Why Outcross?

Taken from "The Genetics of Immunity" By Heather Elizabeth Lorimar

There are many studies and reports available which explain the importance of genetic diversity in any given animal population. One such study, and the best explanation that I have read, was a report written by Heather Elizabeth Lorimar Ph.D at Columbia University. In it she clearly explained why outcrossing is an absolute necessity in order for us to maintain good health and vigor in our breeding stock. The following are excerpts from her excellent paper. I made efforts to find and contact Dr. Lorimar, without success, but because I believe this information is of critical value to breeders of tropical fish, I have decided to use her work as part of this site. The majority of information contained is taken directly from her work. Some small changes were made in order to make the content more applicable to the breeding of angelfish.

We have all heard of (and many of us have seen) the tragic results of doubling up on dangerous recessive genes. One way to avoid these recessive lethal genes is to outcross as much as possible. Unfortunately, the line into which you outcross may carry the very same gene that you are trying to avoid. It is quite possible, however, to weed out genetic faults in even the most heavily inbred lines. Scientists do it all the time. Hundreds of strains of mice, rabbits and other animals exist that are so inbred that each animal of the strain is genetically the identical twin (aside from sex) of every other animal of that strain. This allows scientists to study these animals without the interference of genetic differences between individuals. These animals carry no lethal genes and are extremely healthy in every way but one. They need to be kept in extraordinarily clear, nearly sterile environments because their immune systems are not capable of fighting off a normal range of diseases.

The immune system of all animals is absolutely dependent on genetic diversity. There are basically two kinds of immune responses; each is due to a specific antigen receptor molecule produced by one kind of white blood cell. There are cells called B-cells that release antibody molecules into the bloodstream. Antibodies inactivate or kill foreign particles (such as bacteria or viruses) that enter the body. There is also a second kind of white blood cell called T-cell receptor, which resembles an antibody and recognizes certain sick cells. These cells are very specific; one cell makes only one kind of antigen receptor molecule that will recognize only one virus, or one bacteria, or one kind of cancer. The most startling thing about this system is that for every kind of infection or cancer to which an animal could ever be exposed there is already a T-cell or B-cell in the animal’s body specific to that infection or cancer!

When an animal is immunized, it is exposed to an antigen, like killed or modified live viruses, the B-cell and T-cells in the animal’s body which make the antigen receptors against the antigen are stimulated to multiply and make more antigen receptors. The quantity of antibodies being made can be measured and is called the titer. When the animal is exposed to the actual disease to which it is immunized, it should have a high titer already built up and ready to destroy the virus. If the animal is exposed to a completely different virus, however, the antibody titer against the new virus – being completely independent of the other titer – will probably be insufficient to protect the animal from being infected. Luckily if the animal’s immune system is active and healthy it should be able to build a new immune response quickly enough to fight off most diseases within a reasonable amount of time.

There are probably millions of gene coding for a specific antibody or T-cell receptor in every adult animal. The problem is that there is not enough room on the chromosomes for all these genes. We animals have a very clever method of circumventing this paradox; our chromosomes don’t initially have complete genes for antibodies; instead, they have lots of little gene segments that each B-cell and T-cell cuts and splices together to make a whole gene. Immune system cells are the only cells, which shuffle their own DNA. If it happens elsewhere it would be very dangerous, resulting in all kinds of lethal mutations, otherwise we would not be able to fight many diseases.

In the following discussion I am arbitrarily assuming an original (germline) DNA containing six kinds of gene segments (it actually takes seven kinds of segments to make an antibody molecule), each segment having ten different choices (in antibodies this number varies between four and several hundred depending on segment type), this is to make the math simpler than nature intended. My hypothetical six gene segments can then produce 10x10x10x10x10x10 –one million different antibodies!! If the animal has completely different gene segments on each chromosome, remember that chromosomes come in pairs, and assuming that each cell uses only one chromosome at a time, then the animal has two million possible antibodies. We will for the sake of simplicity, only discuss antibody genes. T-cell receptor genes are very similar to antibody genes so this example can be multiplied by two.

If both chromosomes are the same then the possible number of antibodies is immediately halved to one million. If the animal is further inbred it is possible to start losing individual gene segments, for instance, due to genetic events called crossovers. Each gene segment lost represents the loss of thousands of potential antibodies. The first segment lost immediately cuts the possibilities from one million to nine hundred thousand. When this happens the animal starts losing the ability to fight some diseases. To make matters worse, when all the animals living in one place are closely related, as happens in some inbred lines of guppies or angels, the whole group can be extraordinarily susceptible to one or more diseases. If vaccination were possible, it wouldn’t solve the problem because if there is no antigen receptor (or antibody) to respond to the disease, there isn’t one for the vaccine either.

A well-known example of this kind of sensitivity to disease, caused by a lack of genetic diversity, has occurred in wild and captive populations of cheetahs. There are only about twenty thousand cheetahs left in the world. Captive cheetah breeding programs have been plagued by low birth rates and high infant mortality. To add insult to injury, cheetahs have proved to be very susceptible to respiratory virus called FIP. Most cats that are exposed to the virus that causes FIP do become infected with it, but more than ninety percent of them usually fight off the virus and never develop the symptoms of FIP. Cheetahs that are exposed to the virus experience about a fifty percent mortality rate. Dr. Stephen J. O’Brian (prominent geneticist) and his colleagues examined the causes of the cheetah’s problems (published in the May 1986 issue of Scientific American). They found that individual cheetahs are nearly identical to each other. They are so identical to each other that cheetahs born thousands of miles apart from each other, in the wild, were incapable of rejecting skin grafts from each other – a trait normally seen only in identical twins. They rejected grafts from other species of cats normally, indicating that this was not due to a peculiarity of their rejection ability. Dr. O’Brian concludes, with the addition of other supporting evidence, that the cheetah population must have been narrowed down to far too few breeding individuals at some point resulting in profound loss of genetic diversity. As a result, these cats are in danger of extinction.

We as breeders must be careful not to “fix” immuno-deficiency while we are trying to “fix” type. Fortunately, this is not too difficult. When you want a trait such as eye color, go to more that one source. Remember, you won’t lose type in an outcross unless the animal you cross to lacks type! Most important, watch for the danger signals of excessive inbreeding. They are:

1. Consistently low fertility in males or females.
2. Small spawn size, even in young, healthy females, which are in their prime.
3. Deformities from one line, while other lines are consistently producing healthy fry under the same or similar conditions.
4. Loss of high proportion of fry to one disease. If fifty percent of fry or a group of adults dies of a single infection or defect, there may not be enough immunological diversity in your line.

If we can keep ourselves open to crossing out to other, preferably very unrelated lines and conversely let those unrelated lines cross into our own precious lines, we can do nothing but increase the fitness of our angelfish.