| Abstract: | A common feature of domestic animals is tameness—i.e., they tolerate and are unafraid of human presence and handling. To gain insight into the genetic basis of tameness and aggression, we studied an intercross between two lines of rats (Rattus norvegicus) selected over >60 generations for increased tameness and increased aggression against humans, respectively. We measured 45 traits, including tameness and aggression, anxiety-related traits, organ weights, and levels of serum components in >700 rats from an intercross population. Using 201 genetic markers, we identified two significant quantitative trait loci (QTL) for tameness. These loci overlap with QTL for adrenal gland weight and for anxiety-related traits and are part of a five-locus epistatic network influencing tameness. An additional QTL influences the occurrence of white coat spots, but shows no significant effect on tameness. The loci described here are important starting points for finding the genes that cause tameness in these rats and potentially in domestic animals in general.ANIMAL domestication marked a turning point in human prehistory (Diamond 2002), and domestic animals have been the subject of research for many years (Darwin 1868). Recently, genetic studies have shed light on when, where, and how often a range of animal species were domesticated (Troy et al. 2001; Vila et al. 2001; Savolainen et al. 2002; Larson et al. 2005; Driscoll et al. 2007; Eriksson et al. 2008; Naderi et al. 2008). With the exception of coat color (e.g., Pielberg et al. 2008) and skin pigmentation (Eriksson et al. 2008), little is known about what occurred genetically during animal domestication. At what genes were allelic variants selected for by would-be practitioners of animal husbandry? Although domestic animals differ from each other in many ways, they all share the trait of tameness—i.e., they tolerate and sometimes even seek human presence and handling. Almost nothing is currently known about the genetic basis of tameness.In a series of studies initiated by D. K. Belyaev, researchers at the Institute for Cytology and Genetics in Novosibirsk (Russia) have subjected several mammalian species to a process of experimental domestication (Trut 1999). These studies, some of them ongoing for several decades, involve selection for tame and aggressive behavior in lines of animals derived from wild populations. They include a fox population that has been “domesticated” to such an extent that the tame foxes are now similar to dogs in some respects (Hare et al. 2005). They also include a population of wild-caught rats (Rattus norvegicus) that was selected for either reduced or enhanced aggression toward humans over >60 generations (Belyaev and Borodin 1982). To select the animals, their response to an approaching human hand was observed, and the rats showing the least and the most aggressive behavior were allowed to mate within the two lines, respectively. The initial response to selection was rapid and then slowed, so that little change in behavior from generation to generation has been observed in the last 10–15 generations, although the selection regime has been continued to the present. Today, the “tame” rats are completely unafraid of humans, they tolerate handling and being picked up, and they sometimes approach a human in a nonaggressive manner. By contrast, the “aggressive” rats ferociously attack or flee from an approaching human hand.To study the genetic basis of tameness we have established populations of both rat lines in Leipzig. In their new environment, the rats maintained their behavioral differences in response to humans, and these differences were not influenced by postnatal maternal factors (Albert et al. 2008). In addition, the rat lines differ in a number of other behavioral, anatomical, and physiological traits, raising the question whether these traits are influenced by the same loci as tameness and aggression toward humans.Many domestic animals display conspicuous coat color variations not found in their wild relatives. Prominent examples include the white color variants in dogs, pigs, cows, horses, and chickens. In laboratory rats, it has been proposed that “coat color genes” may account for many of the differences associated with domestication (Keeler and King 1942). It is thus interesting that individuals with white spots appeared in both the tame foxes (Trut 1999) and the tame rats (Trut et al. 2000) at higher frequency than in the corresponding aggressive lines, although they were absent or rare in the founding fox and rat populations, and although they were not selected for. The rat populations studied here provide an excellent opportunity to examine whether tameness is influenced by the same loci as white coat spotting.In this study, we crossed the two rat lines and bred >700 intercross animals. A broad set of behavioral, anatomical, and physiological traits was measured, and a genomewide set of genetic markers was used to identify genomic regions (quantitative trait loci, QTL) that influence tameness as well as other traits that differ between the lines, including white spots. |
