Genetic Diversity, Problematics of Popular Sire Syndrome, and Benefits of Comprehensive DNA testing – Real-life Examples from the Coton de Tulear Breed

The world’s most complex and comprehensive genetic test for dogs, MyDogDNA, has been available for all dog owners for about a year. During this time, Coton de Tulears have been tested to the extent that it is possible to have a first look at the diversity level of this breed. This article also describes a real-life example, encountered in the breed, of how a genetic defect causing a genetic disorder can spread rapidly through popular sires. Today, genetic tests can be utilized for maintaining health-enhancing genetic diversity while simultaneously controlling the occurrence of individual genetic diseases – and the price is certainly no longer an obstacle.

Overview of genetic diversity in Coton de Tulear breed

The focus of modern breeding recommendations is increasingly shifted towards towards maintaining genetic diversity to ensure the breed’s viability and health. When evaluating the breadth of a breed’s gene pool and breeding material, it is important to look at, for example, the average coefficient of inbreeding, the effective population size as well as the over-use of certain individuals for breeding. A breed large in numbers is not necessarily genetically diverse if the amount of dogs used for breeding is small and a small number of dogs produce a significant proportion of all offspring. The breadth of the gene pool can now be estimated not only with comprehensive pedigree databases but also with methods of molecular genetics that enable measuring the level of genetic diversity (level of heterozygosity, i.e. in how large a proportion of the studied genomic sites a dog has inherited different gene forms from its dam and sire).

The Finnish company Genoscoper Laboratories has developed a comprehensive testing concept, MyDogDNA that enables utilization of molecular genetic information when evaluating a breed’s gene pool. Figure 1 demonstrates the heterozygosity level of the Coton de Tulear breed in the MyDogDNA database. The figure gives up-to-date information, currently based on a sample of 46 dogs. Median heterozygosity for the Coton de Tulear is 32.4 % (distribution 25.4 % - 36.0 %) while the median for all dogs in the database is 28.9 % (15.3 % - 40.1 %). This sample thus indicates that Coton de Tulear appears to be more diverse than MyDogDNA-analysed breeds on average. However, variation between individuals abounds and expanding the sample size would clarify the breed's genetic diversity situation furhter.

Figure 1. Genetic diversity distribution in Coton de Tulear. The blue curve reflects the genetic diversity distribution in the Coton de Tulear (N = 46). For comparison, the orange curve illustrates the current distribution in all tested dogs in the MyDogDNA database. Other dogs of the same FCI-subgroup offer another point of comparison (green curve).

Figure 2, in turn, reflects how similar or different tested Coton de Tulears are genetically. In the future, it would be worth viewing the image making use of more detailed information, such as knowledge about the origins of the dogs. For example, information about the countries of origin could reveal potential geographical differences within the breed. MyDogDNA database users also have free access to a breeder tool that, on the basis of MyDogDNA test results, helps identify the matings that best maintain genetic diversity and a wide gene pool. Modern genetic information can thus be a practical supportive tool for breeding, providing one selection criterion among others (such as character and structure).


Figure 2. Genetic relationships within Coton de Tulear breed. Each blue dog in the graph represents an individual dog. Dogs that are closer to one another are genetically more similar, while dogs that are further apart are more different.

Threats of popular sire syndrome - CMR disease in Coton de Tulears as an example

One of the biggest threats to the genetic health of any breed is the over-use of particularly popular sires. Popularity brought by success in dog shows, or general temptation to produce large quantities of puppies, can lead to a situation in which first and second generation descendants of the popular sire represent a significant portion of the annual registrations. The most popular sires in the Finnish Coton de Tulear population have often been dogs imported from other countries. Bringing "new blood" into the Finnish population is important for keeping the subpopulation as open and viable as possible. The import should, however, be done in a controlled and critical way by evaluating whether the dog will genuinely bring new gene forms into the country of import. Evaluating this can be done, for example, with the help of the MyDogDNA Breeder tool that operates on the basis of MyDogDNA test results.

The loss of diversity that occurs through popular sire syndrome and general narrowing of the gene pool is not the only threat for a breed’s health and vitality. The existing essential single gene tests that are relevant for the breed should not be forgotten. Imported dogs should be tested as extensively as possible for inherited problems – especially if they are used copiously for breeding, and strongly leaving their signature on the gene pool of the country of import.

A hypothetical example in the figure 3 illustrates how an inherited disease caused by a genetic defect can become common in the breed before the breeders notice the problem. The same principles leading to enrichment of a disease apply also if popular sires are not examined for known genetic defects of the breed that can already be tested for.

               Each dog carries hereditary genetic defects, undetectable based on its appearance. In this example, the circled individual suddenly becomes a popular sire because of success in the show ring, and passes on its genetic defect to a large amount of descendants.
     
  The genetic defect becomes widespread in only one generation. However, it is not yet noticed because the offspring of the popular sire are phenotypically healthy carriers (envisioning that the mode of inheritance for the disease is autosomal recessive and being affected requires two copies of the genetic defect).
     
  The problems begin in the second generation. The carriers of the first generation have passed on the genetic defect to 50 percent of their descendants and half-sibling matings, or breedings back to the popular sire, are already starting to produce affected offspring as carriers are mated with carriers. At this point, the problems are often refuted as ”bad luck”.
     
  One generation later, the problem may be obvious. The amount of carriers is so large that the probability to combine two carriers and produce affected puppies is very high. Even if close relatives have not yet been mated, the carriership has been widely spread in the breed through the popular sire and its descendants. In this case the disease will emerge later, when dogs that are no longer considered too closely related are mated.

 

Figure 3. Spread of a genetic defect through popular sire syndrome. Based on the article of Dr. Carol Beuchat "The Pox of the Popular Sire" (http://mydogdna.com/blog/pox-popular-sire), freely edited.

Examples of the spread of a genetic defect and its threats to the health of a population can be found also in real life. The breeding committee of the Finnish breed club ’Suomen Coton de Tulear ry’ has, with the consent of the relevant owners and breeders, recently made public a case in which an imported dog called G-Lavanono Pratele Jetynky (Nono) has been found to be affected for CMR. Another popular male, Merihelmen Komisario Palmu (Nitro) was found to be a carrier for the same disease. CMR (Canine Multifocal Retinopathy) is an eye disorder that can lead to retinal atrophy and blindness (Figure 4). The mentioned two dogs are not the only Coton de Tulears that carry CMR, as about one in ten MyDogDNA-tested Cotons carries the disorder. However, more dogs need to be tested to get a more reliable estimation of the population carrier frequency.

The real problem in this case is that both Nono (32 litters, 123 puppies, 177 second generation descendants) and Nitro (28 litters, 105 puppies, and 140 second generation descendants) have been widely used for breeding. CMR follows an autosomal recessive mode of inheritance and all of Nono’s 123 first generation descendants have therefore inherited one copy of the genetic defect and are thus at least carriers for the disorder. According to mathematical probability calculations, around half of Nitro’s offspring are at least carriers. Carriers don’t manifest symptoms, and the carriership doesn’t affect their lives. Carriers can be used for breeding but are recommended to be mated only with individuals tested clear for the defect in question.

Figure 4. A brief fact sheet on CMR.

Towards health and well-being of dogs with the support of genetic information

The breeding committee of the breed club ’Suomen Coton de Tulear ry’ has acted responsibly, and in manner that promotes the breed’s health and well-being by making public the examples described above that illustrate the disadvantages of popular sire syndrome. There is no reason to be afraid of the information provided by genetic tests in dog breeding. In many breeds, genetic tests are a routine part of a breeder’s work. Today, tests are easy to order and use as supportive tool in breeding, together with other selection criteria. Comprehensive panel testing can be considered as a future trend of canine genetics. Testing for as many known disorders of the breed as possible, from the same single, is a cost-efficient way of testing. Genetic diversity can be measured with the same analysis that also provides information on specific genetic traits, such as coat colour or coat type. The extensive MyDogDNA testing concept simultaneously also includes a unique DNA identification profile for each dog, which can be used for parentage confirmation.

The advantage of comprehensive DNA testing is the reasonable price that corresponds approximately to the amount of a stud fee. In addition to the CMR test, the MyDogDNA PASS includes also the PH test (Primary Hyperoxaluria) for the Coton de Tulear breed. The carrier frequency for PH in Coton de Tulears is about 8 % among MyDogDNA tested dogs, and therefore it is important to pay attention also to this disorder. It is a near future goal to bring the Banderas test also into the MyDogDNA PASS.

More information can be found at http://www.mydogdna.com/breeds/251/coton-de-tulear.

 

The article was produced in collaboration with Suomen Coton de Tulear ry.

Takaisin