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The wikipedia article on CTVT says that the tumor cell has 57-64 chromosome while a normal dog has 78 chromosomes. Similarly while all chromosomes in dogs except the X and Y chromosomes are acrocentric, many of the CTVT chromosomes are sub-metacentric or metacentric. Reference is given for the data in Wikipedia though a subscription is needed to view the referred articles themselves, so I was unable to confirm the data. (Wikipedia is sufficient for me, though popular opinion seem otherwise)

In addition there seems to be numerous (in the order of thousands) differences in sequences.

Their study showed that the cancer cells all originated from a common ancestor cell, not from the dogs themselves. Comparisons of the cancer DNA with dog DNA from reference samples held by the US Kennel Club and Crufts showed that it most likely originated at least 250 years ago from a wolf or an "old" Asian dog breed such as a husky or Shih Tzu, and spread like wildfire thereafter.

This was taken from Riddle of Infectious Dog Cancer Solved.

  1. How is the conclusion arrived at in cases like this in general? What percentage / degree of genetic similarity is needed to identify something as belonging to a specific species?

  2. How was the conclusion arrived at in this specific case? (I know I am greedy!)

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    $\begingroup$ I don't quite understand what (or maybe better why) you're asking. If Wikipedia is sufficient for you, why do you want more information? $\endgroup$
    – anongoodnurse
    Jan 26, 2015 at 16:36
  • $\begingroup$ Because what I am asking is not there in Wikipedia $\endgroup$
    – One Face
    Jan 26, 2015 at 21:35
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    $\begingroup$ "How is the conclusion arrived at in cases like this in general?" The same way it was in this case, by DNA analysis. It's in your article. $\endgroup$
    – anongoodnurse
    Jan 26, 2015 at 22:32
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    $\begingroup$ I'm not being antagonistic. You stated that you rely on Wiki. That's fine, then did you read about DNA analysis on Wikipedia? DNA analysis doesn't mean comparing chromosomes. It means looking at, for example, histocompatability loci. In the case of your tumor, mitochondrial DNA was very useful. My questions are usually a prompt to improve questions with a bit of research. The answer is in your article. If that's not helpful, you can do a bit of research to let us know what, exactly, you know or don't know, so we don't need to write a chapter on DNA analysis and guess at what you're asking. $\endgroup$
    – anongoodnurse
    Jan 26, 2015 at 23:27
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    $\begingroup$ @anongoodnurse OK, will read the article again and try to refine my question. Thanks for the pointer $\endgroup$
    – One Face
    Jan 26, 2015 at 23:31

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Your answer to your titular question, How was the origin of CTVT parasitic cancer determined? is that cytogenic studies were done on the tumors. It arose in dogs and was logically assumed to be of dog origin, and was first characterized in 1876. The tumor cells were/are characterized by a rearranged karyotype, which is similar in tumors from in different parts of the world. (1969) Thus it was postulated that that the transmissible agent causing the CTVT is the tumor cell itself and that all worldwide CTVT have a clonal origin. (1985)

Your later question states:

What percentage / degree of genetic similarity is needed to identify something as belonging to a specific species?

When a tumor arises in an animal (say a dog), it is automatically assumed that the cancerous cells, regardless of their ploidy, arose from that animal (in this case, a dog), not a different animal. Not all animals have had their entire genome sequenced, so this question as stated isn't answerable (although the genome of the dog has been completely sequenced). And certainly, not all tumor cell lines have been completely sequenced. The assumption of origin is made by looking at certain genes (e.g. histocompatibility genes) and others known to be species specific.

Since there are relatively few transmissible cancers (that is, where the cancer itself acts as the infectious agent), this is still a safe assumption. While I'm fairly confident more transmissible cancers will be found, I'm also fairly confident that they will be species specific, because in order to escape detection, they will need to be recognized immunologically as "self", as they must survive in the host at least long enough to be transmitted.

This is the case with canine transmissible venereal tumor (CTVT). It is also the case with the contagious tumor Tasmanian Devil face tumor disease (DFTD). Just as with the canine infectious cancer, DFTD cells in different individuals have identical, complex chromosomal rearrangements and are genetically identical to each other but different to the host devil.

Spread of such an infectious cell line between unrelated individuals should be prevented by major histocompatibility complex (MHC) Class I antigens on the surface of the tumour cell, as Class I molecules play a key role in self/non-self-recognition. The recent emergence of the tumor (transmitted by biting) in the endangered marsupial Tasmanian devil (Sarcophilus harrisii) is an unfortunate exception. In the Tasmanian devil, the tumor is very aggressive and kills the host.

In the case of canine transmissible venereal tumor (CTVT), it was shown about five decades ago to be the same dog tumor being spread from dog to dog. The earliest studies were based on histological studies and karyotyping. Paraffin-embedded tumor samples still exist. More recently, with genetic analysis, it is possible to perform much more sophisticated testing on the tumor DNA.

The transmissible agent causing canine transmissible venereal tumor (CTVT) is thought to be the tumor cell itself. To test this hypothesis, we analyzed genetic markers including major histocompatibility (MHC) genes, microsatellites, and mitochondrial DNA (mtDNA) in naturally occurring tumors and matched blood samples. In each case, the tumor is genetically distinct from its host. Moreover, tumors collected from 40 dogs in 5 continents are derived from a single neoplastic clone that has diverged into two subclades. Phylogenetic analyses indicate that CTVT most likely originated from a wolf or an East Asian breed of dog between 200 and 2500 years ago. Although CTVT is highly aneuploid, it has a remarkably stable genotype.

Your quote

likely originated at least 250 years ago from a wolf or an "old" Asian dog breed such as a husky or Shih Tzu...

Has been disputed. Estimates vary widely, from 11,000 years to 250 years.

Canine TVT: Evolution of Somatic Tumour Cell as Transmissible Parasites
MHC gene copy number variation in Tasmanian devils: implications for the spread of a contagious cancer
Clonal Origin and Evolution of a Transmissible Cancer

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  • $\begingroup$ @CRags - Are you an editor? Perhaps you can update the Wikipedia article to reflect the new information? $\endgroup$
    – anongoodnurse
    Jan 29, 2015 at 3:44
  • $\begingroup$ ok, will do today later @anongoodnurse $\endgroup$
    – One Face
    Jan 29, 2015 at 3:54

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