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September 16, 2016 Archives

So, I own a pocket wolf.

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Oh, OK, I own a little black mini-poodle. But, like all dogs, he has the same number of chromosomes as a wolf!

There've been several articles posted recently about the evolution of domestic dogs. While we've tended to think that domestication didn't begin until humans began to settle down & develop agriculture, DNA analysis suggests that wolves and humans may have begun a relationship up to 100,000 years ago. And a paper published in Science back in June presents evidence that there were two domestication events, one in Asia and once in either the Near East or Europe. There's a nice visualisation and explanation of the doggy family tree here.

A few weeks ago, I was discussing domestication with RadioLive's Graeme Hill, and one of the questions he asked was, why do we have so many different body forms in domestic dogs, and so little variation in form in cats? Was it because the dog genome is more 'plastic' & susceptible to change? My answer was that I suspected it had more to do with the length of the species' association with humans. Cats are sometimes described as 'semi-domesticated', and our shared history may go back just 10,000 years (with the possibility, again, of at least two separate domestication events). Whereas 100,000 years gives a lot of time for selective breeding by humans to produce all those different dog breeds.

Which takes us to bulldogs - the subject of a Scholarship Biology question in 2014. Bulldogs were originally bred to drive cattle, and had the strength and the ferocity (and presumably also a high pain threshold!) to subdue an animal by grabbing its muzzle and hanging on. They were subsequently used in bull-baiting, a cruel 'sport' that the UK banned in 1835.

Instead, the dogs were then bred for show. While their physical characteristics remained pretty much the same (short & squat, very muscular, the familiar very short face/muzzle with deeply folded skin) - by 1860 they had already begun to develop, as a result of selective breeding, their now-familiar gentle, non-aggressive temperament.

Unfortunately, selection for that very distinctive body form has brought with it a whole host of inherited disorders. You've probably met a bulldog with that snorfling, stertorous breathing - this respiratory problem is called brachycephalic syndrome, due to the very short face & nasal passage. Other heritable disorders include:

  • hip dysplasia (seen too in other breeds, such as labradors), where the hip joint can partially dislocate - this one is due to polygenic inheritance ie there are a number of different genes involved. 
  • a hole between the two ventricles of the heart. This is called ventricular septal defect (VSD), and an animal must inherit 2 copies of a recessive allele to express this disorder. It's autosomal, which means that male and female bulldogs are equaliy likely to express it.
  • cryptorchidism - one or both testes remains up in the body cavity instead of descending to the scrotum. This one is an autosomal dominant trait - only one copy of the allele is required. 
  • and - with all those wrinkles - dermatitis, also considered to be an autosomal dominant trait. The dermatitis often leads to bacterial infections. 

Poor bulldogs :( 

The actual exam question gave this background information & asked students to discuss two things: how humans may have manipulated the evolution of bulldogs from wolves; and how further selective breeding could be used to try to eliminate EACH of the named disorders AND evaluate how effective this might be. For students intending to sit these exams: remember that when you're answering these questions you need to provide both 'evidential' statements and justifications for them. And to do that, you'll need to integrate the resource materials provided in the exam paper with your own biological knowledge. 

The first part's pretty straightforward: you might consider, for example, why humans would want to select for non-aggressive wolves (eg for help in protecting a human or group of humans from other predators). Or what about the founder effect, which would come into play because of that small population of proto-dogs? The small sample of the wolf gene pool found in those first 'dogs' could mean that particular alleles were simply lost in the dog population, while others could become much more common.

You'd then want to address the sort of things a breeder would focus on to get from generalised doggy form to the highly specialised bulldog: insensitivity to pain; the short, powerful, upwards-facing jaw; the squat, heavily-muscled body. Don't forget the explanation: that the physical features would allow the dog to drive or subdue much larger animals, and that selection for those traits, if they had a genetic component, would see those particular alleles to increase in frequency in the bulldog gene pool.

And of course, once bull-baiting was banned, selection for gentleness/docility came into play. Because that called for further inbreeding in the existing bulldog population, it would likely result in a higher frequency of any existing harmful alleles as well.

The second part of the question tests understanding of students' knowledge of concepts around inheritance. In some cases it's probably possible to remove the deleterious alleles from the bulldog gene pool - but as a result the dogs would diverge from what's currently viewed as the breed standard. For example, selective breeding for a longer, less-wrinkled face could reduce the frequency of brachycephalic syndrome and dermatitis - but the resulting animals would be much less bulldog-like!

The same approach is less likely to be effective for hip dysplasia, however, due to the polygenic nature of this disorder, because it involves multiple genes rather than a single gene locus.

But selective breeding could help with VSD & cryptorchidism. For example, a dog that doesn't express VSD is either homozgyous dominant (with 2 copies of the normal allele) or heterozygous. So at the population level, breeding heterozygous individuals will on average produce 25% of pups with VSD, with the rest not expressing the disorder. Using a test-cross would allow you to breed only from parents homozygous for the normal allele, but it would take more time.  

And with cryptorchidism, you'd avoid breeding from males who had the disorder (which would have very poor fertility anyway, I'm guessing), and from females whose sons had undescended testes (because these females would be 'carriers' for the allele - while cryptorchidism is a dominant trait, you're only going to see it in males). 

Of course, you could also try out-breeding with other dog breeds - but then, the resultant pups may well not conform to the bulldog 'standard'. 

I must say, it does bother me that adhering to a breed standard - a human construct - can perpetuate known health problems in a breed such as this.

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