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Impish Iggies

BRIEF MUSINGS ON DLA HAPLOTYPE DIVERSITY AND DISTRIBUTION

Tatum Clinton Miller

What a title, am I right? :P

I’m doing some research into canine immunogenetics right now, and re-read the chapter on immunogenetics inThe Genetics of the Dog ed. Ostrander & Ruvinsky to brush up on some concepts. While I was reading through it again, some statistics on DLA haplotype diversity and distribution which I had forgotten about caught my eye. Here’s a quote from page 106 of The Genetics of the Dog:

There is clearly much diversity to be found in semi-tame and feral street dogs, as has already been demonstrated in Bali street dogs (Runstadler et al.,2006). […] These dog populations are more outbred than most domestic dog breeds, and this is demonstrated when we assess the number of different haplotypes found in each group (where n = 50-100 dogs) and compare the frequency of the most common haplotypes. The highest haplotype frequency is around 12%, and there are, on average, 15 haplotypes with frequencies of 2-10%, with a further 30 or more haplotypes at lower frequencies.

Since street dogs are subject to much stronger natural selection and virtually no artificial selection compared to purebred dogs, the make-up of their MHC provides insight into the level of genetic diversity required to create a the ideal, healthy, highly effective canine immune system. The key to making the immune system work is the presence of many varied DLA haplotypes with relatively even distribution throughout a population. The fact that genetic diversity in and of itself, rather than deleterious alleles, can make or break the immune system is part of what makes immunogenetics so fascinating to me.

Later in the same chapter, there’s a brief discussion of the high degree of genetic diversity found in Salukis which is also interesting. The Saluki has the most DLA haplotypes found in any breed. One study found 31 haplotypes, while the analysis referenced in The Genetics of the Dog found 28:

There was one major haplotype at a frequency of 37.9%, plus 12 haplotypes with frequencies of 2-10%, and a further 15 with frequencies of <2%. The majority of atypical haplotypes occur in at least two dogs, and often in 4-7 dogs.

The Saluki is a good example of the kind of genetic diversity that remains possible even when breeding for specific traits. The two are not inherently mutually exclusive.

In stark contrast to the healthy diversity of the street dog is the genetic homogeneity of the Italian Greyhound, which suffers from the highest number of immune-mediated diseases of any known breed as a result. According to the research done by UC Davis, the breed is split into two genetically distinct subpopulations of American and European dogs with slightly different DLA haplotypes and allele frequencies. Together, the two subpopulations have 18 DLA haplotypes, with 5 of these being unique to the US and 4 unique to Europe. 

Of the two groups, the American dogs are slightly more diverse and have somewhat more evenly distributed allele frequencies. Of their 14 DLA haplotypes, 2 have a frequency >20%, 2 have a frequency of 10-19%, 5 have a frequency of 2-9%, and 5 have a frequency of <2%. Their most common haplotype has a frequency of 21.5%, and the second most common a frequency of 20.1%. All of the 5 haplotypes that are unique to the US subpopulation are very rare, with a frequency of <2%.

The European IG population has 13 known DLA haplotypes with a notably uneven distribution. 2 of these have a frequency of >20%, 1 has a frequency of 10-19%, 4 have a frequency of 2-9%, and 6 have a frequency of <2%. Their most common haplotype has a frequency of 31.5%, and the second most common a frequency of 24%. All of the 4 haplotypes that are unique to the European subpopulation are exceedingly rare, with a frequency of <1%.

Altogether, there’s a vast divide between the high level of DLA diversity and fairly even distribution found in the street dog and Saluki, and the highly homogeneous, skewed statistics of the IG. Even with the two subpopulations combined, the IG doesn’t even come close to the benchmark of natural canine genetic diversity. The implications of this are the subject of the article I’m currently writing, which will focus on IG health problems and their possible solutions.

In the meantime, enjoy some more tasty statistics from The Genetics of the Dog on DLA haplotype distribution in dog breeds: