Abstract
Domestication involves huge phenotypic shifts via strong directional selection. The resulting changes, often termed the Domestication Syndrome, typically encompass numerous traits; however, the most universal of these are changes in reduced fear of humans (tameness) and brain composition. To assess how early domestication selection may have focused on tameness and its interaction with brain composition, a Red Junglefowl (Gallus gallus) population (the wild progenitor of the domestic chicken) was used to create two lines bidirectionally selected for fear of humans over eight generations of selection. These selection lines were then used to make an intercross population. Using a combination of genome-wide mapping in the intercross and between-line analysis of the selection lines, we show that the genetic loci for tameness co-localise with genetic loci for brain composition and anxiety behaviour. Furthermore, the detected loci for brain composition also co-localise with brain composition loci identified in a separate wild × domestic intercross. These results indicate that tameness and brain composition are either pleiotropic or genetically linked, and that tameness selection appears to recapitulate the same loci that have been selected by domestication itself. Therefore, selection for increased tameness could be the initial selection pressure driving the core of the domestication syndrome.