From Wolves to Dogs: the role of neural crest cells in changing one of man’s biggest fears into his best friend

Dogs are our closest companions. There are currently 77 million dogs in the United states – a number that outpaces the 74 million children under the age of 18. Most people know that dogs descended from wolves, but far fewer of us would welcome a grey wolf into our bed at night. So how exactly did wolf become dog, and how deep does the process of domestication go? Today’s blog post aims to unpack some of the physical, behavioral, neurological, and genetic differences between domesticated dogs and their wild ancestors, and to explain how these changes have come together to create the perfect pet we love today.

“Domestication syndrome” is a term used to describe the physical and behavioral changes animals undergo during domestication. This includes many neotenous traits such as developing smaller bodies, floppy ears, larger eyes, smaller jaws, and shorter faces (Pendleton et al., 2018). It also covers traits such as curly tails and piebald coloration
(Sánchez-Villagra et al., 2016). Some of the behavioral changes include lowered levels of aggression, sustained play into adulthood, and attention-seeking from humans (Wilkins et al., 2014). These are all phenotypic characteristics which differentiate dogs from wolves, but what causes these shifts?

One key player may be the neural crest. Neural crest cells are embryonic stem cells that appear early on in the development of vertebrates, and which later differentiate into a variety of specialized cells throughout the body (Sánchez-Villagra et al., 2016) There is no one “domestication gene,” but because of their far reach, any underdevelopment in the neural crest cells can produce several notable effects, both physically and behaviorally. Early domesticaters were unlikely to be trying to alter the physical appearance of their animals – instead, they were searching for tameness (Saetre et al., 2014). Tameness is the compilation of behavioral traits which enable animals to coexist and work with humans including docility and sociability. It is also necessary for many species to breed in captivity (Pendleton et al., 2018). Tame animals have smaller adrenal glands than their wild counterparts, and these tend to be hypoactive. This reduces both the fear response and production of stress hormones (Saetre et al., 2014). So how is the neural crest related? Some of the neural crest cells migrate to form the adrenal medulla. The adrenal medulla – which is located in the center of the adrenal gland – is then responsible for secreting hormones including adrenaline and epinephrine (Wilkins et al., 2014). Stunted development of the neural crest could lead to delayed formation and reduced functioning of this alert system, making dogs a friendlier, more trusting, and less aggressive companion.

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The physical differences between a grey wolf and a beagle are hard to miss. The beagle shows many traits of domestication syndrome, including piebald coloring, floppy ears, a rounded snout, and a smaller body.

Grey Wolf: [1], European grey wolf in Prague zoo, CC BY 2.0
Beagle: The original uploader was Soccersmp at English Wikipedia., Beagle puppy Cadet, marked as public domain, more details on Wikimedia Commons

After temperament, coloration is perhaps the most noticeable change in domesticated animals, many of whom show decreased pigmentation. Domesticated animals often have white patches, or a piebald pattern, with paler areas on the chest, above the eyes, the paws, and on the tip of the tail (Wilkins et al., 2014). These areas are almost always deficient in or lacking melanocytes. This is in line with the neural crest hypothesis because melanocytes are derived from neural crest cells (Cieslak et al., 2011) . Suppressing these would lead to more variation in coloring and the appearance of white patches.

A large proportion of neural crest cells are responsible for forming the skull and teeth (Wilkins et al., 2014) A decrease of neural crest cells leads to an overall smaller head size, with a flatter face and rounder muzzle. The jaws and teeth are particularly downsized (Pendleton et al., 2018), which helps explain why most pups are “cute” and not “terrifying” when they yawn or nibble on our fingers.

And as for their adorably floppy ears? We have a mild deficiency in neural crest cells to thank for those, too. The neural crest cells are responsible for forming much of the cartilage required to support the tissues of the pinna, or the outer ear. When this production is suppressed, the ear no longer has enough support to stand correctly, and instead flops to the side (Sánchez-Villagra et al, 2016).

Of course, this is only the beginning of the story. Human selection for individuals with mild neural crest cell deficits helps explain how we ended up with tamer, playful, and more trusting animals, and it explains how many of the physical traits we now recognize in domesticated dogs followed suit. In future posts, we’ll look at how this early dog transformed into the Boxers, Poodles, and Labradors of today, and how the unique relationship between man and dog has altered their evolutionary path – and our own.

References

Cieslak, M., Reissmann, M., Hofreiter, M., & Ludwig, A. (2011). Colours of domestication. Biological Reviews, 86(4), 885–899. https://doi.org/10.1111/j.1469-185X.2011.00177.x

Pendleton, A. L., Shen, F., Taravella, A. M., Emery, S., Veeramah, K. R., Boyko, A. R., & Kidd, J. M. (2018). Comparison of village dog and wolf genomes highlights the role of the neural crest in dog domestication. BMC Biology, 16(1), 1–21. https://doi.org/10.1186/s12915-018-0535-2

Saetre, P., Lindberg, J., Leonard, J. A., Olsson, K., Pettersson, U., Ellegren, H., Jazin, E. (2004). From wild wolf to domestic dog: gene expression changes in the brain. Molecular Brain Research, 126(2), 198–206. https://doi.org/10.1016/j.molbrainres.2004.05.003

​​San Filippo, M. Where the (not-so) wild things are: AVMA releases data on top, bottom states for dog, cat and overall pet ownership. (Dec. 2018). https://www.avma.org/news/pressroom/pages/2018-top-and-bottom-states-for-pet-ownership.aspx

Sánchez-Villagra, M. R., Geiger, M., & Schneider, R. A. (2016). The taming of the neural crest: a developmental perspective on the origins of morphological covariation in domesticated mammals. Royal Society Open Science, 3(6), 160107. https://doi.org/10.1098/rsos.160107

United States Census Bureau. (Dec. 2018). State Population by Characteristics: 2010-2017.https://www.census.gov/data/datasets/2017/demo/popest/state-detail.html

Wilkins, A. S., Wrangham, R. W., & Fitch, W. T. (2014). The “Domestication Syndrome” in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics. Genetics, 197(3), 795–808. https://doi.org/10.1534/genetics.114.165423