Genetic Diversity in a Collection

[Krynn]

I've read up quite a bit about inbreeding in captivity and the wild, so I hope I can add to this conversation. Feel free to criticize:

I think the suggestion that reptiles can tolerate more inbreeding than mammals is slightly misleading. Most wild animals inbreed to a small degree without consequence, and almost all animals have some sort of method for avoiding inbreeding because of the negative consequences. Most inbreeding avoidance strategies include kin recognition (being able to tell who your relatives are), and male dispersal. There are of course a few exceptions to this rule (naked mole rats, ants and bees are the best examples of this), but I see no reason to believe that inbreeding does not effect reptiles in the same way as mammals.

Why is inbreeding bad?

As you all know, animal populations accumulate mutations. Some of these mutations are advantageous to the animal, but most of them are disadvantageous and decrease the survival of an animal. Many genetic traits work the exact same way as the color and pattern morphs that we all love. There are dominant traits, co-dominant traits, and recessive traits. I will give an example of how inbreeding causes problems, using kinking as an example of a disadvantageous trait. (Disclaimer: Im not entirely sure that kinking is caused by inbreeding, but I think it is an entirely possible explanation at least in some lines. Feel free to substitute kinking in this example with any other disadvantageous trait you can think of).

Lets pretend we are looking at a wild population, and a developing snake acquires a dominant mutation that causes kinking. Chances are that the snake will die very early on in its life. In the rare chance that the snake manages to survive and reproduce, 100% of its offspring will have kinking and also likely die. Natural selection quickly eliminates the dominant kinking gene from the population. You can imagine the same scenario where the snake instead has a co-dominant mutation, and the heterozygous form has mild kinking. The snake might survive and reproduce, but 50% of the offspring are mildly kinked. In the event that two kinked snakes mate, 25% of the offspring are the completely unviable super form. Before long, the co-dominant kinked snakes are outcompeted by the normal unkinked snakes and the gene is still removed from the population.

Now imagine the snake is born with with a recessive kinked gene. The snake is born completely healthy, and has 50% het kinked offspring that are also completely healthy. Every so often, a het kinked snake will mate with another het kinked snake and produce kinked young. For this reason, normal snakes still slightly out-compete kinked snakes, but often not enough to remove it from the population because it can always hide in the het form. Lots of disadvantageous recessive alleles occur at low frequencies in all populations, including us. Although the chance of having any particular mutation is incredibly low, the chance of having ANY such mutation is actually quite high. When mating with an unrelated individual, the chances that you are both het for the same mutation is incredibly low. The problem with inbreeding is that you share a proportion of your genes with your relatives, and the chance that you will express disadvantageous genes is much higher.


How bad is inbreeding?

So inbreeding is bad because it increases the chance that you express rare, disadvantageous, recessive alleles. However, as mentioned already in this post, most animals inbreed at least some of the time and are somewhat resilient to low levels of inbreeding. For the most part, inbreeding problems manifest themselves when you have many generations of inbreeding without introducing new genes from unrelated individuals. The likelihood of producing offspring that suffer from inbreeding depression comes down to what proportion of genes you share with your mate. Everyone shares 50% with their parents and siblings, 25% with their half siblings, 12.5% with cousins etc. By the time you get down to someone who is your third cousin you actually only less then 1% of your DNA with them.

So although I am certainly not advocating having children with your cousins, having inbreeding events every once and a while is not necessarily a bad thing (genetically speaking).

As far as snakes are concerned, inbreeding can be a problem when you are inbreeding snakes that are inbred to begin with. Inbreeding causes offspring to be more genetically related then you would expect (from the percentages above). If you bred half siblings together, normally they would share 25% of their genes, but in an inbred strain they may share 35-40% of their genes instead. Paired full-sibs that are inbred may share 75-80% of their genes.

I dont want to read all this. Just answer my question. Is it ok to inbreed my snakes?

This post is basically a long and comprehensive way of saying what several people have already said in this post. A little bit of inbreeding isn't all that bad. After all it is the only way that a breeder can start producing the recessive mutations that we all love. With that being said, its always good to throw in some genetic variation whenever you can (even if its a third or fourth cousin). When starting a breeding project from scratch, perhaps try to get your snakes from several different breeders rather then all from the same clutch. This will give you some opportunities to inbreed for a generation or two down the line with less risk.

Remember, the reason that sexual reproduction evolved was not so that animals could enjoy the orgasm. It was to promote genetic diversity. Let us all try to keep captive snake populations as diverse as possible, so we can avoid inbreeding depression now and in the future.

Cheers,
-Krynn