|The ancient tumour genes are most similar to husky genes|
Scientists have discovered that the genes in a transmissible cancer, canine transmissible venereal tumour (CTVT), which infects the genitalia of dogs and which is passed on during mating, are all from a single dog in which the tumour first arose, some 11,000 years ago. It is one of only two known mammalian cancers which can be passed directly from one individual to another. The other is a tumour transmitted between Tasmanian devils when they bite one another.
Since every tumour is effectively a clone of this original one, the genes have remained close to those of the original dog and can be found in dogs all over the world. They can thus shed light on the origin of the domestic dog although the dog is believed to have been domesticated far earlier - about 33,000 years ago. This discovery was made by a team led by Elizabeth Murchison of the Wellcome Trust Sanger Institute in Hinxton, UK, who analysed DNA from tumours from dogs from Australia and Brazil.
It's such a strange entity. It's a cancer that's become a parasite, and yet it came from the same species that is now its host.Despite the fact that they are clones, over the intervening 11,000 years they have accumulated some 2 million mutations due to mistakes in cell division. The tumour is thus showing signs of diversification by evolution from the original tumour. It was by analysing the rate of these mutations and the order in which they arose that the team arrived at the 11,000 year age for the tumour. Selection pressure comes from the method by which it is transmitted and grows in the new host. The easier this occurs the more likely that mutation is to survive and be passed on. Mutations which inhibit this or prevent successful transmission altogether will be quickly eliminated. To be successful, the cancer needs to keep its host alive and fit to breed until it is passed on and the longer it manages this the more likely it is to be passed on. This means the cancer has evolved a reasonably mild form which consequently makes it relatively easy to treat, treatment being successfully in 90% of cases.
The cancers found in the Australian and Brazilian dogs seem to have diversified only about 500 years ago, coinciding with the height of European exploration. It is thought that the cancer had a fairly limited geographical distribution for some 10,500 years until Europeans took infected dogs all round the world. It probably arose in an isolated and therefore highly inbred population with little genetic diversification so the tumour cells would not be recognised by the immune system as foreign cells. Comparing the non-mutated DNA with that of living dogs it seems the tumour arose in a dog which was similar, though not the same as, huskies and Alaskan malamutes.
A similar process is thought to be responsible for the tumours in Tasmanian devils which, due to population decline, are now also highly inbred and so lack genetic diversity. The cancer which is threatening them only arose within the last 30 years.
I'd love to see an 'Intelligent Design' explanation for this and the Tasmanian devil cancer, if any creationist feels like attempting one.
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