Interesting idea. However, morphologically the cells are of mammalian type rather than protist. Furthermore the genome of a TVT has been fully sequenced, and it is typical canine, albeit with many mutations. The cells cannot survive outside of a host tumour. So it would probably take a lot of convincing to have it accepted as a new species.

CTVT as well as the Tasmsnian Devil Transmissible Facial Tumors (there are two separate diseases described so far) for me represent a phenomenon of an escaped modified cell clone/s which can be considered parasitic/infestive but represent a population of cells originating from the same species. Therefore, the consideration for separate species might be quite a challenge.

These neoplasms do behave like a species, they also exchanged mitochondria with their host, at least in case of CTVT. Hence, capable of reproduction (division) these neoplasms still have close relation to their host in terms of genome, metabolism etc.

The most plausible option in my opinion is to classify them as "parasitic" cell populations capable of forming neoplasms in their hosts.

Best,

Ivan

Thank you for your comments.

Yes CTVT and other transmissible tumors are limit cases for the species concept.

There are several examples of multicellular ancestors giving rise to unicellular descendants. Yeasts for example in the kingdom Fungi, or myxozoans in the kingdom Animalia (from a cnidarian ancestor). Myxozoans aren't tumors, but are still parasitic unicellular animals that behave like protists, but remain cytologically cnidarian (even with the typical cnidocyst).

Unlike HeLa cells, which are a laboratory artifact, I can't help thinking that CTVT may be regarded as a species of "unicellular dog" which evolved from a species of "multicellular dog", like yeasts or myxozoans. Many species are obligatory parasites, but are still species on their own right.

The only strong argument I have found out is that in order to be considered an individual species, a daughter species must be able to survive the death of its mother species. If it is not the case, then it means the individuation process was incomplete. In the case of CTVT (and the Tasmanian Devil's one), the only host of the parasitic putative daughter is the mother itself, and so can't survive its mother's death.

What do you think? Is it necessarily an evolutionary dead end for this lineage of cells? Or may they be able one day to infest another species (foxes, cats, or even maybe non-mammalians or non-vertebrates) and then complete their individuation process?

You're right, Damien. In the case of CTVT there is a dog cell which uses dog as a host ans shares similarity with it. If CTVT is transmitted to cats in natural conditions, for example, that will be very convincing. I have to do my own research but I think CTVT can be also found in some other canids, excluding the wolf as the closest relative. Devils Facial Tumor has a bigger chance to infest other species due to the fact that devils fight not only within themselves but with other animals too. So, passing of the cell clone in immune compromised animals of different species will be a way to go and to gain resistance to a foreign species immunity. Then the things might become very interesting. There are already couple of publications for interspecies transmission of hematocyte neoplasms in cockles. I do have a theory around that but I would like to apply for scholarship with it.

Cheers,

Ivan