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Canid genomics: mapping genes for behavior in the silver fox.

T. Spady,E. Ostrander

Published 2007 in Genome Research

ABSTRACT

Domestication is the condition and/or process of genetically and environmentally induced developmental adaptation to man and captivity (Price 1984). Several trends are commonly observed in domesticated animals. Most notable are the changes in morphology including pigmentation, size, and relative skeletal proportion (Belyaev 1969; Price 1984). These may be hugely divergent between domestic animals and their progenitor counterparts (Belyaev 1969, 1979; Price 1984; Trut 1999, 2001). Domestication is also associated with marked changes in reproductive physiology, accelerated sexual maturity, increased fecundity, loss of reproductive seasonality, and longer periods of reproductive receptivity. However, it is the behavioral adaptations associated with domestication that are the most dramatic and that have most captured our attention. Domestic animals are said to be “tame”, responding to humans in a less-aggressive, often even affable manner. Humans form bonds and often develop lifelong relationships with domestic animals, frequently becoming valued members of the family community. Using the resources presented by Kukekova et al. in this issue of Genome Research, we take a giant step forward in our ability to localize the genes controlling the process of domestication in the canine system (Kukekova et al. 2007). Kukekova et al. (2007) have published the first meiotic linkage map of the silver fox (Vulpes vulpes), which is a color variant of the red fox. It last shared a common ancestor with the domestic dog (Canis familiaris) ∼10–12 million years ago (Vila et al. 1999a). A colony of silver foxes has been established at the Institute for Cytology and Genetics (ICG) at the Russian Academy of Sciences in Novosibirsk, Russia, with the purpose of developing lines of animals suitable for studying the genetics of domestication. The animals have been selectively bred for nearly half a century for one of the key components to domestication, tame behavior. To date, 34 fox pedigrees have been developed by breeding foxes from tame and aggressive strains (see Fig. 2, below), and then backcrossing F1 progeny to the tame strain. Two additional pedigrees have been produced by crossing tame individuals alone, and one more pedigree by crossing tame to aggressive animals. Crosses and behavioral status of animals used in the crosses are described in more detail below. Genetic analyses of these crosses are likely to provide behaviorists with insights into the genetic underpinnings of complex mammalian behaviors as well as an understanding of the changes associated with the early stages of the domestication process. Because of the close phylogenetic relationship between the dog and the red fox species complex (Fig. 1A), genomic resources developed previously in the dog have proven useful in the construction of the fox meiotic linkage map. In the late 1990s, both Yang and Graphadatsky and their collaborators independently showed that syntenic relationships between the genomes of canid species were largely conserved, despite the marked karyotypic differences (Graphodatsky et al. 1995; Yang et al. 1999). The dog has a karyotype of 78 acrocentric chromosomes, compared with 34 metacentric chromosomes and a variable number of B chromosomes observed in the red fox. Kukekova et al. (2007) used 320 microsatellite-based markers to construct the fox map. The markers were all either previously published dog microsatellites (Guyon et al. 2003; Breen et al. 2004; Clark et al. 2004), a set of dog markers previously optimized for the fox (Kukekova et al. 2004), or markers made available through the Mammalian Genotyping Service of Marshfield Laboratories (Madison, WI). Markers were also included that were recently developed using data from the 7.6x whole-genome assembly of the domestic dog (Lindblad-Toh et al. 2005). All markers were genotyped using the 37 silver fox pedigrees from the ICG described above. The resulting linkage map covers the entire haploid set of 16 fox autosomes as well as the X chromosome, with an average marker spacing of ∼6.8 cM, a total map length of 1480 cM, and an average Polymorphic Information Content (PIC) in the silver fox of 0.5 (Kukekova et al. 2007). The initial map was composed 43 linkage groups, each of which corresponded to a unique block in the 7.6x whole-genome assembly of the dog. Alignment of the markers with the dog assembly allowed for consolidation of fox linkage groups into individual chromosomes. Interestingly, the B chromosome segments of the fox are not included in the linkage map presented by Kukekova et al. (2007). B chromosomes were generally believed not to contain genes of major importance, and fluorescence in situ hybridization (FISH) experiments using fox probes against dog chromosome spreads (Yang et al. 1999) support this belief. However, the recent discovery of a highly conserved and seemingly functional C-KIT proto-oncogene copy on a fox B-chromosome suggests that this hypothesis should probably be revisited (Graphodatsky et al. 2005). The Farm-Fox Experiment, as it has become known, is, in essence, a fast-forwarded reconstruction of man’s first exercise in domestication: the domestication of the dog from the gray wolf (Canis lupus). Dogs were the first domesticated species, and studies of canid phylogenetic relationships show that gray wolves are the sole progenitor to the domestic dog (Clutton-Brock 1995; Vila et al. 1997, 1999b; Wayne et al. 1997) (Fig. 1B). The study of genetic diversity across canids indicates that dogs first originated in East Asia and subsequently spread across the prehistoric globe. Archeological evidence places the time of domestication about 14,000–17,000 yr before the present (ybp) (Clutton-Brock 1995; Sablin and Khlopachev 2002). However, molecular evidence suggests it could have occurred much earlier. Mitochondrial data suggests that dogs diverged from wolves, at the earliest, about 135,000 ybp (Vila et al. 1997). However, an analysis of more dogs 1Corresponding author. E-mail eostrand@mail.nih.gov; fax (301) 480-0472. Article published online before print. Article and publication date are at http:// www.genome.org/cgi/doi/10.1101/gr.6055807. Commentary

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