Domestication, an accelerated evolution driven by man, puts pressure on organisms to adapt to their new environment. This has generated a large variety of phenotypes in a short time-span, as seen in a number of domestic animals including the domestic chicken.
Our hypothesis is that domestic phenotypes may to have developed partly as correlated responses to early selection for tameness. By reenacting early selection pressure on a population of wild-type animals we can study the associated phenotypic and genetic changes. We have bred three lines of the wild ancestor of the domestic chicken, the Red Junglefowl (Gallus gallus). By selecting solely for the trait fear of humans, we have two lines displaying high or low fear, and one unselected showing intermediate fear levels. Currently five generations of offspring have been bred within each line, and several phenotypic changes, correlated to the selection trait, can be observed: low fear birds gain weight faster, show higher feeding motivation and more aggressive behaviour, while the high fear birds show significantly more avoidance behaviour. Low fear birds also display higher dominance ranking, lay larger eggs with heavier chicks, and have better plumage condition.
We performed genetic analysis by means of transcription microarray and PCR on hypothalamus and cerebral hemisphere from selected animals. Whilst the unselected group did not differ from the original parental population, the high and low fearful groups in generation S5 differed from each other significantly for a number of genes in both tissues. Amongst the differentially expressed genes, we detect functions ascribed to immunology, weight and behaviour. We also detected a number of transcripts previously annotated with sperm-associated functions. The results support the hypothesis that domesticated phenotypes may evolve because of correlated effects related to reduced fear of humans.