Genetic architecture of tameness in a rat model of animal domestication

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.

Figure 1.—

Behavior in the glove test. (A) The glove test measures the level of tameness or aggression toward an approaching hand. (B) PCA scores derived from glove test behaviors of tame F₀ (blue), aggressive F₀ (red), F₁ (purple), and F₂ (black) animals. Circles, females; squares, males.

Figure 2.—

Correlations between phenotypes. Positive correlation coefficients are shown in red and negative ones in blue. Red boxes mark correlations within the same test or group of traits.

Figure 3.—

QTL for tameness and aggression. (A) Evidence for linkage to tameness across the genome. High F-values indicate the presence of a QTL. The dashed horizontal line represents the genomewide significance threshold. Solid horizontal bars are 2-LOD drop confidence intervals for QTL position. Chromosome boundaries are indicated by upward tick marks on the x-axis. (B) Several traits map to the two QTL for tameness. Only significant QTL are shown. Black and red upward tick marks are microsatellite and SNP marker positions, respectively.

Figure 4.—

White coat spotting and tameness. (A) Tameness level of F₂ animals with (n = 190) and without (n = 393) white ventral coat spots. (B) A QTL for spotting (black line) does not show linkage to tameness (red line). The vertical dashed line indicates the location of the Kit gene. Black and red upward tick marks are microsatellite and SNP positions, respectively.

Figure 5.—

An epistatic network for tameness. (A) Overview of QTL (circles) and epistatic interactions (lines). Only QTL in pairs with significant epistatic interactions are shown. Bold (nonbold) solid circles: the QTL was significant (suggestive) in the scan for single loci. Dashed circles: the QTL was significant only as part of an epistatic pair. Numbers in circles indicate QTL chromosome and position (centimorgans). (B–F) Phenotypes for two-locus genotypes. Circles indicate the mean phenotype for the respective two-locus genotypes; error bars show the standard error of the mean. The strength of the QTL effect at the first locus in the pair is indicated by the slope of the line connecting the homozygous genotypes [both alleles from the tame line (TT) vs. both alleles from the aggressive line (AA)]. An effect at the second locus in the pair is indicated by nonoverlapping allele effects at a given genotype of the first locus.