspring produced in home aquariums can have serious health problems, but they may not have any issues at all and actually do just fine. Erica's example of the Xiphophorus is one of the better known and well studied cases of hybridization in the lab and by aquarium nuts (I'm an academic and an aquarium nut). There are books that explain the genetics and the crosses required to get specific colors and fin development. Additionally, these hybrids do just fine even after many generations of backcrossing and line breeding, but without occasionally introducing an unrelated line (even if it's still an aquarium strain) and/or not paying attention to the genetics one is more likely to have health issues arise. Line breeding is eventually detrimental, whether it's in an aquarium or nature. I think Fundulus' point comes more from inbreeding issues than actual hybrid issues. If you took any species and started with one pair and exclusively line bred it, there would eventually be health problems. Some species would probably last more generations than others before health issues started popping up. The same thing goes for hybrids in the aquarium. Some crosses, like in Xiphophorus, can probably last longer without health issues from line breeding than say Lepomis macrochirus X Lepomis cyanellus. It would be interesting to line breed hybrids and parental species to look at any differences in the rate at which health issues and fecundity change.

To hybridization in nature: Most hybridization ends at the first generation, but sometimes genetic material can become incorporated (or introgressed) into one or both of the parental species, and even less frequently the hybrid offspring will selectively mate with each other and form a hybrid, or chimaeric, lineage. Introgression often happens when certain genes change some aspect of the parental lineage, like tolerance to a lower temperature, and the introgressed gene allows those individuals to do better. Although, if one species greatly out numbers another and hybridization occurs, introgression is often just a result of the numeric bias. There is a massive amount of literature on the causes and results of hybridization and I'm not discussing most of it, but there are plenty of examples ranging all the extremes. Rarely do chimaeric species form and most examples are from the plant world, but it happens in fish as well. Usually the chimaeric species uses a different habitat than either of the parental species. In sunflowers the chimaeric lineage uses a soil type that the parents cannot use, and in European Cottus the chimaeric species uses a different river habitat than those of the parental species. There are examples of chimaeric species of North American fishes, including the western Gila, but most instances have not been fully described.

It's true that under the allopatric speciation model, gene flow between diverging species is often thought to lead to homogenization, reduced fitness, and erosion of species boundaries. However, what about all the species that remain distinct despite numerous hybridization events? This heterospecific mating (hybridization) could facilitate speciation via the shuffling of existing variation, leading to the creation of novel allelic combinations. In turn, these novel allelic combinations could enable hybrids to take advantage of changing environmental conditions. One interesting paper I read discussed how hybridization has facilitated range expansion among many species of North American toads (Anaxyrus).

BTW, check your own genome for some evidence of this -- ~2-4% of our genome is from Neanderthals, including some important immunological genes.

Mixing genes -- what's the point?

To the original question, thank you bpkeck.

The rest of the debate is interesting, but it seams we are trying to solve questions with multiple variables.

Using the swordtail example, it wasn't the magic of mixing platy and swordtail genes that created colors, it was the selective breeding that kept intensifying the colors that already existed. Because Xiphophorus species interbreed so readily, the breeders had a wider spectrum of genotyps to work from. Selective breeding has been used to enhance or suppress certain characteristics for years. Years ago, you couldn't hope to keep discus unless you had elaborate water conditioning setups because they needed the high tannin, soft water found in the Amazon basin. Now, through selective breeding, we have discus varieties that flourish in almost any water quality. They also come in any color of the rainbow. If you had both wild and domestic discus in the same aquarium, the domestic would probably fare better, not because they were better fish, just better adapted to aquarium life. Put them in the wild, and things would probably not work out so well for them, as they are not well adapted to the wild environment (which includes more than water quality), and the more vibrant colors would work against their survival as well.

In the world of natural selection, success is measured by the number of offspring carrying your genetic code that gets passed on to successive generations. The more generations, the more successful.

Having bright red colors is going to work against you in the wild, but if your native habitat has pretty stable water chemistry, tolerance of nitrates is not going to increase your odds of success.

I agree with Rjmtx, the question is: better adapted to aquarium life, or life in their native habitat? I would bet that a domestic swordtail (X. helli X X. maculatus, or whatever its genetic makeup), would not have too many generations in the wild. But releasing this variant into the wild could really screw up a true X. helleri population by throwing in some genes that would not have been prominent without selective breeding, and could collapse the native stocks.

This is a big question without simple answers. I am interested to see what data you come up with, Robbie.

I diverged from the original question because it's so wide in breadth that there's no way to answer it. Hell, at least provide the genus you're wanting to know about. I was simply replying to the topic question -- "Hybridization Harmful?" and Ben's secondary response to hybridization in nature. Once fish are put into an aquarium they're already removed from their biological purpose anyways -- do with them what you will, after all you're the species in control....

Edited by blakemarkwell, 12 September 2011 - 12:29 PM.

BTW, check your own genome for some evidence of this -- ~2-4% of our genome is from Neanderthals, including some important immunological genes.

That's true, except that probably Neanderthals weren't really a distinct species but rather a northern population of modern humans. If you want to see a Neaderthal and you're of European descent, look in the mirror. (Yes, it's true, I'm breaking with the established orthodoxy on this. But that's a whole 'nother story.)

Bruce, we can't even decide what constitutes a species with extant populations, let alone extinct ones.... All I'm saying is that European and African Homo mixed genes. Whether you want to consider them the same species or different is all conjecture. Sorry, back to the original topic.

Edited by blakemarkwell, 12 September 2011 - 12:38 PM.

*Edit: Although I do know the point you are making -- if that much of our genome is from Neanderthals, than there were fertile hybrids and thus not a separate species under BSC.

If you took any species and started with one pair and exclusively line bred it, there would eventually be health problems. Some species would probably last more generations than others before health issues started popping up.

Inbreeding itself does not necessarily cause health problems. Inbreeding can't expose a recessive disease if there aren't the genes for recessive diseases in the founding fish you start the line with. There are fish at the Xiphophorus Genetic Stock Center that have been brother-sister paired for over 80 generations and are still going strong. Source: http://www.xiphophor...troduction.html

Several of the original genetic strains of platyfish and swordtails developed by Dr. Gordon in the 1930s still are available today; they are virtual genetic clones, the products in some cases of more than 80 generations of brother-to-sister matings.

What you might be thinking of when you mention inbred fish being more susceptible to health problems is that because the line is basically one fish cloned to make multiple individuals (like identical twins), any disease that can wipe out one fish has the potential to take out the entire population. In a diverse population, the death toll from any single disease usually maxes out at less than 100% of the population. Look at HIV; the people whose genotype has two copies of the Delta 32 mutation cannot be infected with M-tropic HIV and would not get sick. But when your population is all one big clone of itself, you don't have those rare mutations who would have survived the disease, and it is possible for 100% of the population to be wiped out by a single pathogen. Line and in breeding increase the homogenity and decrease the diversity of the population, making the population's disease resistance go down. But there don't necessarily need to be health problems in an inbred line just because it's inbred. Like with guppies. If you start out with genotypically straight spined fish and cull any random mutation curved spine fish you get, there's no reason for the population to get deformed looking. Inbreeding wasn't what caused the deformity; allowing a deformed fish to breed was.

Hybridization itself is also not necessarily harmful. Yes, hybridizing a certain X. hellerii with a certain X. maculatus will result in nearly all of the offspring developing melanoma. In that case, that hybrid offspring suffered for its hybridization. But are all hybrids of hellerii and maculatus necessarily worse off? No; if the black spotted gene is not held by the parents, then the hybrid can live quite a successful life. Define success as you will; I define it as not being genetically programmed to die at a small fraction of the otherwise possible lifespan due to an unavoidable melanoma that the fish is guaranteed to get due to its hybrid genotype.

Edit:
That is, of course, because X. hellerii and X. maculatus are very similar fish. A 'blood parrot' is an example of a hybrid that is physically deformed because its parents were not closely related. They get misshaped mouths and have problems eating because the mouth openings of the two parents were not the same shape, and the blood parrot gets something kind of in the middle. That is one example of a hybrid that is impaired simply because it is a hybrid.

Edited by EricaWieser, 12 September 2011 - 02:31 PM.

Sorry for taking so long to respond. Just so you know, the fish I was looking to breed were Orangespotted Sunfish and Northern Longears. What do you think?

Sorry for taking so long to respond. Just so you know, the fish I was looking to breed were Orangespotted Sunfish and Northern Longears. What do you think?

Lepomis humilis (orange spots) get to be around 15 cm (source 1). Lepomis megalotis (longear) get to be around 24 cm (source 2). Because one is almost twice the size of the other, I doubt a hybrid between the two would be a good idea. Research indicates that hybrid longear/pumpkinseed exist, though (source 3), with pumpkinseeds reaching a maximum length of 40 cm (source 4). So that's interesting. I'd like to hear a sunfish expert's opinion on this.

Sources:
1. http://www.fishbase....ary.php?id=3374
2. http://www.fishbase....ary.php?id=3377
3. http://www.dto.com/f...ciesProfile/319
4. http://www.fishbase....ary.php?id=3372

Edited by EricaWieser, 12 September 2011 - 04:16 PM.

Northern Longear (Lepomis megalotis peltastes) are smaller than Central Longear (Lepomis megalotis megalotis) so the size difference you reported won't be the same. Even with the size difference (I don't know what that has to do with hybridization to begin with -- heck, look at the size difference in parental species involved in Pararhinichthys bowseri), Nate just posted a putative L. humilis and L. macrochirus hybrid that he caught. I don't see any problem in trying out that combination, as Lepomis have already tried out damn near every one of them in nature.