Abstract
Practical relevance:
When compared with the number of individuals that make up a dog breed, the population within a given cat breed is very small. Therefore, to maintain a breed standard, a certain degree of inbreeding is necessary. However, when inbreeding reaches a certain threshold, it can lead to decreased fertility, which manifests as failure to conceive, smaller litter size, increased neonatal illness and neonatal mortality. Breeders should be encouraged to keep comprehensive records on breeding outcomes, including number of kittens born, neonatal vitality, daily kitten weights and kitten health at weaning. Commercially available DNA panels are available to inform and facilitate excellent breeding choices and can estimate the coefficient of inbreeding. Clinicians should include a review of the degree of inbreeding in the work-up for any cattery or cat colony experiencing decreased fertility.
Aim:
The objective of this article is to provide clinicians, especially those working with cat breeders, with an easy-to-understand guide to genetics and to demonstrate how inbreeding influences fertility and neonatal survival.
Equipment and technical skills:
Medical records and the pedigree of the cats in question are required to investigate cases of infertility that may be related to inbreeding. A DNA analysis kit that measures genetic diversity and health parameters can also be helpful; those that have been developed by geneticists and veterinarians at universities are preferable, as they include access to highly skilled genetic counselors and researchers who are open to working up newly discovered genetic diseases.
Evidence base:
The material provided is based on current literature and the author’s own studies examining outcomes in a closed cattery.
Introduction
Feline genetics, as well as the physiology of cat reproduction, has been catching up with the dog at a rapid pace over the past 10 years. Even so, a review by the author of small animal abstracts submitted to the European Veterinary Society for Small Animal Reproduction and the Society for Theriogenology meetings in recent years revealed that only 10-15% involved feline reproduction.
While the genetics of fertility has been extensively studied in endangered cat species, there are few studies in domestic cats. This may partly be due to breeding practices aimed at producing specific outward traits (coat color, hair length, facial features, etc) without regard to reproductive fitness, and in part because of the lack of understanding of inbreeding. Any purebred cat is a product of inbreeding. The question becomes how closely related can the potential parents be to maintain reproductive fitness, or does it matter?
What is inbreeding?
Inbreeding is defined as ‘the interbreeding of closely related individuals especially to preserve and fix desirable characters of and to eliminate unfavorable characters from a stock’ (merriam-webster.com) (Figure 1). Inbreeding is performed – in plants and animals – to increase the number of individuals with desirable physical traits within a population.
(a) Example of a typical four-generation horizontal pedigree provided by breeders. By convention, males are listed above females. The first column represents the offspring (Frank and Mitzi von Katzenjammer) of the parents Tom von Katzenjammer and Queen B in the second column. The paternal grandparents (Ed von Katzenjammer and Mirabella) and maternal grandparents (Philippe Le Chat and Catalina by the Sea) are listed in the top two and bottom two cells, respectively, of the third column. The great-grandparents are listed in the fourth column. Note that Ed von Katzenjammer was bred with both Georgia Bear and Mirabella, which makes him the common ancestor. (b) The horizontal pedigree in (a) converted into a simplified vertical pedigree. The numbers represent the cats in (a) and the pedigree shows outcrosses and inbreeding. Squares are males and circles are females. Individuals 4, 5 and 7 are products of outcrosses; however, individuals 4 and 5 are half-siblings as they share the same father. Individuals 8 and 9 are a product of inbreeding as they share the same ancestor – individual 1; their inbreeding coefficient is 6.25%
Inbreeding results in increased homozygos-ity, which means an increase in identical-by-descent alleles of any given gene (alleles inherited from the common ancestors). For example, the Siamese coat pattern (Figure 2) is a recessive trait, and two copies of the cS allele are needed for the trait to be expressed. 1 In order for all cats in a litter to have Siamese markings (dark points; ie, ears, face, tail and feet), the parents of the offspring must have two copies of the cS allele, genotype cScS. This is a very old mutation that dates back to before the creation of breeds, whereby one copy of the normal C gene mutated into the cS variant. This ancestral cat, with its CcS genotype, still looked ‘non-Siamese’. Being that it was the only cat in the beginning with this variant, it would have bred to a ‘normal’ CC cat, resulting in half of its offspring having the CcS genotype. Backcrossing the offspring to the first CcS cat, or breeding the offspring with each other, would have produced 25% CC, 50% CcS and 25% cScS (ie, 75% non-Siamese looking and 25% Siamese-patterned) cats (Figure 3). This demonstrates how inbreeding is necessary to keep an autosomal recessive desired trait fixed within a population. While this example only examines a single trait, this principle applies to all autosomal recessive and complex traits. To create a purebred animal, a certain degree of inbreeding is necessary to double up on desired traits.
A certain degree of inbreeding is required to create a purebred animal. The Siamese coat pattern is a recessive trait. Image by Andreas Lischka from Pixabay
Pedigree demonstrating inbreeding to preserve a desired trait within a population. Squares are males and circles are females. C is the normal allele, cs is the Siamese allele. Green symbols indicate normal (base color) cats, pink symbols are heterozygous (carriers) and blue symbols are homozygous for the recessive trait (ie, Siamese markings). Whole litters of Siamese kittens can only be produced by mating a Siamese tom with a Siamese queen
Coefficient of inbreeding
The coefficient of inbreeding is a measure of how closely related the offspring of a potential mating are, and is typically given as a percentage, with 0% being completely unrelated and over 60-70% being seen in inbred strains of laboratory mice (Table 1). In 1922, Sewall Wright developed a mathematical method to calculate the coefficient of inbreeding (F), called Wright’s equation (Figure 4). Today there are online calculators (such as the one provided by the Laboratory of Veterinary Genetics at the University of Montreal: labgenvet.ca/en/inbreeding-calculator). Additional information on relatedness, the inbreeding coefficient and genetic diversity can be obtained with DNA testing (eg, Wisdom Panel Complete for Cats; Mars).
Inbreeding coefficient of potential offspring of related individuals
The more closely related the individuals in the first column are, the higher the coefficient of inbreeding. This table assumes that the common ancestors are not themselves inbred; if they were, the coefficient of inbreeding would be higher
Wright’s equation. Fx = coefficient of inbreeding of the animal(s) in question; n1, n2 = number of generations from the sire’s and the dam’s side to the common ancestor, respectively; FA = coefficient of inbreeding of the common ancestor
Inbreeding and fertility
The advantage of inbreeding is that by increasing the homozygosity by descent (same alleles inherited from the common ancestor[s]) of desired traits, the uniformity of the breed and the ability to pass on desired traits is increased. The downside is that it may decrease fertility, litter size and neonatal via-bility, 2 and increase the homozygosity of deleterious genes, leading to a greater prevalence of genetic diseases, especially recessive ones. Most breeders breed for a specific phenotype (physical attributes) first, and fertility second. In a Swedish study, British Shorthair, Birman, Persian and Exotic Shorthair cats were over-represented when it came to dystocias and stillborn kittens. 3 This suggests that breeding cats for specific traits such as brachycephalism may not directly influence fertility but may lead to increased occurrences of dystocia, with potential loss of kittens.
So-called ‘inbreeding depression’ has been shown in multiple dog breeds and is defined as an increase in homozygosity of potentially deleterious genes, leading not only to a decrease in fertility but potentially also having a negative impact on immunity, size, growth rate and overall health. 4
To evaluate the link between inbreeding and fertility, the heritability of several reproductive traits may be relevant. Studies of Labradors and German Shepherd Dogs have shown that the likelihood of a puppy surviving to 7 weeks of age is more controlled by their genetic make-up than the likelihood of puppies being alive at birth. Genetic studies in Arctic foxes (Vulpes lagopus) also demonstrated that when the focus was on improving pregnancy and whelping rates, success increased dramatical-ly. 5 Studies in Boxer dogs demonstrated a relatively high heritability of birth weight, which is important as higher birth weights typically lead to increased survival of the neonate. A study in foxhound males demonstrated that higher inbreeding coefficients were associated with lower sperm concentrations, lower average total sperm counts per ejaculate, lower conception rates and a decreased number of live births. 4
From these studies it can be learned that fertility should be evaluated by parameters beyond simply sperm quality and pregnancy rates, including pregnancy length, incidence of caesarean sections, birth weights of the kittens, viability of the offspring at birth, 6 and 12 weeks of age, and kittens born with malformations or genetic disorders, as has been suggested previously; 6 and ultimately by inbreeding coefficients too. A high coefficient of inbreeding in pigs has been shown to be associated with low libido, 7 suggesting that the anecdotally low libido seen in Persian toms may also be correlated with a high coefficient of inbreeding.
A preliminary study (unpublished data) by the author on a single founding tom and his 245 offspring in a research breeding colony illustrates the impact of inbreeding depression on reproductive traits and that the health and viability of the offspring are directly correlated. The study, which examined the relationship between inbreeding coefficients over the tom’s entire breeding career of 7 years and 9 months until adoption, found that he produced a steady number of litters (ie, there was no decline with his advancing age). While not all findings were statistically significant owing to the small numbers of samples in some groups, there was a trend towards increased numbers of neonatal kitten deaths and malformations (Figure 5) with higher inbreeding coefficients (Table 2). Kitten litter sizes also decreased as the coefficient of inbreeding increased (Figure 6). Interestingly, this tom spent an equal amount of time with the queens that produced the kittens with the highest coefficient of inbreeding (F = 56.2%, Table 2) as he did with the other queens, yet far fewer litters, as well as kittens per litter, were produced. This suggests that a higher coefficient of inbreeding resulted in fewer viable pregnancies.
A 9-week-old kitten with hydrocephalus, eyelid agenesis and microphthalmia that was born into a feral cat colony. Inbreeding coefficients can be high if the colony has limited range. Courtesy of Julianne Grady, VMD
Correlation of inbreeding coefficient with kitten numbers produced by a single tom throughout his breeding life spanning 7 years and 9 months
The coefficient of inbreeding (F value) was 0% when the tom was a new arrival to the colony and rose to 56.2% as the colony became more inbred
*Percentage of total number of kittens per group
Box and whisker plots showing the correlation between the size of kitten litters and coefficient of inbreeding (F) for 74 litters sired by a single tom over a 7-year and 9-month period. There is a decrease in the median litter size (indicated by arrows) as the F value increases. N = number of litters per group; x = mean litter size; whiskers = highest and lowest numbers of kittens per litter; box = 75% of all litter sizes
Indeed, inbreeding depression has been clearly demonstrated in the Florida panther (Puma concolor coryi) as an example of a distant relative of the domestic cat. 8 Through molecular studies the investigators were able to show how inbred this cat population had become, and the consequences were abysmal semen quality, increased incidence of cryptorchidism and some non-fertility related genetic diseases. Another example of severe inbreeding depression concerns cheetahs from east Africa (Acinonyx jubatus raineyi) and southeast Africa (Acinonyx jubatus jubatus). 9 Over 10,000 years ago cheetahs were spread throughout the world but became separated, for unknown reasons, into small groups (bottlenecked) on the African continent, resulting in inbreeding.
A second bottleneck occurred during the 1800s when the cheetah populations were further diminished by hunters and farmers. Both subgroups of cheetahs are so severely inbred that the majority of the heterozygosity has been lost. Compared with the domestic cat, which has a general heterozygosity of approximately 4.6%, the east African cheetah has a heterozy-gosity of 1.4% and the southeast African cheetah only 0.04%. The effects on reproductive health are characterized by reduced fertility, high neonatal mortality and extremely poor semen quality. In this study, all cheetahs combined had an average of 20 x 106 motile sperm per ejaculate (vs 147 x 106 for the domestic cat). The percentage of morphologically abnormal spermatozoa in the cheetahs was upwards of 75% in this study, compared with 29% in the domestic cat. 9
Studies examining the dispersal patterns of cats since domestication and genetic diversity have shown that diversity is decreased in purebred cat populations by increasing homozygosity. 10 However, owing to domestication being a fairly recent phenomenon, genetic diversity within both the feline population as a whole and within breeds is still greater than in purebred dogs. 11 A more recent study 12 demonstrated less diversity overall in Eastern breeds of cats (Birman, Burmese, Oriental Shorthair, Peterbald and Siamese) vs Central or Western cat breeds. This may, in part, reflect the fact that certain breed standards permit different colors, markings and haircoats.
Coefficients of inbreeding have been determined in domestic cats13,14 and range from 1.98% to 5.5%, with Persians and Exotic Shorthair cats having the highest coefficient of inbreeding. Fertility was not examined, but in one of the studies 14 a decrease in births was noted among the Persian and Exotic Shorthair breeds over time. This is perhaps more likely due to a decrease in popularity of these breeds rather than infertility per se. However, what these studies do show is that inbreeding coefficients are much lower in cat breeds than they are in some dog breeds, a fact that can only benefit a feline breeding program.
Another study compared the co efficients of inbreeding of the Thai breed – an old-style Siamese – and the thinner-looking Western Siamese breed. 15 The population of Thai cats is much smaller than that of Western Siamese cats; however, as care was taken to use a large number of founder cats and cats from different regions when reintroducing the old-style Siamese, they have a lower coefficient of inbreeding than the Western Siamese breed. Breeding practices for the Thai breed are stringent; for instance, they are not to be outcrossed, as this may introduce genetic diseases from the other breed. In addition, in the Netherlands Thai cats must have an inbreeding coefficient of 0% over four generations and should not have a common ancestor over 10 generations, although this is very difficult to achieve as there are a limited number of sires available. While the study addressed methods of maintaining a healthy breeding population, it did not discuss fertility.
It is difficult to determine what constitutes a detrimental coefficient of inbreeding in terms of a precise number. In cattle, one study suggested that 12.5% should be the limiting coefficient of inbreeding. The same study demonstrated that there was a 0.14% reduction in a given trait such as milk production for every 1% increase in the inbreeding coefficient. 16 A study that examined the Swedish breeding cat population found that litter sizes varied by breed (ie, each breed had typical litter sizes); while the coefficient of inbreeding is unknown, this suggests a genetic basis to litter sizes. 3 Another very large study, conducted in France, involving 5303 queens and their 28,065 kittens, demonstrated that certain breeds had far lower pregnancy rates than others. 17 The authors suggested that either a higher coefficient of inbreeding or a genetic basis was responsible for the fewer litters born. Particularly affected were the Oriental Shorthair, Turkish Angora, Peterbald and Highland Fold breeds. As the inbreeding coefficients are not very high in these breeds, a genetic cause is more likely.
Role of genetic testing
Initial sequencing of the feline genome was published in 2007 18 and the quality of this sequence has since steadily improved. This has led to commercially available genetic testing panels (eg, Wisdom Panel Complete for Cat and Optimal Selection Feline; Mars), which not only allow genotypes for morbid or interest traits to be determined, but also average genome heterozygosity for use when choosing a breeding pair. Traditional pedigree analysis and calculations of coefficient of inbreeding are laborious and at times complicated. Genetic testing panels offer a simple, convenient method to screen DNA extracted from cheek swab samples. The DNA is examined for variants (changes in the DNA) linked to genetic diseases and some physical traits, allowing breeders to plan their breeding programs. The submitted DNA sample can also be compared with DNA results from other cats of the same breed to assess diversity and similarity.
Key Points
✜ Fertility data should include pregnancy rates, queening rates, litter sizes, kitten vitality at birth, and kitten viability at birth and weaning. The data should be carefully reviewed and detailed pedigree analysis and calculations of coefficients of inbreeding performed when designing a breeding program or working up cases with infertility concerns.
✜ A four or five generation pedigree may show a low coefficient of inbreeding while a 10-generation pedigree may reveal that there are many more inbred individuals within the breeding line, which would in turn increase the coefficient of inbreeding.
Footnotes
Conflict of interest
The author declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The author received no financial support for the research, authorship, and/or publication of this article.
Ethical approval
This work did not involve the use of animals and therefore ethical approval was not specifically required for publication in JFMS.
Informed consent
This work did not involve the use of animals (including cadavers) and therefore informed consent was not required. No animals or people are identifiable within this publication, and therefore additional informed consent for publication was not required.