Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
MHC haplotype diversity alone should not be the sole criteria.
#2
It is axiomatic that making analogies from one species to another is often a dubious activity.

We are rightly warned against anthropomorphism when observing animal behavior. We are told to see animals as they are and to do all we can to avoid projecting our human values and cognitive abilities onto other species.

In my hamster-crazed days, I had a hamster escape– as they are wont to do from time to time. One night, I was sitting on the floor watching television, and something crawled up onto my lap. I was bit startled, of course, but when I looked down, I discovered it was my errant hamster.

My 11-year-old mind decided that he came to me because he was bonded to me in the same way dogs were.

I was wrong. Syrian hamsters have no concept of social bonds. The only social bonds they have are between mothers and their nursing offspring. When they are done nursing, she cares for them not one bit.

This particular hamster had associated me with food. I had often fed him from my hands, and he grew quite tame. (He bit me only a few times, which was saying something for those hamsters). He had spent a few days foraging behind the walls, which I’m sure were not well-provisioned. Hunger set in, and because he associated my scent with being fed, he crawled onto my lap.

Now, it is easy to debunk the assumptions I made when I thought my hamster was bonded to me. Comparing hamsters to dogs is folly. It is what children do.

However, when it comes to the science on inbreeding, it is very common for people to make assumptions based upon different species.

It is just as dubious.

Jess Ruffner at DesertWindHounds writes about this issue in her latest installment that critiques the closed registry system.

One of the things one often sees in defenses of inbreeding and the closed registry system are quotations of studies that involve other species.

However, dogs have a very different natural history from other animals. Other animals have evolved different ways of dealing with genetic diversity issues. Some animals can handle very, very low genetic diversity and low MHC diversity. Others cannot.

The examples that different apologists misuse, misunderstand, or even misrepresent are fairly common. Cheetahs have low genetic diversity and low diversity within their MHC haplotypes, but because they have evolved through this genetic bottleneck and natural selection has been able to work on them, they are able to continue. It is not an ideal situation, but the cheetahs have been able to survive the past 10,000 years with very low levels of genetic diversity. Dogs in closed registry breeds have not evolved within this framework. Natural selection does not affect most Western dogs, which whelp indoors and get regular veterinary care.

Then there is the island fox (Urocyon littoralis) canard. Supposedly, dogs can do fine with reduced genetic diversity because the heavily inbred population of island foxes still has diversity within its MHC. Well, it turns out that those inbred foxes likely exhibited balancing selection. These foxes, like all wild dogs, have very strong inbreeding avoidance behavior, which would have allowed them to choose mates with different MHC genes. Those foxes that were heterozygous in the MHC were better able to survive over those who were homozygous.w And that process would have continued the diversity of the MHC in these foxes, despite being quite inbred. These animals are also not strictly monogamous, even though they do pair bond. That means that litters sometimes have different sires, which means litters can have littermates with very different MHC haplotypes.

(Many domestic dogs have also have inbreeding avoidance behavior. I remember when Cabbage, the long-legged JRT came in season, and Timmy, her four-month-old son, discovered what his equipment was for. Cabbaged savaged him every time he came near. Even when she was receptive to other males, she wouldn’t let Timmy near her.)

Of course, there are animals that have low diversity in the MHC and low genetic diversity all around. Jess mentions the inbred Swedish population of the European beaver, which has very low genetic diversity and very low diversity in the MHC I and MHC II. The beavers are thriving despite being derived from only 80 individuals. Although the European beaver population in Russia is not inbred, there are plenty of examples of beavers inbreeding in the wild. Young beavers do not go far from their parents’ territories, and thus, they often breed with relatives. European beavers lack the inbreeding avoidance behavior of wild dogs, and it is not that unusual to see pairs made up of parent and offspring. Because they have evolved with these behaviors, the animals have evolved a very high tolerance for inbreeding.

Inbreeding tolerance is not universal across species. Each species has its own peculiar tolerance for reduced genetic diversity.

Dogs in closed registries do not have the evolutionary heritage that resembles any of these often quoted examples.

Dogs are derived from wolves that have very strong inbreeding avoidance behavior. All cases of inbreeding in wolves have happened when the breeding female has died and one of the female offspring pairs off with her father. Wolves disperse from their natal packs– usually before they are three– and many individuals travel great distances to find their mates and territories. This behavior lowers the likelihood that wolves will mate with a relative. Wolves evolved as a high genetic diversity species, as the study that found the Goyet Cave dog discovered. Since the Pleistocene, wolves have lost entire MtDNA lineages. it was once a very genetically diverse species.

And as domesticated animals, dogs have also had a gene flow across vast areas. The camp wolves of the Pleistocene hunter-gatherer bands likely gave up living in territories, so they could easily follow a ready food source. This change meant that camp wolf genes could spread even more than normal wolf genes. Later, when man became pastoralists, there were gene flows from dogs that were following flocks of sheep and goats over vast distances. After all, shepherds always took their flocks into the high mountains during the spring and into the valleys during the winter. Some of their dogs likely ran off and joined local farm and village dog populations, and in this way, domestic dogs retained an unusually high amount of genetic diversity for a domestic animal.

Dogs did not evolve to have closed registries. They evolved to have diverse and dynamic gene pools, and that is why we have so many issues with inbreeding in this species. It has relatively low tolerance for inbreeding.

It was once thought that wolves did fine with low genetic diversity. The wolves of Isle Royale were trumpeted as the best example of a healthy population of wolves that had thrived in spite of a genetic bottleneck that came from being founded by a small founding population that wandered across the ice to colonize the island. However, inbreeding has exposed what is likely a deleterious recessive gene or series of genes that causes bone deformities. The majority of wolves on the island nowhave these deformities, and they are now having a very hard time killing moose.

But wolves can put up with more inbreeding than dogs can. That is because dogs are a domestic species and are not subject to natural selection. The wolves of Isle Royale might be able to make it if the ones with the deformed backs die off, but if the remaining wolves are all carriers in some way for that deformity, then it natural selection cannot save them in the way that it did for the cheetah. Because dogs are typically born in houses and well-cared for in the West, dogs with genetic disorders or even weird conformation that prevent them from surviving in the wild are capable of surviving long enough to reproduce.

The natural selection effects on cheetahs cannot happen to domestic dogs– unless you want to risk animal cruelty charges. They can be mimicked by breeding dogs outdoors, but it still doesn’t do much for their MHC diversity.

Finally, some animals are able to survive inbreeding because they evolved within very narrow range that has only so many different pathogens. The pathogens and the organism are able to co-evolve. That why species that have very small ranges don’t need a diversity in MHC genes.

I don’t have to tell you that doesn’t apply to either dogs or wolves. Wolves once had a vast range in Eurasia and North America (perhaps also North Africa). Dogs were domesticated somewhere in Eurasia and then spread throughout the rest of the world. They need diverse MHC genes.

So one must be very careful when reading articles about the viability of inbreeding in domestic dogs– especially when they make comparison to relatively successful wild populations that have experienced a genetic bottleneck. Each species has its own peculiar tolerance for inbreeding that results from its own peculiar evolutionary history and behavior. Comparing closed registry domestic dogs to cheetahs, island foxes, or beavers is simply not valid.

It is as bad as comparing a friendly domestic dog that comes to you for an ear scratch to an escaped hamsters that comes you for a sunflower seed. The difference in behavior of the two is the result of how these animals have evolved.

And as it is in behavior, so it is for inbreeding tolerance.

Anyone who makes claims otherwise is either misunderstanding the concept or is misrepresenting it.

See also:

MHC, DLA, WTF?
Reply


Messages In This Thread
RE: MHC haplotype diversity alone should not be the sole criteria. - by Versi - 04-12-2017, 05:16 PM

Forum Jump:


Users browsing this thread: 1 Guest(s)