2019 Breed REPORT
“What is needed is for breeders to think in population terms… to look at each breed genetically as a population and each breeder involved with that particular population as a conservator of that breed in partnership with others.” ~ J. Bragg
The purpose of this report is to help English Shepherd breeders produce healthy dogs today and for generations to come. The report contains data on English Shepherd breed health and assesses how current breeding practices compare to some recommended conservation breeding principles. Our hope is that this information will assist breeders as they consider both their personal breeding priorities and the needs of the breed.
ENGLISH SHEPHERD POPULATION: BRIEF HISTORY
The English Shepherd breed developed as working farm dogs in the United States. The breed’s versatility and popularity allowed a large, diverse group of founders to contribute to its development. The generous population size and diversity that characterized English Shepherds in the early to mid-20th century was not retained over time, however. By the late 1980s, the breeding population of English Shepherds had dwindled and the dogs were becoming hard to find. Starting in the mid-1990s and extending to the present, the breed population has been rebounding. Growth in numbers is necessary to maintain a healthy viable breed but growth in numbers is not enough. Growth must be balanced by health breeding practices so that genetic diversity is maintained and founder lines are not lost. More about this below:
SUMMARY OF KEY FINDINGS
➢ Steady population growth over the past 12+ years
➢ Litter sizes generous; fertility maintained to older ages (8+ years)
➢ Majority of breeding dogs produce only 1-2 litters
➢ Founder imbalance following a bottleneck in the 1980s – 1990s
➢ Average level of inbreeding exceeds recommended threshold
➢ Loss of genetic diversity due to lack of litter & sibling contribution
➢ Low generation time accelerating loss of genetic diversity
DEFINITION OF GENETIC DIVERSITY
Genetic diversity is an attribute of groups; it refers to the amount of genetic variation found in the group. The amount of genetic diversity in a group is influenced by the population size and the relationships (kinship) between members of the group. A large group of unrelated individuals will have a lot of genetic diversity; a small group from one family will have little genetic diversity.
There are several different ways to look at population size. One is to do a head count – how many dogs are there in the breed? When we study the genetics of a breed, however, what we really need to know is the number of breeding dogs – and, more specifically, the number of males and females contributing to the gene pool. As with all things related to genetics, there are some mathematical equations and assumptions that are necessary to determine what geneticists refer to as “effective population size”. Suffice it to say that the “effective population size” (designated “Ne”) depends on both the number and proportion of males and females in the breeding population, and can be used to predict the rate of inbreeding and loss of genetic diversity.
BREED POPULATION SIZE
Data Source: English Shepherd Club Registry, litter registrations, 2007 through 2018
The number of breeding dogs and litters registered more than doubled over the past 12 years. The numbers available do not include the entire breed population but there is no indication that the overall population growth trend is different for the subset of the breed registered with the ESCR than for the registered population as a whole over the past two generations (2011 – 2018).
Effective Population Size (Ne) Data
|Interval||# Sires||# Dams||# Litters||Ne|
|2007 thru 2010||114||146||228||256|
|2011 thru 2014||208||276||441||474|
|2015 thru 2018*||288||352||596||634|
*all intervals span 4 years to allow comparison – each dog is only counted once/interval; 2018 numbers are as of Feb. 2019
BREED FITNESS INDICATORS
Data source: ESCR, random sample of 100 litters from 2007 – 2017
Biologists use the term “fitness” to describe the reproductive characteristics of a breed. Common measures of fitness include litter size, viability of pups, and proportion of successful breedings.
English Shepherds produce good-sized, healthy litters and they retain their reproductive strength to 8 years for dams and to 9+ years for sires.
• Range: 1 – 14 pups
• Average = 7.5 live pups
• 50% of litters included 8 or more puppies
• 15% of litters included one or more stillbirths
• Overall, < 1% of pups were stillborn
DEATHS BEFORE 6 WEEKS OF AGE
• Average litter size at weaning = 7.1 pups
• 94% of pups survive to weaning
• 81% of litters had no puppy deaths
• 11% of litters lost 1 pup
• 8% of litters lost 2 or more pups
FERTILITY AND AGE
The age distribution for English Shepherd stud dogs and dams was calculated from the random sample of 100 ESCR registered litters. Roughly 50% of E.S. sires and dams were 3 years of age or less; relatively few dogs were over 6 years of age when bred. English Shepherds appear to remain fertile until older ages.
FOR STUD DOGS:
Range in age = 10 months – 12.5 years
Median age = 3. 25 years
Distribution of sires by age:
< 3 years at breeding = 43%
< 4 years at breeding = 64%
< 5 years at breeding = 75%
< 6 years at breeding = 84%
For sires age 6 – 8 years, average litter size = 8.2 pups
For sires aged 9+ years, average litter size = 7.6 pups
Range in age: 9 months – 11.3 years
Median age = 3 years
Distribution of dams by age:
< 3 years at breeding = 47%
< 4 years at breeding = 70%
< 5 years at breeding = 82%
< 6 years at breeding = 88%
For dams age 6 – 8 years, the average litter size = 8 pups
For dams age 9+ years, the average litter size = 4.6 pup
VARIATION IN NUMBER OF LITTERS
Data Source: ESCR litter registrations from fall 2013 – fall 2018
The genetic composition of the breed changes with each generation. Most English Shepherd sires and dams produce only one or two litters in their lifetime. The following charts illustrate the size of the parent population (blue bars) and their contribution to the next generation (red bar); the information is grouped into three categories – dogs (sires or dams) producing 1 -2 litters, dogs producing 3 – 5 litters, and dogs producing 6 or more litters. Dogs producing more than 2 litters have a disproportionately large impact on the genetic composition of next generation. For example, the 238 sires producing 1 – 2 litters over the 5-year interval contributed 315 litters whereas the 94 sires producing 3 or more litters contributed 404 litters in the same period of time.
Whether the change in composition of the gene pool is good or bad for the health of the breed depends on several factors including the overall size and diversity of the gene pool, the quality and characteristics of the dogs, and the number of offspring from each litter that go on to reproduce. It is important for breeders to be aware of shifts in the population, however, and to monitor the overall size and diversity of the breed to ensure that growth remains genetically balanced.
BALANCE OF SIRES AND DAMS
Effective population size (the breeding population, used to predict rates of inbreeding and loss of genetic diversity) depends on the ratio of unique sires to dams. A ratio of 1:1 – equal numbers of sires and dams – maximizes the effective population size and minimizes loss of genetic diversity. Any significant imbalance between the number of sires and dams automatically restricts the effective population size and increases loss of genetic diversity. Imbalances most often result from the use of fewer individual sires than dams. The so-called “Popular Sire” syndrome occurs when one or a few widely used stud dogs produce a large proportion of the litters in the breed.
The ratio of Sires to Dams in English Shepherds has been relatively stable over the past 12 years at roughly 3 sires for every 4 dams.
“Founders” are the individuals who establish the genetic foundation for a breed – the dogs from which current members of the breed descend. The genetic diversity of a breed is determined by the size and diversity of the founding population and the number of descendants representing them. Breeds do not have a single “founding” generation; often, populations undergo cycles of growth and decline. When breed numbers fall, descendants of the original founding group are lost; when the population starts a new growth cycle, the dogs present in the breed at that time become the new “founders” of the breed.
Population declines are referred to as “genetic bottlenecks” because of the constricting effect they have on genetic diversity. In order to conserve as much genetic diversity as possible, it is important to balance the contribution to the breed (offspring added to the gene pool) of the most recent founding generation as it emerges from a genetic bottleneck. The English Shepherd population experienced a genetic bottleneck in the 1980s. The breed has been recovering from that bottleneck over the past 15 – 20 years.
SIRE AND DAM LINE DIVERSITY
“There are two unique points of canine pedigree diversity that are not always paid much attention. These are the topmost and bottommost lines of the pedigree – the tail-male or sire-line and the tail-female or dam-line. They represent unique genetic content, held by the mitochondrial DNA and the sex chromosomes, much of which is transmitted only by those pedigree lineages… diversity in sire and dam lines is often quite scarce in purebred dog genomes.
Since these lines are not consistently scrutinized and conserved by breeders (because they are unknown if the breeder has not researched the pedigree all the way back to breed foundation), they are subject to changes in the frequency of their occurrences… Most breeds begin with a fair number of unique sire and dam lines. But some drift into prominence and others into obscurity, scarcity, and finally extinction.” ~ J. Bragg
The data and diagrams that follow are based on English Shepherd pedigree data; these data will soon be augmented by genetic information from the ES Diversity Study. The specific sample used for this analysis included all sires, dams, and registered litters over a 5-year period (2013 – 2018). These charts show certain founders are in danger of disappearing from the gene pool. The five most common sire founders are represented in 55% of the litters (399) while the five least common are found in only 2.4% of the litters (17 total). Similarly for dam lines, the five most influential founders are represented in 56% of the litters (403) while the five least common founders are represented in less than 1% of the litters (5 total).
Sire Line Representation:
Unique Sire Line Founders = 21 individual founders
Present Generation Descendants = 332 stud dogs
Litters produced over the past 5 years = 719 litters
The single most influential sire line founder is behind 66 of the 332 present generation stud dogs. His descendants produced 18% of the litters in this 5-year interval (132 of 719). The top five most influential sire lines combined account for 186 of the 332 stud dogs and 55% of the litters.
For more detail on sire lines, please see this appendix to this report.
MATERNAL (DAM) LINE REPRESENTATION:
Unique Maternal Line Founders = 35 individual founders
Present Generation Descendants = 423 dams
Total number of litters produced = 719 litters
The single most influential dam line founder is behind 89 of the 423 dams in this sample. Her descendants produced 21% of the litters in this 5-year interval (153 of 719). The top five most represented dam lines account for 232 of the 423 dams and 56% of the litters.
For more detail on dam lines, please see this appendix to this report.
Inbreeding refers to the breeding of closely related individuals. The precise degree of relationship that represents “too much” inbreeding in dogs is a matter of debate. Studies recommend keeping the “coefficient of inbreeding”(COI) less than 6%, which is roughly the degree of relationship between first cousins, to reduce the risk of genetic disease. Risk for genetic disease increases as inbreeding increases.
TRENDS IN COI FOR ENGLISH SHEPHERDS
The chart below is based on COI calculated from pedigrees for a random sample of 40 English Shepherd litters. The litters represented by the 20 blue bars were born in 2007; those in red were born in 2017.
|Inbreeding Coefficient (COI)||2007 Litters||2017 Litters|
This increase in inbreeding deserves further study. It is possible for a breed to maintain both high average levels of inbreeding and high genetic diversity if the breed population consists of numerous unrelated but inbred family lines. We can determine if this is the case for English Shepherds by analyzing the kinship (relatedness) of dogs from different family lines via genetic testing. One of the goals of the ES Diversity Study is to evaluate the kinship of different lines and, in turn, suggest strategies for preserving existing diversity.
PEDIGREE COI VS. GENETIC COI
English Shepherds breeders face a challenge when estimating levels of inbreeding:
• English Shepherd pedigree records contain many gaps (missing ancestors)
• Lack of full pedigree information generally conceals the extent of inbreeding
• To calculate an accurate COI based on pedigree requires accurate information on 10+ generations of ancestors
Many English Shepherds lack sufficient pedigree information to allow calculation of an accurate COI. In general, pedigree-based COI calculations underestimate actual inbreeding. An alternative method for looking at COI is to use genetic testing to directly measure the percentage of a dog’s genome that is homozygous due to inbreeding. Homozygosity exists when a dog inherits the same form of a gene from both parents; the more closely related the parents, the more similar their genetics and the higher the likelihood that they will pass on identical copies of each gene to their puppies. The “genetic COI” calculated from a genetic test tells us how inbred our dogs really are.
Results so far indicate that English Shepherd pedigree-based COIs are generally significantly lower than the measured genetic COI. The table below includes a sampling of COI from English Shepherds participating in ESBC study. Overall, the average measured (genetic) COI is more than twice the pedigree-based COI. This difference primarily reflects common ancestors (behind both sire and dam) that are not apparent from available pedigrees.
|Dog||Pedigree COI||Genetic COI|
WHY DOES COI MATTER?
The coefficient of inbreeding is related to the likelihood that a dog will be affected by genetic disorders. Most genetic mutations that damage the health of dogs act in a “recessive” manner meaning that a dog needs to inherit two copies of the mutation – one from each parent – to experience negative health effects. The COI predicts the likelihood of that happening. The higher the COI, the greater the likelihood that a dog will inherit two identical copies of a gene. Since dogs with high COI have more homozygous (identical) gene pairs, they are more likely to be affected by negative recessive traits.
While breeders are sometimes admonished to cull (remove from breeding) dogs that are discovered to carry a genetic mutation, the reality is that most genetic disorders cannot be eradicated by culling, and that culling of dogs that carry (have one copy of) genetic mutations has the unintended effect of reducing the essential genetic diversity within breeds. The most effective strategy for reducing the number of dogs affected by genetic disease is to maintain genetic diversity and to monitor inbreeding, and to avoid breeding carriers of a particular genetic mutation to other carriers of that same mutation.
“You really can not eradicate genetic diseases because they are mostly recessive… the best way to manage the health of a population is through maintaining genetic diversity.” ~ Laurie McConnell, PhD.
Genetic losses occur with every generation of purebred dogs. Many factors contribute to these losses – random chance, too few progeny contributing to the next generation, inbreeding and selection, genetic bottlenecks, etc. Because of this, the longer the generation time (interval between one generation and the next), the slower the loss of genetic diversity in the breed. A breeding population that turns over every 2 years is going to lose genetic diversity much faster than one that turns over every 6 years.
Put another way: genetic diversity is at its maximum in the founding generation of a breed, and declines with every generation that follows. Breeders can slow the loss of diversity – which, eventually, will reach a point where the breed requires an infusion of new genetics (outcross) to survive – by maintaining a high generation time for each litter (age of the sire plus age of the dam at breeding, divided by two). To quote Jeff Bragg, “four years should be considered an appropriate minimum (generation time)… five or six is better.”
Many different parental age combinations of can result in a generation time of 5 years: a 3 year old dam bred to a 7 year old stud dog, or two 5 year olds bred to each other, or a 4 year old stud dog bred to a 6 year old dam, and so on. If one parent is younger than 5 years, the other must be older to maintain a generation time of 5 years. The goal should be for an average generation time of 5+ years across all litters – not every litter will achieve that goal but genetic diversity is retained longer with a high average generation time.
In addition to the advantage of retaining genetic diversity, longer generation times provide breeders with an advantage when it comes to breeding healthy dogs. Serious genetic diseases often do not manifest until a dog is 3 or 4 years old. Extending the generation time past this threshold allows breeders to select for health and vitality more effectively.
It is encouraging to note that although litter size declines slightly with age, the average litter size at weaning remains approximately 7 puppies even with longer generation times.
|Summary Data||Generation Time|
|Range||14.5 months to 11.5 yrs|
|Generation Time||Proportion of Litters||Litter Size at Weaning|
|Under 4 years||69%||7.4 pups
|4 – 6 years||19%||6.9 pups|
|Over 6 years||12%||6.6 pups|
LITTER AND SIBLING CONTRIBUTION
Two conservation breeding principles that work in tandem to help maintain genetic diversity are: (1) avoid repeat breedings, and (2) ensure contribution of more than one sibling per breeding cross. While it is sometimes tempting to use the same sire and dam pairing over and over, doing so reduces the breeding combinations available in the future – particularly if the puppies of a repeated cross are sought after as breeding stock. The breeding that produced those offspring can dominate the gene pool in the future, resulting in a genetic bottleneck.
In order to retain genetic diversity from one generation to the next, breeders should avoid repeat breedings and strive to ensure that at least two pups from every litter – barring obvious defects – contribute to the next generation (“sibling contribution”). When only one puppy from a given mating contributes to the next generation, one half of the available genetic diversity in that cross is lost since each pup inherits half of its parent’s genes. In theory, if two puppies contribute to the next generation, the average loss is reduced to 25%. If more littermates contribute, the loss is diminished further. Lack of sibling contribution appears to be an important factor in the reduction of genetic diversity in English Shepherds (see figures below).
Data source: ESCR litter registrations
Study sample included all puppies born in 2010 and 2011
Age at follow up 7+ years
Sample size = 126 unique litters (1165 puppies total)
Group 1 (blue) – No Sibling Contribution: 43% 0f the 2010-11 Litters had NO puppies go on to reproduce = 0% of Next Generation
Group 2 (red) – One Pup Contribution: 36% of the 2010-11 Litters had ONE puppy that later reproduced = 36% of Next Generation
Group 3 (green) – Sibling Contribution: 21% of the 2010-11 Litters had TWO OR MORE puppies bred from = 64% of Next Generation
Total # of puppies from that group that subsequently produced a litter = 125
Sex distribution of puppies later used as breeding stock: 39% male, 61% female.
# Male = 49 (from 37 litters), meaning that 71% of litters did NOT contribute a male to the gene pool
# Female = 76 (from 55 litters), meaning that 56% of litters did NOT contribute a female to the gene pool
The skewed sex distribution, 2 sires to 3 dams in this study group, suggests that the ratio of sires to dams (discussed previously) may be shifting away from the desired ratio of 1:1.
These data indicate that:
(1) There is a potential loss of as much as 61% of available genetic diversity due to lack of sibling contribution.
(2) The loss of genetic diversity is worse for males than females.
(3) There is a reduced effective population size due to fewer sires than dams contributing to the breed.
A more complete analysis of kinship within the breeding population will help to determine more precisely how the lack of sibling contribution is affecting genetic diversity. The fact that 218 individual parents (unique sires & dams that produced the 2010-2011 litters) only contributed 125 puppies to the gene pool (breeding stock for next generation) means that these parents did not replace themselves, genetically, in the next generation.
“Breeding should be guided in such a way as to avoid reinforcement of known recessives whilst maintaining genetic diversity in the population.” ~ Jeff Bragg
The “genetic load” of a breed consists of the genes within the population that can damage the fitness of individual dogs. Some of these genes are known but many have not been identified; some will be common to many breeds (hip dysplasia, for instance) and others will be breed-specific or associated with particular bloodlines within a breed.
It is unlikely that the genetic load can be eliminated – the function and interaction of genes within the genome is not known and, even if it were possible to pinpoint purely negative genes, breeders cannot add or remove single genes from a dog or breed. Breeders can monitor genetic defects, however, and avoid breeding combinations that increase the risk of genetic disorders being expressed.
GENETIC MUTATIONS IDENTIFIED IN ENGLISH SHEPHERDS
Screening your breeding stock for common mutations is recommended if you do not know the genotype of your dog’s parents. The frequency of most of the following mutations in English Shepherds is not known; the only study to date suggested that 15% of English Shepherds may be carriers of the MDR-1 mutation.
MDR-1 – results in sensitivity to certain medications
Collie Eye Anomaly – may result in thinning of the retina and vision problems
Progressive Retinal Atrophy-prcd – causes deterioration in the retina with age
Degenerative Myelopathy – associated with risk for neuromuscular disease
Dilated Cardiomyopathy – associated with risk for heart disease in some breeds
Hyperuricosemia – increases risk of developing urinary stones
Trapped Neutrophil Syndrome – increases susceptibility to baceterial infections
Von Willebrand Disease, Type II – can increase risk for bleeding disorders in some breeds
In the past, English Shepherd breeders have had to make decisions without the benefit of information on the breed population as a whole. The breed’s unique history has resulted in a population that appears to experience relatively few genetic disorders. The diversity and vitality that has characterized the breed may be in danger, however, due to increased levels of inbreeding, dwindling numbers of founders, and failure to intentionally cultivate diversity in breeding stock. Fortunately, each of these problems has a solution and breeders can reverse the trends that threaten breed health.
Recommended Breeding Practices
The following list is excerpted from Jeff Bragg’s article, “Population Genetics in Practice.” You can (and should!) read the whole article online:
- Maintain balance of sires and dams
- Avoid incestuous matings (e.g., closer than first cousins)
- Understand and monitor coefficient of inbreeding (COI)
- Pay attention to the trend in COI
- Know the genetic load but don’t obsess over it
- Conserve sire and dam-line diversity
- Maintain high generation time
- Avoid repeat breedings
- Ensure sibling contribution
- Monitor fitness indicators (e.g., fertility and litter size)
- Avoid unfit breeding stock
- Monitor population growth
- Attempt founder balancing (in coordination with other breeders)
The next steps in our research will involve studying the genetic relationship between dogs in the breed. Pedigree is a useful guide but genetic testing of a representative sample of the breed will give us more information. We need your help in order for this to be a success!
If you would like to participate in the English Shepherd Genetic Diversity Study, please email English Shepherd Breed Conservancy.
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