Leslie A. Lyons, PhD
Department of Population Health & Reproduction, School of Veterinary Medicine, University of California, Davis
Davis, CA, USA
The age of genetic testing is upon us and it is time to teach "old cats new tricks". The population dynamics of breed development, whether for a cat, dog, chicken, or whatever other breed, leads to decreased genetic diversity within a population. This is part of the usual development of a breed. Decreased genetic variation tends to compromise overall health of a population and or individual, and can cause the increased frequency of genetic traits. On the one hand (or paw) these genetic traits are desired and help define the breed, on the other hand, unwanted traits can also tag along, accidentally presenting as common, often breed-specific diseases, defects and or anomalies. This essay will describe the dynamics used to construct a breed and how the balance and perhaps reversal of some of these dynamics need to be considered if a genetic test is used to eradicate traits from a population.
The population dynamics of breed development include: founder effects, reduced migration, bottlenecks, selection, inbreeding and genetic drift. A founder effect generally occurs at the initiation of a breed. All the other dynamics continue during the existence of a breed and need to be balanced to reduce severe health issues. Each type of dynamic will be explained as to how it reduces genetic variation and how it can be reversed to increase genetic diversity.
Breeding practices and registry rules can reduce the migration of genetic variation into a population. Migration is the influx of new genes into a population from another population by "migrating" individuals. Migration can basically be considered outcrossing to non-breed individuals. One common practice that reduces migration is the requirement of "long" pedigrees, meaning 5-8 generation pedigrees or even deeper. The deeper the pedigrees have been used to promote the concept of breed "purity", to separate variants of breeds, and to reduce the potential transmission of recessive traits. Genetic testing can help alleviate these fears. Parentage testing via DNA profiling (fingerprinting) can support the "true" pedigrees of animals. Recessive traits can pass through a breed regardless of the depth of a pedigree; however, genetic tests can identify carriers. Hence, depth of pedigree is less important since with a genetic test, one can prevent the breeding of two carriers, which will ensure the undesirable trait will NOT be produced. Hence, DNA profiling and carrier testing will be far more accurate for the identification of carriers or unwanted cats than the implementation of 8-12 generation pedigrees.
The "purity" issue remains a slippery slope and there is no way to directly answer, when is a Burmese a Burmese, or any other breed for that matter. If one unknown cat is present in the great-great grandparent generation, then an average of 6.25% of the current generation is contributed by that cat's genes. However, this is an average, and selection can influence that range from 0-50%. If we assume some 30,000 genes in the mammalian body, at 6.25%, still nearly 1,875 of the present day cat's genes are contributed by that great-great grand-parent. If extended to another 3 generations, the contribution drops to 0.78 %, but the range is the same, 0-50%. Hence, the contribution of the cat is influenced more by the selection by the breeder than the length of pedigree behind the cat. In the end, what breeds true is what makes a breed!
Deeper pedigrees often are used to separate varieties of a breed, such as American vs. European Burmese. Both varieties are recognized as specific breeds in the CFA, differing in conformation and accepted color varieties. Breeds will also become fractionated within themselves, often based on color varieties in cats. Sable Burmese are registered in the 400 series, while dilution colors and carriers in the 1400 series. This numbering system is meant to assist breeders who do not prefer dilution colors from having undesired offspring. Genetic testing can help to correctly register trait carriers. If a genetic test for dilution existed, testing can help breeders cross the color lines since only the breeding of two carriers causes the undesired trait. Hence, breeders could safely use only one carrier, leading to more variation but not the production of the undesired colors.
Selection is the prominent and consistent threat to genetic variation within a breed. Breeders need to select cats that will not only conform to type and be competitive in the show hall, but also be good queens, have good temperament, and overall good health. Unfortunately, breeders need to realize that disease traits may need to be maintained in a population in order to prevent selection-based bottlenecks. If a genetic test existed for the feline craniofacial defect that is common to contemporary Burmese cats, it would be likely that a high percentage of these Burmese will be carriers. Undoubtedly, there would be selection against many carriers. However, as for the reduction of many disease traits, neuter and spaying all carriers is not the best approach, as this generally throws the baby out with the bathwater.
The intensity of selection against an undesired trait needs to consider a variety of breed aspects. Strong selection against a highly prevalent trait can cause severe genetic variation loss in any breed, large or small. However, strong selection of a trait with a low frequency in a breed with a small population size can be just as detrimental. Two good examples are the Persians and Korats. Persians are an enormously popular breed through-out the world and approximately 38% of Persians have PKD. Even though the population is large, a swift eradication of all PKD cats would intensely bottleneck this breed, and other detrimental traits, such as progressive retinal atrophy, are awaiting their chance in the limelight. Conversely, Korats are a very small breed and some cats carry "points'. Even though the frequency of carriers is small in this breed, because the breed is also small, any eradication of cats may be too strong of a selection pressure. The Burmese breed is somewhere in between the Persian and Korat spectrum, hence selection against the head defect in the Burmese breed could be very detrimental if the eradication is to intense. The Burmese breed is moderately popular but in regards to the population size, is fairly small. American vs. European standards has prevented migration, Contemporary vs. Traditional lines has increased line breeding, as has color varieties. Thus, even though the number of individuals may be higher than Korats, these other sub-fractions cause additional concern for management.
Inbreeding is all inherent in a breed by definition; however, line-breeding is more severe. Although this method may more quickly result in desired type, if combined with popular sire or queen type bottlenecks, line-breeding can be highly detrimental to a breed's genetics. Burmese are highly susceptible to line-breeding effects due to the fractionation of the breed; American vs. European, Contemporary vs. Traditional, Sables vs. other colors.
Genetic bottlenecks occur when particular sires and queens contribute greatly to the ongoing generations. If not monitored, repeated bottlenecks can quickly alter the frequency of traits and greatly reduce variation. Electronic databases and an organized breed group can monitor and limit "over-breeding", hence reducing the loss of genetic variation. Breeders could use these databases to help define limits, thereby controlling how much of the ongoing population comes from only one or a few cats. This would take a coordinated effort by the breed councils and potential regulations to set limits and standards.
Genetic drift is the random loss of variation, which can be substantial in small populations. Generally, small litters prevent the production of individuals that represent all combinations of genetic types. Hence, all genetic variation is not passed onto subsequent generations. Breeders need to realize that large, healthy litters are also important to the maintenance of genetic variation and that small populations have a high risk of accidental loss of genetic variation. Thus, breed groups may need to recognize that there is a lower limit to the size of a population for it to survive and the smaller the population, the more intensive management may need to be required.
If you had a genetic test, what do breeders do? Balance, balance balance! Some breeders may quickly eliminate a trait, others may do nothing. Hopefully, a majority will balance and consider the good traits with the bad.
1. Open communication and education. Only the breeders can make a breed management program work.
2. Breed-wide health surveys so that all health concerns are considered in the breed management.
3. Database studies that identify popular sires and dams, hence potential bottlenecks and drift. Popular sires could be limited and contributions from all cats would be encouraged.
4. Consider improving migration (outcrossing) by reducing pedigree depth and crossing with related breeds. Use DNA profiling to confirm pedigrees.
5. Consider color carriers in breeding programs. Carriers can be detected with genetic tests.
6. Increase migration (new genes) with more foreign imports.
7. Slowly reduce carriers for diseases over many years. Carriers can be maintained in population with no risk of producing defects if testing is implemented.
8. Bank DNA now: Not only to find historical carriers but also for the development of future health and research programs. Cheek swabs are stable are room temperature or in a frost-free freezer. Banking DNA may help your areas of interest or other research projects in the future.
Proper use of genetic testing by Burmese breeders could lead to breeder cooperation, interaction and better breeder unity. Te Burmese breeders could become the leaders in proper genetic management and health programs and become the showcase breed for others. DNA testing results can be shared through semi-private webpages, health registries or even as part of registrations, as has been used by other species breeding programs. Advertise your DNA testing as part of a positive commercial aspect of your cattery. Parentage and color trait testing (agouti, brown, color) exist and dilution and the craniofacial defect tests are around the corner. Hence, Burmese breeders throughout the world should be rallying to develop a logical plan for the elimination of the production of kittens with head defects, or any other undesired traits. However, this plan must have a balanced approach that will lead to the flourishing and not the demise of a beautiful breed. Start with a few meetings at popular shows to open the discussions and formulate the concerns and potential approaches. If the breeders are willing to interact, certainly geneticists will help develop management plans.