Inbreeding to Improve Fish Strains

I often get questions regarding the inbreeding relationship of the tropical fish I sell. The typical aquarist wants to be sure that each strain consists of a group of fish that are unrelated, because inbreeding causes so many problems. Certainly, none of us want fish that will easily degrade with each generation. So, the answer must be to "always breed to an unrelated fish", or "if you must breed to a sibling, then certainly for no more than a generation or two." I"m not sure how these types of perceptions come about, but they are very common. In fact, I see experienced aquarists frequently warning against inbreeding and recommending frequent outcrosses to keep lines vigorous and fast growing. Are you ready for the shocking truth? This is untrue! Inbreeding does not automatically make for worse fish, or stunted fish. The inbred fish will not automatically inherit anything that makes them less vigorous or slower growing. Color is not always going to degrade and neither is the ability to breed prolifically.

So what really happens when we inbreed? Is there some gene that is mysteriously created that destroys the line? We know for a fact that some inbred lines of tropical fish exhibit many problems. Why is it? Well, the answer is really quite easy. When inbreeding, we double-up many genes. In other words, they end up in homozygous pairs at the same location on the chromosome. If the gene is recessive, then the trait is hidden if the fish is heterozygous for that gene. The trait expresses once the recessive gene is homozygous on that location. If it's a trait you like, that's great. If it's an undesirable trait, then that's bad, right? Well, not necessarily. If you want to eliminate undesirable recessive traits from your line, then one way to eliminate them is to make them appear, so you can select against them.

If the undesirable gene is a dominant gene, then eliminating it is easy. Every fish containing one, will express the trait and you can select against it. So, when inbreeding, the only way for a line to express undesirable traits, is for the person to fail to test for and then select against the trait. That's right, inbreeding can result in problems when in the hands of an inexperienced person, because they fail to select against the undesirable traits. In the hands of the knowledgeable breeder, it is a tool that brings out undesirable traits so they can be selected against. This is not exactly an easy process. It requires raising fish until the trait would normally be expressed, and sometimes you have to test-cross to determine if the gene is present in your breeding pairs. However, rest assured that in the hands of the knowledgeable aquarist, inbreeding is a useful tool. In theory, you could select against these undesirable genes, until you have a line of fish that only contain the genes you want and are virtual clones. In practice, this wouldn't happen, but the object is to come as close as you can to creating a clone with only the traits you want.

The biggest reason to inbreed is because it can be very beneficial when trying to "fix" a trait... If the trait is a desirable one that is determined by a recessive gene or maybe even multiple recessives, then the quickest and sometimes the only way to acquire its expression, is to inbreed. To outcross to new and unrelated lines, would make it less likely that they contain the same recessive positive genes. There are many instances where an outcross is almost guaranteed to eliminate the expression of a desirable trait for a generation or two, and possible even longer. Consequently, inbreeding is a useful tool to "fix" recessive traits that you want in your line.

Inbreeding is a tool that can improve a line when used properly. If you select fish with only desirable genes, then unwanted traits cannot appear as the result of the inbreeding. There is one negative that is sure to happen, given enough time with any line of inbred fish. The genetics of the immune system does not work the same as the other genes in the fish. A large combination of antibodies are produced as the result of the shuffling of immune system genes. The immune system genetics depends on a diverse number of genes to be shuffled. In this way, a great number of antibodies can be produced, enabling a defense against a larger number of pathogens and other invaders. Inbreeding leads to the tendency to lose some of these genes over time. This reduces the diversity of the immune system and good immune systems depend on diversity of genes. To counteract these losses, most breeders will set up separate lines of the same stock, and then make periodic crosses among the lines to maintain the immune system diversity. If one line loses a gene, another will give it back.

The use of careful selection of the next generation combined with the modified inbreeding we call line-breeding, has resulted in some very nice lines of fish that have been inbred for dozens of generations with no apparent degradation. In fact, many of these lines are continually improved along the way.

The reason inbreeding can be so effective with fish and probably not so effective with many other animals is because of the sheer numbers of fry that most fish produce. As fish breeders, we get to choose from many hundreds of offspring, which enables us to find the individuals (even if few in number), that contain the traits we desire and to "fix" them. It can require a lot of tank space and some very careful consideration combined with good judgment, but it is done all the time.

Next time I'll write about out-crossing and how it can also be used as a breeder's tool to improve their stock. Until then...

Enjoy your fish,

Steve Rybicki

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