Genetic Diversity

Genetic diversity helps give Mastiffs a future as well as a past!

Way back in a distant history, part of my profession was to look at conserving wild animal breeding populations and their genetics. Maintaining small, closed populations of rare, often endangered species. So, thinking an article on this subject for the readership of the Mastiff Association's Newsletter may be of some interest, and possibly encourage debate & forward planning. Converging in a brief introduction to small population genetics and its management, with contemplation for our Mastiff folk and the longevity of our much loved breed and household companions.

Well; what could possibly be the issues, associated problems, and viable solutions? Recently in the evolution of our domesticated dog breeds the incidence of genetic ailments seems to have increased dramatically, despite the prevalence of genetic testing with positive endeavours of owners, breeders, canine organisations and with wide access to new information technology Why is this?

Breeding programmes, despite evidently being proactive in eliminating; health issues, fertility problems, loss of type, soundness and extending longevity, could they be having an adverse, opposite and detrimental effect in the long-term?

As genetic diversity from a closed breeding programme is lost generationally, only considered on a line-by-line basis (here and now) with more and more individuals bred from each population being removed from breeding, which in turn affects the long-term sustainability of a species/breed.

Naturally of course we are very selective with the use of youngsters in our breeding programmes using only the genotype/phenotype that fits the required breed, and eliminates less idealised individuals from the gene pool (breeding stock), for many different and often personal reasons. Unfortunately in selecting only our perceived best much less than a quarter of youngsters produced will possibly contribute towards the future gene pool, which could eliminate much more than three-quarters of the bred population. Ultimately this results in the un-bred individuals not passing on their individual genes, removing the possibility of genetic improvement and diversity, with the result that those genes (perhaps) being lost forever narrowing the gene pool into a bottle neck, year after year and creating more relatedness/homozygosity of the remaining (breeding) population.

This also can be compounded by popular sire syndrome (individual males being used exclusively). Getting round this issue in an endangered breeding programme, the breeding ‘herd’ ideally should have 50-50 individuals of both male and female sexes being used for breeding equally, often pertaining in endangered species breeding programmes, that isolated bachelor populations are set up to be utilised. As most herd/packs only have one dominant male breeding with several females (as male dominance aggression can be a welfare issue) hence the use of bachelor groups where different males are rotated within the female herd/packs to maintain healthy genetic diversity.

The founder population, which generally starts with a small, but open genetic sample size and then annually continues to reduce with the subsequent generational in-breeding, linebreeding to fix type, size and temperament required for the original job/niche in each individual breed standard. Fortunately to some degree Mastiffs did come from a large genetic baseline (due to the differing types used for different uses/jobs) and have had influxes post war-time of new blood from other similar breeds relatively recently, in comparison to their long ancestral lineage. (although, additionally its breeding selection has been narrowed a lot longer being an ancient breed).

Furthermore, there is the subsequent generational loss of diversity from individual littermates being denied from breeding programmes. With the exclusion of their genetic makeup, and since all dogs carry hundreds of recessive mutations, which generally cause no problem, if there is a single copy for the individual that carries them, and the other copy of the allele is normal. When two copies of those mutations are inherited from both parents concentrated by them sharing similar genetic makeup parentage (in-breeding), more youngsters are homozygous which increases the chances and frequency of recessive mutations.

Loss of diversity has the effect of weakening and bringing forth an inbreeding depression affecting and producing low fecundity (birth), high mortality (death) rate especially of neonates (babies), and reduced longevity. This also can cause polygenic issues (many similar but different genes to culminate, causing possible health issues): premature organ failure, immune disorders, even cancers. This in turn results in the general population failing becoming less adaptable and unable to maintain vigour. Animals that are very closely related can often produce offspring which are homozygous (two copies of the same gene from both parents) which in turn could increase disease-causing deleterious mutations to manifest.

Historically, this was witnessed and researched by scientists on an isolated pack (being shut off from the mainland) in the 1980s of 50 wild Wolves ‘Canis lupus’ (wild cousins of our Mastiff) over a period of time which dwindled down to a handful in 2017 these had limited new blood and low genetic diversity within the pack, which after a number of years were observed finally failing, having pups being born with congenital abnormalities, even despite having the natural selection process of survival of the fittest (only the strongest/fittest survive and breed). With the onset of a project in 2019 to introduce new genes/blood of 19 (healthy, naturally selected) individuals the pack now consists of 30+ individuals, a healthy population for this closed Island living pack. Again, into the future new blood will have to be introduced at a later date.

As modern dog breeders we have at our disposal health testing and DNA tests that are valuable tools to see disease trends which can help with reducing or eliminating the incidence and concentration of the specific issues. But it cannot be solely relied on or the only tool to utilize (only short term, quick results are produced). Holistically on a long-term basis, to protect the health and longevity of a closed population/breed, we need to use broader long-term solutions and management of a breed’s genetics, one that maintains diversity which therefore promotes and protects the general health and immunity of our dogs over an extended period of time.

Other ways to assist and maintain diversity, in addition to and alongside other techniques- Semen freezing/banking (frozen zoo) could be an alternative to maintaining diversity into the future without using all the present population’s individuals within a live breeding programme, hence maintaining genes without building a huge living population, and relatively cost effective when considering the importation of a stud dog, puppy, or frozen semen which can end up being very costly and on occasion little or no guarantee of fertility or offspring.

Additionally, and with the important proviso that these populations are genotypically clean, (free from disease) keeping small populations apart (island populations) especially of rarer genes/genetics for protracted periods of time with periods of gene concentration, periodically interspersing/outcrossing with other unrelated island populations can have the effect of producing healthier offspring (sustainable vigour) for a short limited period, before the whole process needs to be done again and again to maintain diversity, the rare genes and halt genetic drift. Yet with such small numbers of large kennels of our expensive giant breed, this means it’s rarely available and or practical in our modern financial world. And would need to be heavily health tested with all available tests to stop doubling up and concentrating on health issues.

Alternatively, into the future a possibility could be Biobanking/cloning from suitable skin material that has been collected years previously, either from living specimens or early post-mortem (less than 5 days) Resurrecting genes that have been lost to the past. A further controversial and possibly undesirable technique could be outcrossing to a similar related breed of healthy ancestry. Which would be necessary if diversity was lost on a dramatic scale.

Pedigrees and pedigree analysis with the associated inbreeding coefficients can often give a misleading result to genetic breeding and diversity, as ancient pedigrees can be missing or incorrect as well as genes/DNA being random and as different within sibling pups from the same litter, than as dogs from distant pasts (showing less relatedness). This brings us onto getting dogs DNA diversity checked with a simple buccal DNA cheek swab, to see which individuals are showing rare and valuable diversity within the populous, helping to keep the genetic loss down and maintain genes which could be advantageous and useful into the future.…DON’T THROW BABY OUT WITH THE BATH WATER!

Obviously, this is a personal and simplistic view of population genetics and genes can vary in such an irregular and random way. All we can do is our best to reduce the frequency and concentration of homozygous recessive traits (duplicate genes carried by related ancestors) in our long-term breeding programmes. Without losing important genetic diversity for that short-term fix. Removing healthy dogs from a breeding programme reduces genetic diversity, with the exception, importantly of when it’s to assist in reducing certain deleterious/detrimental genes, which thankfully is made more simplistic, aided by tools like; DNA health testing, with a known outcome which will factor into the future and over many generations, reducing these and many other testable issues. Conceivably much needed in a small generational time span.

Conceivably (pun intended), we could use all these techniques combined, maybe this will be the way forward for long-term survival of our precious, ancient, vulnerable and native breed.

NB: Originally the majority of this article was written several years ago, some subsequent information/research may have been updated.

Colin R Hill

MA Health & Committee Member