Snapshot Issue 10 May 2005
« No tail, no tax. » Such was the motto dog owners went by in 17th century Great Britain. The Old English sheepdog, also known by the nickname ‘Bob’ or ‘Bobtail’, is a perfect example of a dog whose tail was systematically docked so that farmers could shun taxes. Indeed, only the owners of dogs who enjoyed the lap of luxury were subjected to tax. Hence caudectomy, or the docking of an animal’s tail within days of its birth, was performed on puppies that were intended for purposes other than those to be purely decorative.
How about Mother Nature? Does she have the power to bob a tail the way men do? Yes she does… There is a protein which, like the vet’s scalpel, has the capacity to shorten a dog’s tail: the T protein, also known as the Brachyury protein from the Greek ‘brakhus’ meaning short and « oura » meaning tail. The Brachyury protein is a transcription factor and thus regulates the synthesis of proteins. It does this by binding to the genes it controls via a specific domain: the T-box domain. Once bound to a gene sequence, T protein induces the synthesis of a protein. Unfortunately, to this day no one knows which proteins are under T protein’s influence. However, what is known is that they are involved in the early stages of embryonic development and essential for the development of the posterior part of the primitive spine and hence the tail.
How is it then that Brachyury protein can also be involved in the making of a tailless dog? By the swapping of one amino acid for another in the protein’s sequence. And when the mutation hits the T-box domain, the Brachyury protein becomes dysfunctional. Consequently, all the proteins which depend on Brachyury’s correct function are not produced and the animal is born either without a tail, or with one that is shorter than it would normally be.
Brachyury protein is not unique to dogs and has proved to have the same role in other mammals – such as rabbits or mice. And how about humans? Besides our not sporting tails, can Brachyury protein be found in our systems? Yes. Logically, we would expect the human Brachyury protein to differ from the other mammal T proteins in some way in its T-box. But no such thing… There is no mutation and yet we bear no tails. It is a surprising discovery which only stresses the fact that, as is the case in so many biological systems, Brachyury protein is not the only one to decide on the appearance of our posterior.
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