Genetic Influences on Brain Gene Expression in Rats Selected for Tameness and Aggression

Inter-individual differences in many behaviors are partly due to genetic differences, but the identification of the genes and variants that influence behavior remains challenging. Here, we studied an F2 intercross of two outbred lines of rats selected for tame and aggressive behavior towards humans for more than 64 generations. By using a mapping approach that is able to identify genetic loci segregating within the lines, we identified four times more loci influencing tameness and aggression than by an approach that assumes fixation of causative alleles, suggesting that many causative loci were not driven to fixation by the selection. We used RNA sequencing in 150 F2 animals to identify hundreds of loci that influence brain gene expression. Several of these loci colocalize with tameness loci and may reflect the same genetic variants. Through analyses of correlations between allele effects on behavior and gene expression, differential expression between the tame and aggressive rat selection lines, and correlations between gene expression and tameness in F2 animals, we identify the genes Gltscr2, Lgi4, Zfp40 and Slc17a7 as candidate contributors to the strikingly different behavior of the tame and aggressive animals

Facial shape differences between rats selected for tame and aggressive behaviors

Domestication has been consistently accompanied by a suite of traits called the domestication syndrome. These include increased docility, changes in coat coloration, prolonged juvenile behaviors, modified function of adrenal glands and reduced craniofacial dimensions. Wilkins et al recently proposed that the mechanistic factor underlying traits that encompass the domestication syndrome was altered neural crest cell (NCC) development. NCC form the precursors to a large number of tissue types including pigment cells, adrenal glands, teeth and the bones of the face. The hypothesis that deficits in NCC development can account for the domestication syndrome was partly based on the outcomes of Dmitri Belyaev’s domestication experiments initially conducted on silver foxes. After generations of selecting for tameness, the foxes displayed phenotypes observed in domesticated species. Belyaev also had a colony of rats selected over 64 generations for either tameness or defensive aggression towards humans. Here we focus on the facial morphology of Belyaev’s tame, ‘domesticated’ rats to test whether: 1) tameness in rats causes craniofacial changes similar to those observed in the foxes; 2) facial shape, i.e. NCC-derived region, is distinct in the tame and aggressive rats. We used computed-tomography scans of rat skulls and landmark-based geometric morphometrics to quantify and analyze the facial skeleton. We found facial shape differences between the tame and aggressive rats that were independent of size and which mirrored changes seen in domesticated animals compared to their wild counterparts. However, there was no evidence of reduced sexual dimorphism in the face of the tame rats. This indicates that not all morphological changes in NCC-derived regions in the rats follow the pattern of shape change reported in domesticated animals or the silver foxes. Thus, certain phenotypic trends that are part of the domestication syndrome might not be consistently present in all experimental animal models

Quantitative analyses of sexual dimorphism in the dataset.

<p>(A) Plot of multivariate regression of Procrustes shape coordinates on Centroid size showing differences between males (open green triangles and open blue squares in the respective tame and aggressive groups) and females (solid triangles and solid squares in the respective tame and aggressive groups) in the tame (green triangles) and aggressive (blue squares) rats; (B) PCA plot showing similar shape variation between the sexes in the two groups.</p

Landmarks used in the study.

<p>(A) Rostral view; (B) Diagonal view; (C) Lateral view; (D) Ventral view; (E) Doral view with wireframe; (F) Lateral view with wireframe. Definition of landmarks corresponding to the number in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175043#pone.0175043.t002" target="_blank">Table 2</a>.</p

Landmarks used in this study.

<p>Landmarks used in this study.</p

PCA plots of the overall variation in the dataset.

<p>(A) PCA plot in shape-space showing a distinct separation between the tame (green triangles) and aggressive (blue squares) groups along PC1 and within group variation along PC 2; (B) Surface morphs and wireframes depicting the shape changes along the scores of PC1 and PC2 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0175043#pone.0175043.g001" target="_blank">Fig 1(A)</a>. The surface morphs are computed by warping the respective PC scores onto the mean shape of all the specimens in the sample and illustrate the shape changes from the negative to the positive end of the PC axes. The same shape changes are also represented by the wireframe diagrams, the dotted lines showing the changes along each PC axis against the mean shape in solid lines, of the sample. PC1 captures shape changes, primarily in the face, between tame (green triangles) and aggressive (blue squares) rats. The tame rats in green triangles (on the negative end of PC1) show retraction in the anterior aspects of the snout and dorso-ventral contraction of the anterior cranial vault, compared to the aggressive rats in blue squares (on the positive end of PC1); PC2 accounts for the within-group and shared shape changes in the two groups, capturing variation in the position of the incisors and lateral aspects of the infraorbital foramen; (C) PCA on regression residuals showing little change between and within the groups when size is regressed out from the analysis.</p

A Comparison of Brain Gene Expression Levels in Domesticated and Wild Animals

Domestication has led to similar changes in morphology and behavior in several animal species, raising the questionwhether similarities between different domestication events also exist at the molecular level. We used mRNA sequencing toanalyze genome-wide gene expression patterns in brain frontal cortex in three pairs of domesticated and wild species (dogsand wolves, pigs and wild boars, and domesticated and wild rabbits). We compared the expression differences with thosebetween domesticated guinea pigs and a distant wild relative (Cavia aperea) as well as between two lines of rats selectedfor tameness or aggression towards humans. There were few gene expression differences between domesticated and wilddogs, pigs, and rabbits (30–75 genes (less than 1%) of expressed genes were differentially expressed), while guinea pigs andC. aperea differed more strongly. Almost no overlap was found between the genes with differential expression in thedifferent domestication events. In addition, joint analyses of all domesticated and wild samples provided only suggestiveevidence for the existence of a small group of genes that changed their expression in a similar fashion in differentdomesticated species. The most extreme of these shared expression changes include up-regulation in domesticates of SOX6and PROM1, two modulators of brain development. There was almost no overlap between gene expression in domesticatedanimals and the tame and aggressive rats. However, two of the genes with the strongest expression differences betweenthe rats (DLL3 and DHDH) were located in a genomic region associated with tameness and aggression, suggesting a role ininfluencing tameness. In summary, the majority of brain gene expression changes in domesticated animals are specific tothe given domestication event, suggesting that the causative variants of behavioral domestication traits may likewise bedifferent.funding agencies|Max Planck Society||European Research Council|233297|German Science Foundation (DFG)|AL 1525/1-1|CAS young scientists fellowship|2009Y2BS12|National Science Foundation of China research grant|31010022||SFRH/BPD/65464/2009|</p