Canine Behavioral Genetics: Pointing Out the Phenotypes and Herding up the Genes
2008; Elsevier BV; Volume: 82; Issue: 1 Linguagem: Inglês
10.1016/j.ajhg.2007.12.001
ISSN1537-6605
AutoresTyrone C. Spady, Elaine A. Ostrander,
Tópico(s)Neuroendocrine regulation and behavior
ResumoAn astonishing amount of behavioral variation is captured within the more than 350 breeds of dog recognized worldwide. Inherent in observations of dog behavior is the notion that much of what is observed is breed specific and will persist, even in the absence of training or motivation. Thus, herding, pointing, tracking, hunting, and so forth are likely to be controlled, at least in part, at the genetic level. Recent studies in canine genetics suggest that small numbers of genes control major morphologic phenotypes. By extension, we hypothesize that at least some canine behaviors will also be controlled by small numbers of genes that can be readily mapped. In this review, we describe our current understanding of a representative subset of canine behaviors, as well as approaches for phenotyping, genome-wide scans, and data analysis. Finally, we discuss the applicability of studies of canine behavior to human genetics. An astonishing amount of behavioral variation is captured within the more than 350 breeds of dog recognized worldwide. Inherent in observations of dog behavior is the notion that much of what is observed is breed specific and will persist, even in the absence of training or motivation. Thus, herding, pointing, tracking, hunting, and so forth are likely to be controlled, at least in part, at the genetic level. Recent studies in canine genetics suggest that small numbers of genes control major morphologic phenotypes. By extension, we hypothesize that at least some canine behaviors will also be controlled by small numbers of genes that can be readily mapped. In this review, we describe our current understanding of a representative subset of canine behaviors, as well as approaches for phenotyping, genome-wide scans, and data analysis. Finally, we discuss the applicability of studies of canine behavior to human genetics. The domestic dog displays greater levels of morphological and behavioral diversity than have been recorded for any land mammal (Figure 1) and holds the unique distinction of being the first species to be domesticated.1Clutton-Brock J. Origins of the dog: Domestication and early history.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with People. CUP, Cambridge1995: 7-20Google Scholar The phenotypic radiation of the dog has been the product of restricted gene flow and generations of intense artificial selection.2Ostrander E.A. Kruglyak L. Unleashing the canine genome.Genome Res. 2000; 10: 1271-1274Crossref PubMed Scopus (123) Google Scholar These factors have generated the astounding level of diversity noted among the more than 350 breeds of dog recognized worldwide, many of which were developed for highly specialized tasks such as herding, hunting, and retrieving.3Wilcox B. Walkowicz C. Atlas of Dog Breeds of the World. T.F.H. Publications, Neptune City, NJ1995Google Scholar Indeed, breeds are often defined by a combination of their specialized morphological and behavioral traits4American Kennel ClubThe Complete Dog Book. Howell Book House, New York1998Google Scholar (Figure 2).Figure 2Herding BehaviorShow full captionDogs have been bred for a large number of behaviors including hunting, pointing, herding, guiding, etc. Shown is an example of the border collie herding livestock. Photo by Dan Weber.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Dogs have been bred for a large number of behaviors including hunting, pointing, herding, guiding, etc. Shown is an example of the border collie herding livestock. Photo by Dan Weber. The American Kennel Club (AKC) in the United States recognizes 157 distinct breeds of dog. For a dog to be a registered member of a breed, both of its parents must have been registered members of the same breed, meaning that many modern breeds, although all members of the same species Canis familiaris, represent closed breeding populations, often characterized by high levels of genetic homogeneity. Domestic dog breeds are thus ideal for studying the genetic basis of morphology, disease susceptibility, and behavior. Indeed, captured within the 157 U.S.-recognized breeds are heights that vary from nine inches (Pekingese) to three feet (Irish wolf hound); snouts that may be long and pointed (greyhound and collie) or short and flattened (pug and bulldog); coat colors, length, and textures galore; and variation at every level imaginable. Indeed, tail position alone has over a dozen recognized descriptors (plumed, ringed, snapped, whipped, sickle, curled, double curled, etc.). Behavioral variation is similarly captured within different breeds. Differences between breeds that herd versus guard livestock illustrate this point particularly well. Herding breeds, such as the border collie, are used to manage the movement and behavior of livestock. As their name implies, guarding breeds, such as the kuvasz, live among the livestock, usually unattended, and guard against predators. Both types of dog have been developed to work with livestock; however, they present radically divergent behavioral responses to their charges. Herding breeds strongly express predatory motor patterns such as stalking. More advanced aspects of the canine hunting sequence (grabbing) are differentially developed among herding dogs, with breeds like the Australian cattle dog, which is used to work typically stubborn cattle, strongly expressing grab-biting behaviors.5Coppinger R. Scheider R. Evolution of working dogs.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with people. CUP, Cambridge1995: 21-50Google Scholar In contrast, livestock-guarding breeds only weakly express predatory motor patterns. Good livestock-guarding dogs do not chase, stalk, or even attempt to play with livestock.5Coppinger R. Scheider R. Evolution of working dogs.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with people. CUP, Cambridge1995: 21-50Google Scholar Consideration of other breeds defines an array of additional behaviors, such as such as pointing, retrieving, tracking, and drafting, that are presumably controlled, as least in part, by strong genetic components. In addition, dogs display an amazing range of emotions to which humans respond, including loyalty and affection, for which a genetic basis has often been postulated.6Ostrander E.A. Lindblad-Toh K. Giger U. The Dog and Its Genome. Cold Spring Harbor Press, Cold Spring Harbor, New York2006Google Scholar With recent completion of a 1.5× survey sequence of the standard poodle, a 7.5× high-quality draft sequence of the boxer,7Kirkness E.F. Bafna V. Halpern A.L. Levy S. Remington K. Rusch D.B. Delcher A.L. Pop M. Wang W. Fraser C.M. et al.The dog genome: Survey sequencing and comparative analysis.Science. 2003; 301: 1898-1903Crossref PubMed Scopus (410) Google Scholar, 8Lindblad-Toh K. Wade C.M. Mikkelsen T.S. Karlsson E.K. Jaffe D.B. Kamal M. Clamp M. Chang J.L. Kulbokas 3rd, E.J. 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Breed relationships facilitate fine mapping studies: A 7.8kb deletion segregates with collie eye anomaly across multiple dog breeds.Genome Res. 2007; (in press)Google Scholar as well as how variation in the dog genome is organized,11Kirkness E.F. Sines of Canine Genomic Diversity. Cold Spring Harbor Press, Cold Spring Harbor, NY2006Google Scholar, 12Fondon 3rd, J.W. Garner H.R. Molecular origins of rapid and continuous morphological evolution.Proc. Natl. Acad. Sci. USA. 2004; 101: 18058-18063Crossref PubMed Scopus (424) Google Scholar allow us to hypothesize that we can unravel the genetic basis of both simple and complex canine behaviors with currently available tools. In the following sections, we first review dog domestication and describe ongoing experiments to identify behavioral genes. We discuss phenotypes of interest and highlight the features of the canine population that make it amenable to mapping studies. We also discuss what is known about genetic variation within the canine genome and how that may relate to behavioral genetics. Finally, we discuss examples of both normal and aberrant behaviors of interest to both human and companion-animal geneticists and the potential for identifying causative genes via the canine system. Domestication is both the process and condition of genetic and environmentally induced developmental adaptation to humans and captivity.13Price E.O. Behavioral aspects of animal domestication.Q. Rev. Biol. 1984; 59: 1-32Crossref Scopus (437) Google Scholar Initial studies of mitochondrial DNA (mtDNA) from domestic dogs suggested the potential for multiple domestication events and an origin of perhaps >100,000 years ago.14Vila C. Savolainen P. Maldonado J.E. Amorim I.R. Rice J.E. Honeycutt R.L. Crandall K.A. Lundeberg J. Wayne R.K. Multiple and ancient origins of the domestic dog.Science. 1997; 276: 1687-1689Crossref PubMed Scopus (687) Google Scholar More recent studies, however, suggest that dogs were domesticated 15,000 to 40,000 years before present (YBP).15Savolainen P. Zhang Y.P. Luo J. Lundeberg J. Leitner T. Genetic evidence for an east asian origin of domestic dogs.Science. 2002; 298: 1610-1613Crossref PubMed Scopus (641) Google Scholar Although fossil records show an association between prehistoric man and wolves, which are precursors of domesticated dogs,15Savolainen P. Zhang Y.P. Luo J. Lundeberg J. Leitner T. Genetic evidence for an east asian origin of domestic dogs.Science. 2002; 298: 1610-1613Crossref PubMed Scopus (641) Google Scholar, 16Vila C. Maldonado J.E. Wayne R.K. Phylogenetic relationships, evolution, and genetic diversity of the domestic dog.J. Hered. 1999; 90: 71-77Crossref PubMed Scopus (148) Google Scholar as far back as 400,000 YBP,1Clutton-Brock J. Origins of the dog: Domestication and early history.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with People. CUP, Cambridge1995: 7-20Google Scholar most of archeological data support a true domestication event date of about 15,000–20,000 YBP. Domestication brought about several distinct changes in the appearance of the wolf, as demonstrated by the remains of the earliest dogs found in Russia and Germany, dated at 13,000–17,000 and 14,000 YBP, respectively,1Clutton-Brock J. Origins of the dog: Domestication and early history.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with People. CUP, Cambridge1995: 7-20Google Scholar, 17Sablin M.V. Khlopachev G.A. The earliest ice age dogs: Evidence from eliseevichi 1.Curr. Anthropol. 2002; 43: 795-798Crossref Scopus (142) Google Scholar as well as fossils from Iraq and Israel dated at 12,000 YBP.1Clutton-Brock J. Origins of the dog: Domestication and early history.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with People. CUP, Cambridge1995: 7-20Google Scholar During the early Holocene period, between 10,000 and 7,000 YBP, dogs spread across much of the globe and were found even in the Americas. Remains of these early dogs are characterized by their smaller cranial volumes and mandibles, compacted teeth, and smaller auditory bulla (bony enclosure of the inner and middle ear). Other morphological changes included widened snouts, decreased tooth size, decreased body size, altered coat color and pattern, and altered tail and ear carriage. The development of cooperative hunting techniques together with the utilization of projectile hunting implements are believed to have significantly increased hunting efficiency during the Mesolithic period.1Clutton-Brock J. Origins of the dog: Domestication and early history.in: Serpell J. The Domestic Dog, Its Evolution, Behavior and Interactions with People. CUP, Cambridge1995: 7-20Google Scholar Hence, humankind's relationship with the dog increased in sophistication, resulting in selection of dogs exemplifying more refined traits adapted for specialized aspects of the hunt. As a result, modern hunting dogs have been selectively bred to point, track, chase, hold at bay, retrieve, and flush. A remarkable resource for understanding the behavioral and morphological changes associated with early domestication is the so called “farm-fox experiment,” which has been conducted for the last 50 years at the Institute of Cytology and Genetics of the Russian Academy of Sciences (ICG) in Novosibirsk, Russia. In the 1950s, Dmitry Belyaev and colleagues established a colony of silver foxes (Vulpes vulpes) with a goal of domesticating the animals so that they would be easier to handle by furriers seeking to develop products from the animal's unique silver coat.18Belyaev D.K. Domestication of animals.Science. 1969; 5: 47-52Google Scholar Foxes were selected on the basis of a key component of domestication, tameness. Yet, despite rigorous selection based solely on behavior, several morphologic traits that typically distinguish domestic dogs from their wild progenitors began to appear in the foxes,19Trut L.N. Early canid domestication: The farm-fox experiment.Am. Sci. 1999; 87: 160-169Crossref Scopus (467) Google Scholar, 20Trut L.N. The Genetics of the Dog.in: Ruvinsky A. Sampson J. CABI Publishing, New York2001: 15-41Crossref Google Scholar including widened skulls, shortened snouts, floppy ears, shortened tails, curly tails, and altered coat color patterns. In aggregate, these data suggest a link between selection for behavior and generation of a subset of morphological traits observed in modern domestic dogs. ICG researchers began their breeding program with 100 females and 30 males.19Trut L.N. Early canid domestication: The farm-fox experiment.Am. 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Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York2006: 515-537Google Scholar Newly developed quantitative measures are now used to define both tame and aggressive strains. These measurements rely on the assessment of video-recorded behavioral tests and can be reproducibly measured and quantified.23Kukekova A.V. Acland G.M. Oskina I.N. Kharlamova A.V. Trut L.N. Chase K. Lark K.G. Erb H.N. Aguirre G.D. The genetics of domesticated behavior in canids: What can dogs and silver foxes tell us about each other.in: Ostrander E.A. Giger U. Lindblad-Toh K. The Dog and Its Genome. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York2006: 515-537Google Scholar Examples include the frequency of occurrence of specific vocalizations and the relative positions of highly communicative body parts, such as the tail. Each of these has been used in a principal-component analysis (PCA), which classifies the variation of correlated traits into linear combinations. Principal components (PCs) are, thus, genetically accessible phenotypes. Nearly 50 traits have been defined that distinguish the tame and aggressive fox populations and can be summarized into two PCs, explaining 47.3% and 6.4% of the variance between the populations.23Kukekova A.V. Acland G.M. Oskina I.N. Kharlamova A.V. Trut L.N. Chase K. Lark K.G. Erb H.N. Aguirre G.D. The genetics of domesticated behavior in canids: What can dogs and silver foxes tell us about each other.in: Ostrander E.A. Giger U. Lindblad-Toh K. The Dog and Its Genome. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York2006: 515-537Google Scholar Recently, a fox meiotic linkage map was constructed that covers the entire haploid set of 16 fox autosomes as well as the X chromosome.24Kukekova A.V. Trut L.N. Oskina I.N. Johnson J.L. Temnykh S.V. Kharlamova A.V. Shepeleva D.V. Gulievich R.G. Shikhevich S.G. 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Alternatively, do canids have a mechanism for rapid generation of nonlethal mutations that are then available for selection? Experiments by Fondon and Gardner address this issue.12Fondon 3rd, J.W. Garner H.R. Molecular origins of rapid and continuous morphological evolution.Proc. Natl. Acad. Sci. USA. 2004; 101: 18058-18063Crossref PubMed Scopus (424) Google Scholar, 25Fondon 3rd, J.W. Garner H.R. Detection of length-dependent effects of tandem repeat alleles by 3-d geometric decomposition of craniofacial variation.Dev. Genes Evol. 2007; 217: 79-85Crossref PubMed Scopus (32) Google Scholar These investigators measured skulls of 20 breeds of dog as well as several mongrels. They then analyzed the DNA sequences of 37 microsatellite-repeat-containing regions from 17 genes hypothesized to be involved in craniofacial development. Strikingly, they found fewer perfect repeats in dogs than in humans, suggesting that evolution of microsatellite repeats might occur faster in dogs, thus accelerating the development of new alleles available for selection.12Fondon 3rd, J.W. Garner H.R. Molecular origins of rapid and continuous morphological evolution.Proc. Natl. Acad. Sci. USA. 2004; 101: 18058-18063Crossref PubMed Scopus (424) Google Scholar, 25Fondon 3rd, J.W. Garner H.R. Detection of length-dependent effects of tandem repeat alleles by 3-d geometric decomposition of craniofacial variation.Dev. Genes Evol. 2007; 217: 79-85Crossref PubMed Scopus (32) Google Scholar Given these results, it is easy to imagine how rapid mutation among strategically positioned microsatellite repeats in genes encoding neurotransmitters and their receptors, ion channels, synaptic-vesicle proteins, and axon-guidance molecules could play a role in behavioral variation as well. 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