Domestication Syndrome: Why Cute Looks Come with Breeding for Tameness
Compared to their wild ancestors, domesticated animals are cuter and tamer, and researchers behind a new study propose that a type of embryonic stem cell is the reason why breeding for tameness causes changes in such diverse traits.
More than 140 years ago, famed scientist Charles Darwin was the first to notice that domesticated species are not only more tame, but also tend to display a suite of other characteristic features, including floppier ears, patches of white fur, and more juvenile faces with smaller jaws.
The explanation for this pattern remains unknown, but this study is the first to come up with a unified hypothesis for "domestication syndrome."
The authors suggest that the underlying link between these features could be the group of embryonic stem cells called the neural crest. Although the theory has yet to be tested, it could apply not only to mammals like dogs, foxes, pigs, horses, sheep and rabbits, but it may even explain similar changes in domesticated birds and fish.
"Because Darwin made his observations just as the science of genetics was beginning, the domestication syndrome is one of the oldest problems in the field. So it was tremendously exciting when we realized that the neural crest hypothesis neatly ties together this hodge-podge of traits," Adam Wilkins, one of the researchers, said in a statement.
Neural crest cells are formed near the developing spinal cord of early vertebrate embryos. As the embryo matures, the cells migrate to different parts of the body and give rise to many tissue types, including parts of the skull, jaws, teeth and ears, as well as the adrenal glands.
However, domesticated mammals may show impaired development or migration of neural crest cells compared to their wild ancestors, according to the newly proposed theory.
"When humans bred these animals for tameness, they may have inadvertently selected those with mild neural crest deficits, resulting in smaller or slow-maturing adrenal glands," Wilkins explained. "So, these animals were less fearful."
Among other effects, neural crest deficits can cause depigmentation in some areas of skin (white patches), malformed ear cartilage, tooth anomalies and jaw development changes, all of which are seen in the domestication syndrome.
"This interesting idea based in developmental biology brings us closer to solving a riddle that's been with us a long time," added Mark Johnston, Editor-in-Chief of Genetics, the journal in which this study was published.