Where do Transmissible Tumors fall in the tree of life (if at all)?

There are some tumors that have developed the ability to spread from one individual to another. Apparently, the one that has the longest known history is the Canine Transmissible Venereal Tumor (CTVT), but there are others like the Tasmanian Devil Facial Tumor Disease (DFTD). I suppose that in terms of genetics, something like CTVT would be derived from Canis sp. genes. The tumor spreads clonally, but it appears to change over time via different mutations.

I see some folks classifying these tumors broadly as parasites, and some folks seem to be saying that each of these types of tumors is effectively an individual cell line or even an individual organism. But given that they seem to mutate and differentiate over time, wouldn’t that mean that each line is more like a species? (I don’t see any folks saying these are species though.)

What’s the right way to look at these tumors within the tree of life? Are they life? Can they be thought of as species?


The same way you look at cancer (which it is), which is made of mutated cells, it’s still a human or animal it grows in. The whole fact that there’s no immune responce on those cell in animals that have transmissible cancer shows that those cells aren’t a different taxon.


SciShow did a good short video a couple months ago musing about the difference in goals between a single cell of an organism and the organism as a whole, through the lens of CTVT.


that was a good video. so in a nutshell, they say that CTVT can be thought of as a single-celled dog. so does that mean it’s effectively just a different phenotypic expression of the genes in a particular Canis species, or is it its own species, or is it something else?

Interesting question, and I think others have given good answers philosophically speaking, but I would add that the idea of documenting the occurrence of this strange transmissible cancer on iNat seems super worthwhile. Seems like a great use-case for the tag system available when posting an observation, to make the observations (of the devils with the cancer) searchable.

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Well, not single-celled dog because the tumor is a multicellular mass. It can be thought of (philosophically) as a multicellular dog which does not look like a dog (it’s an amorphous mass). But one wouldn’t call a sirloin steak a cow. And no one would call a cell culture of Henrietta Lacks’ cervical cancer (still alive and growing in labs around the world) a person or a human–much less Henrietta herself. So it’s best to avoid these philosophical musings and just call a CTVT what it actually is–a living tissue mass derived from a dog that once lived a long time ago. The only difference between it and Henrietta Lack’s tumor is that it can self-propagate from dog to dog whereas HL’s tumor requires humans to propagate it in a cell culture medium.

Now, is it a different species? That’s a bit tricky. I suppose one could argue that this living, amorphous mass–which is reproducing asexually on it’s own accord–is a species of it’s own with it’s own evolutionary trajectory. But I doubt that way of thinking is going to catch on.


i think “single-celled” in this context is meant to refer to a unicellular organism vs a multicellular organism. a multicellular organism should have specialized cells that depend on each other, whereas a unicellular organism can go about its business completely as a single cell. a unicellular organism can exist in colonies.

i actually don’t know whether the cells in any CTVT tumor are specialized or not, but i don’t have any reason to believe that they couldn’t exist on their own. so if that’s the right way to think about it, it would be a unicellular organism – or single-celled dog – and a mass would just be a colony of single-celled dogs, right?

that’s a big difference when it comes to life, but i don’t want to stray too far from the main path so i’m going to leave this alone.

so if i’m reading in between the lines correctly, the definition of a species is just so squishy that in the end it’s all just subjective, and no one is really willing at this moment to stake their reputation by writing a paper that classifies these kinds of tumors as their own species. that territory is just too uncharted. is that more or less correct?

Thing is dog one is pretty stable genetically, devil one is too new for that and is more dangerous because of lack of genetic variation in current devils’ populations. With dog one being transmitted sexually it also raises a point of difference of it and normal parasite, as it’s pretty much divided by dog’s actions, appearing in a new host that is the same as previous one.

I had given thought to making a forum post about this previously. Genetically, the organism is still whatever it originated from, albeit with many mutations (most of the time). I imagine, on iNaturalist, we might designate an infrataxon without a proper scientific name. This method is used with some viruses. Alternatively, staff may decide simply using observation annotations is enough, if the taxon isn’t used frequently. This does limit discoverability, though.

Don’t be so sure. If you’ve ever seen some of the controversy over embryonic stem cells…

Now, since this tumor tissue is, in fact, dog tissue, that would preclude calling it another species – just as our human cancers are not classified as other species. It seems to make the most sense to say that these do not fall anywhere in the tree of life; that they are abnormal growths within the affected species.

Another odd one that fits here is disseminated neoplasia in shellfish described in this article which can affect multiple species.

Edit: An off topic, but also equally odd topic for taxonomical categorization are prions discussed in a few comments here.

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Since the tumors self-propogate by asexual cell division (vs. sexual reproduction) one would need to apply the same sort of concepts for species as one does for bacteria. And those concepts are as subjective as they come. But I don’t think that’s why calling these tumors species will catch on. I just don’t think that transmissible tumors are the sort of thing that people have in mind when they think of species–even if they might technically pass as one using a species concept definition. I’m not sure why, though. Perhaps because they don’t have a “life cycle” or don’t have a recognizable shape with body parts?

This does, unavoidably, get into some challenging territory–which I don’t think most humans are capable of navigating. What is a dog? Is a dog cell which is alive the same as a living dog being even if it doesn’t have a tail and doesn’t bark? Or must a dog (a dog being or a dog life) be able to develop into something that barks and wags its tail? And if it can’t, it’s just a mass of dog cells? This philosophical challenge lies at the heart of the abortion debate and will never be resolved to everyone’s satisfaction.

Putting those philosophical challenges aside, I don’t think that one can argue that these transmissible tumors (each tumor, not the collective of all tumors because they’re not part of an interbreeding population) are asexually self-propagating cells which are each on their own evolutionary trajectories. That gets awfully close to being what we think of as asexual species (such as bacteria). But see above–I just don’t think this is what people (even biologists) are something that they would want to call species.

i think this is because cancers generally don’t reproduce beyond the original host. so cancers are definitely living cells, but it’s not entirely clear if they’re organisms (functioning as individual entities), and the concept of species is also less relevant for cancers because their trajectories as cell lines generally do not extend beyond the space or life span of their original hosts.

regarding virus names, i think they just have a different naming rules than animals. that said, if we were to put a given transmissible tumor into its own taxon, i think it’s interesting that it would make sense genetically to place it near or within the original species, but it would lack all the usual characteristics that place the original species within its branch in the tree of life (ex. a dog fits under vertebrates because it has a backbone, and it fits under mammals because it has mammary glands, but CTVT has neither).

maybe we don’t call it dog then? maybe CTVT is so different as a lifeform that it deserves its own entirely different common name?

yup. that’s why i think these are so interesting if you try to fit them into our existing taxon frameworks.

Maybe this is the botanist in me speaking, but taxonomic frameworks aren’t built around consistent characteristics. With plants, we have so many “rule-breakers” that making rules makes no sense. An example would be the taxonomic nightmare that was (and is) Asparagaceae.

The position of CTVT depends the definition of a species used in describing it. The organism is clearly not accurately described by the species Canis familiaris, despite its recent ancestry. It cannot interbreed with C. familiaris, and isn’t geographically isolated, so the ranks of form, variety, and subspecies don’t really function either. I do think it would be interesting to create a new category for such aberrations, and have CTVT be, for example, Canis familiaris aberr. canine-transmissible-venereal-tumor.

Maybe, given the asexual nature of CTVT, taxonomists don’t believe in describing it as its own separate entity yet. It clearly functions as a pathogen, but doesn’t fit into the rules typically used for describing organisms of vertebrate ancestry. One could, in theory, consider the pathogen to simply be the dog it originated from, like a tissue culture. How many generations of asexual tissue culture need to pass before that culture is considered its own species? Would that culture be differentiated by its habit, by genetics, or by its unusual lineage?


That raises another interesting point: did these tumors originate just once, from one dog (monophyletic), or is there parallel evolution going on, with unrelated tumor lineages originating from several different dogs (paraphyletic)?

my understanding is that CTVT would have originated from one individual dog, though it could have picked up stuff from other infected individuals and other infected species over time.

the other transmissible tumors also should have originated from individual animals. if i’m remembering correctly, there are actually two separate transmissible tumor lines in the Tasmanian Devil population (which would have come from two different individuals, probably).

i haven’t read much about the other transmissible tumors in mollusks and other animals. i don’t have any reason to believe they would have a much different kind of evolutionary origin, but i suppose anything is possible.

If I become a transmissible tumor in humans, I can live forever.

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Sadly for you for know known human cases of transmition are artificial, maybe the rarity of those tumors is the result of unique qualities of those taxa, but more likely they require uniqely complicated mutations to become such entities.

if i’m remembering correctly, folks believe that transmissible tumors are most likely to arise in populations with low genetic diversity. so i guess @jasonhernandez74’s best chance for becoming the source of a new transmissible tumor is to have a sister wife and be exposed to lots of mutagens.

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It is said for devils, but it affects how hosts react to it, their receptors are too similar and all are failing to notice the tumor (and tumors also synthetize agent that neutralize them), if devils weren’t in that poor state as a species, they probably wouldn’t be so affected by it, with dogs we see it’s not an often problem. But does it mean those tumors appear in other species and just go unnoticed and die out?