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| A free-ranging dog living at a Trinidad dump. JCM |
More than 140 years ago, Charles Darwin noticed something
peculiar about domesticated mammals. Compared to their wild ancestors, domestic
species are more tame, and they also tend to display a suite of other
characteristic features, including floppier ears, patches of white fur, and
more juvenile faces with smaller jaws. Since Darwin's observations, the
explanation for this pattern has proved elusive, but now, in a Perspectives
article published in the journal Genetics, a new hypothesis has been
proposed that could explain why breeding for tameness causes changes in such
diverse traits.
The underlying link between these features could be the
group of embryonic stem cells called the neural crest, suggest the authors.
Although this proposal has not yet been tested, it is the first unified
hypothesis that connects several components of the "domestication
syndrome." It not only applies 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," says Adam Wilkins, from the Humboldt University of Berlin.
Wilkins is an editor at Genetics and one of the paper's authors.
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. These tissues
include pigment cells and parts of the skull, jaws, teeth, and ears -- as well
as the adrenal glands, which are the center of the "fight-or-flight"
response. Neural crest cells also indirectly affect brain development.
In the hypothesis proposed by Wilkins and co-authors Richard
Wrangham of Harvard University and Tecumseh Fitch of the University of Vienna,
domesticated mammals may show impaired development or migration of neural crest
cells compared to their wild ancestors.
"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 says. "So, these
animals were less fearful."
But the neural crest influences more than adrenal glands.
Among other effects, neural crest deficits can cause de-pigmentation in some
areas of skin (e.g. white patches), malformed ear cartilage, tooth anomalies,
and jaw development changes, all of which are seen in the domestication
syndrome. The authors also suggest that the reduced forebrain size of most
domestic mammals could be an indirect effect of neural crest changes, because a
chemical signal sent by these cells is critical for proper brain development.
"This interesting idea based in developmental biology
brings us closer to solving a riddle that's been with us a long time. It
provides a unifying hypothesis to test and brings valuable insight into the
biology of domestication," says Mark Johnston, Editor-in-Chief of Genetics.
Tests of the neural crest hypothesis may not be far off, as
other scientists are rapidly mapping the genes that have been altered by
domestication in the rat, fox, and dog. The hypothesis predicts that some of
these genes will influence neural crest cell biology.
If so, we will have a much deeper understanding of the
biology underlying a significant evolutionary event, Wilkins says. "Animal
domestication was a crucial step in the development of human civilizations.
Without these animals, it's hard to imagine that human societies would have
thrived in the way they have."
The full article is available on-line.
A. S. Wilkins, R. W. Wrangham, W. T. Fitch. The "Domestication Syndrome" in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics. Genetics, 2014; 197 (3): 795 DOI: 10.1534/genetics.114.165423
