The following article is a small part of a seminar
given by myself for EACA this year. Having been asked to submit a
part of the seminar for publication in ‘The Round Up’ I felt the
subject of HMC Haplotypes is relatively new and yet of great
importance to the modern Breeder.
Has anyone reading actually heard of MHC Haplotypes?
Well until recently neither had I. However the more
I have come to read on this subject the more intrigued I am, MHC Haplotypes are essential to the continuation of life, and not only
that but it is essential we keep a high variation of such Haplotypes
within the breed.
I appreciate that a lot of people will find an
explanation of MHC way too complex and I have tried to simplify
things, however if I leave you behind don’t worry and
I apologise to
those that are ahead of me, I will use an example to help in a few
moments, so please bear with me.
The immune system is governed by the Major Histocompatability Complex (MHC). This group of genes is referred to
as a “complex” because they are all positioned close together on one
chromosome. This positioning virtually guarantees that the genes
will be inherited as a unit called a haplotype. The haplotype will
be passed to offspring without the usual shuffling that occurs as
genes are distributed into sperm or eggs.
Every individual possesses two MHC haplotypes, one
inherited from each parent. These haplotypes enable the immune
system to respond to and fight viruses and bacteria and they exist
in all species of mammal.
In any wild species and in humans, there are so
many variations within these haplotypes that many individuals have
MHC genes have a high mutation rate; this is for
good reason, because their diversity is VERY important to species
survival. MHC is natures answer to the problem of infectious
disease. A mere handful of allele would not allow the necessary
flexibility to face down an ever-evolving array of pathogens.
In most cases, each haplotype a dog has will differ
from the other, thus increasing its odds of having something in its
immune arsenal that will work against whatever nasty bug it may
encounter. Each individual will have two haplotypes but the overall
population of a species will have a great many. So when a new
disease comes along the species will survive even though individuals
will die out, because some individuals will always have the correct
combination of MHC alleles to fight of the attack. Survivors of
epidemics have the “right” combination of MHC alleles to combat that
particular infectious disease.
The same plague may occur again and again, but as
time goes by it becomes less virulent because those with inadequate MHCs will have died and been removed from the breeding population.
The high MHC mutation rate guarantees that there will be plenty of
ammunition for any new plagues that occur.
Now i shall try and explain this in a less
Lets us say each parent has two doctors bags filled
with equipment for fighting infection, and when a puppy is born he
inherits a Doctors bag from each parent.
OK let us open our doctor’s bags ...
Between the two bags we might get something to help
fight 6 viral infections, 7 bacterial infections and an
antihistamine to help combat allergies. But if both Doctors bags are
pretty much identical, the new puppy will have less medication to
choose from, if he becomes ill there is less chance of him finding a
cure in his Doctors bags.
Now we get to the important part.
To ensure MHC variety within a species, all
naturally reproducing species will avoid or significantly limit
inbreeding, and this includes line breeding.
Studies in mice have shown that females show a
preference for mates with dissimilar MHCs, thereby ensuring the
offspring sired by those males will have flexible immune systems.
Whilst no studies have been done on dogs to date, there are lots of
reports of bitches that refuse to mate with closely related dogs. In
an inbred individual, the chance that both parents have passed on
identical genes within the MHC increases. This situation diminishes
the body’s capability to mount an effective immune response. Such
dogs are more prone to infections and are more likely to suffer
autoimmune disease or allergies.
Going back to our Doctors bags, the closer the
breeding behind our puppies’ pedigree, the lower the chance of him
having variety in his Doctors bags. Overall the canine population
probably contains as much MHC diversity as it ever did, BUT when we
begin dividing the species into individual breeds, and then we line
and inbreed within these breeds the available MHC allele become
limited. Without diversity within the MHC, the dog is much more
vulnerable to disease. If the disease is bad enough, the dog may
die. If there were only a few possible MHC haplotypes in a breed or
species, the risk of an entire population being wiped out by a
virulent plague would be very high.
The regular use of popular sires over several
generations can play havoc with MHC diversity.
Since any individual can only have two MHC haplotypes, if a significant portion of a breed descends from a
relative few individual dogs the population may not be able to
respond effectively to the next canine plague that comes along.
Inbreeding has been the norm in domestic dogs for more than a
century. As we have said the technique is used quite effectively to
“fix” traits deemed desirable.
The practice of inbreeding to improve breed traits
has inadvertently led to a reduction of MHC diversity within the
individual breeds. Unfortunately most Breeders do not appreciate
that continued line Breeding is a form of Inbreeding!
Popular sire use is especially pernicious because
each such sire can have only two MHC haplotypes--nowhere near the
hundreds that exist in the canine genome. Therefore, when a
significant portion of a breed descends from one individual, those
dogs’ resistance to infectious disease or susceptibility to
autoimmune disease can be seriously affected.
No dog affected with chronic autoimmune disease or
serious allergies should be bred. Auto Immune diseases include
“Collie nose” which is an autoimmune dermatitis of the face. Another
growing problem created by a faulty immune system is generalised Demodectic Mange.
My seminar goes on to explain the amount of
in-breeding within the Breed today, and i think the truth of this
would alarm most of us, but that is another story!