A Genetic Assessment of the English Bulldog Niels C
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Pedersen et al. Canine Genetics and Epidemiology (2016) 3:6 DOI 10.1186/s40575-016-0036-y RESEARCH Open Access A genetic assessment of the English bulldog Niels C. Pedersen1*, Ashley S. Pooch2 and Hongwei Liu1 Abstract Background: This study examines genetic diversity among 102 registered English Bulldogs used for breeding based on maternal and paternal haplotypes, allele frequencies in 33 highly polymorphic short tandem repeat (STR) loci on 25 chromosomes, STR-linked dog leukocyte antigen (DLA) class I and II haplotypes, and the number and size of genome-wide runs of homozygosity (ROH) determined from high density SNP arrays. The objective was to assess whether the breed retains enough genetic diversity to correct the genotypic and phenotypic abnormalities associated with poor health, to allow for the elimination of deleterious recessive mutations, or to make further phenotypic changes in body structure or coat. An additional 37 English bulldogs presented to the UC Davis Veterinary Clinical Services for health problems were also genetically compared with the 102 registered dogs based on the perception that sickly English bulldogs are products of commercial breeders or puppy-mills and genetically different and inferior. Results: Four paternal haplotypes, with one occurring in 93 % of dogs, were identified using six Y-short tandem repeat (STR) markers. Three major and two minor matrilines were identified by mitochondrial D-loop sequencing. Heterozygosity was determined from allele frequencies at genomic loci; the average number of alleles per locus was 6.45, with only 2.7 accounting for a majority of the diversity. However, observed and expected heterozygosity values were nearly identical, indicating that the population as a whole was in Hardy-Weinberg equilibrium (HWE). However, internal relatedness (IR) and adjusted IR (IRVD) values demonstrated that a number of individuals were the offspring of parents that were either more inbred or outbred than the population as a whole. The diversity of DLA class I and II haplotypes was low, with only 11 identified DLA class I and nine class II haplotypes. Forty one percent of the breed shared a single DLA class I and 62 % a single class II haplotype. Nineteen percent of the dogs were homozygous for the dominant DLA class I haplotype and 42 % for the dominant DLA class II haplotype. The extensive loss of genetic diversity is most likely the result of a small founder population and artificial genetic bottlenecks occurring in the past. The prominent phenotypic changes characteristic of the breed have also resulted in numerous large runs of homozygosity (ROH) throughout the genome compared to Standard Poodles, which were phenotypically more similar to indigenous-type dogs. Conclusions: English bulldogs have very low genetic diversity resulting from a small founder population and artificial genetic bottlenecks. Although some phenotypic and genotypic diversity still exists within the breed, whether it is sufficient to use reverse selection to improve health, select against simple recessive deleterious traits, and/or to accommodate further genotypic/phenotypic manipulations without further decreasing existing genetic diversity is questionable. Keywords: English bulldog, Health, Genetic assessment, Short tandem repeats (STRs), Maternal haplotypes, Paternal haplotypes, Internal relatedness (IR), Dog leukocyte antigen (DLA), DLA class I and II, Runs of homozygosity (ROH) * Correspondence: [email protected] 1Center for Companion Animal Health, School of Veterinary Medicine, University of California, One Shields Avenue, Davis, CA 95616, USA Full list of author information is available at the end of the article © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Pedersen et al. Canine Genetics and Epidemiology (2016) 3:6 Page 2 of 16 Plain English Summary aggressive temperament than the common indigenous The English bulldog is one of the most popular breeds dogs of the period. Therefore, their ancestors were pre- in the world because of its child-like appearance and de- sumablymastiff-typedogsoriginallybredinAsiafor meanor. The alterations in body type and behavior needed their strength and aggressiveness. Controversy exists as to create the breed have required physical changes well to whether these Mastiffs were crossed with breeds beyond its village dog ancestors. These changes have oc- such as the Pug to make them more effective at bull curred over hundreds of years but have become particu- baiting [1]. The first description of a Bulldog as a dis- larly rapid over the last decades. Unfortunately, popularity tinct entity from the Mastiff was in 1631 in a letter does not equate to health and there have been increasing writtenfromMr.EatoninSpaintoafriendinEngland pressures on breeders to moderate the extreme physical [1]. The bulldog was further genetically altered over the changes that now affect the breed and its health. Im- 500 year period of bull baiting by “selection of the fit- proving health through genetic manipulations presumes test,” with emphasis on increasing agility and putting that enough diversity still exists to improve the breed more power and weight into the head and front end to from within, and if not, to add diversity by outcrossing minimize spinal damage when they were shaken by the to other breeds. The present study was an assessment bulls [1]. of genetic diversity that still exists in a representative Attempts to legislate against bull baiting began in the number of individual English bulldogs using DNA ra- UK in 1802 and the sport was finally abolished by an ther than pedigrees. The results confirm that the breed Act of Parliament in 1835, which led the breed to the has lost considerable genetic diversity through such brink of extinction [1]. The Bulldog endured by the efforts things as small founder population and artificial genetic of a small group of aficionados and the breed underwent bottlenecks resulting from highly focused selection for even more change in size and temperament after 1835 to specific desired physical traits. This is manifested by a ultimately make them into the shorter-faced, squatter and narrowing of allele diversity in many parts of the genome, more affable companion dog that we know today.1 The and the creation of numerous large regions of the genome first Bulldogs appeared in show rings in the UK in 1860, that are essentially identical within the breed, which are and the Bulldog was first recognized by the American significantly different from other dogs. Loss of genetic di- Kennel Club in 1886.2 versity is also pronounced in the region of the genome A number of modern breeds use “bull” or “bulldog” in that contains many of the genes that regulate normal im- their names and all have evolved from the original Bull- mune responses. The loss of genetic diversity and extreme dogs and Mastiff-type dogs [1, 2]. Some of these breeds changes in various regions of the genome will make it very are of more ancient origin, while others are reconstruc- difficult to improve breed health from within the existing tions of breeds that no longer exist. The modern Olde gene pool. Loss of present genetic diversity is further English Bulldogge is a reconstruction of the original threatened by rapid integration of new coat color muta- bulldog based on crosses between English bulldog, tions, increased wrinkling of the coat, and attempts to American bulldog, American Pit Bull Terrier and Mastiff.3 create a more compact body type. Contrary to current be- The Miniature bulldogs, French bulldog and American liefs, brachycephaly and the resulting breathing problems bulldog are also constructed breeds that nonetheless trace in the breed are the result of complex changes in head some of their ancestry to the original English bulldog. structure, and cannot be corrected by merely lengthening Although there are several “bulldogs,” the ideal English the face. Furthermore, other issues in English bulldogs bulldog is easily differentiated by its huge head with need to be addressed, including many serious health prob- wedge-shaped body, short and folded ears; stocky build lems that are not associated with brachycephaly, but are with deep furrows of the skin, especially of the face; intrinsic to inbreeding. short or corkscrew tail; short thick legs with equally broad paws; and a gentle, child-like appearance and dis- Background position [2]. The first mention of what might be a contemporary type The outward appearance of many dog breeds change bulldog was reported in 1632 [1]. The “bull” refers to the with time, and this is also true of the English bulldog.4,5 breed’s use in the sport of bull baiting in England. Bull Photographs of English bulldogs from the nineteenth baiting, which had its beginning even centuries earlier century depict dogs with less pronounced brachycephaly, with the Egyptians, Greeks and Romans, became a na- less chondrodystrophic skeletal structure, a long tail, and tional sport in England from the 13th to 18th centuries without excessive skin folds on their face or body.6 A [1]. The objective was for the dog to latch onto the bull’s picture of an English bulldog from 1935 can be found nose and force it to the ground, with the first dog to do on an orange crate label advertising California oranges so the victor. These original bulldogs had stockier bodies, (Fig. 1) and this dog was already differing in appearance larger heads and stronger jaws, and a more ferocious and from its ancestors pictured a century earlier and from Pedersen et al.