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

even with CTVT, it’s thought that the source dog would have most likely lived in an inbred population. the tumor would have spread in this population before acquiring additional mutations that would have then allowed it to move to other less related individuals / populations.

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It sounds very dangerous for other rare animals.

I think I understand what you mean, but sister wife is just a little ambiguous.

hmmm… i guess it could be a little ambiguous, but i would have assumed that a woman’s sister wife would be a co-wife (polygamy), but a man’s sister wife would be his sister + wife (incest)… although i suppose it could refer to a situation where he is widowed and marries his deceased wife’s sister (i guess that’s neither incest nor polygamy). i guess a man marrying his biological sister is such an uncommon pairing that there aren’t great words or phrases to describe that pairing. suggestions?

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But you won’t have a differentiated brain that can appreciate your immortality.

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These transmissible tumors are clearly members of the taxa that they originated from. In the case of CTVT, they are mammals in the genus Canis. Are they Canis domesticus (dogs)? They don’t interbreed with dogs. They are very morphologically distinct. Although genetically similar, they have accumulated some new mutations. I think one can make a very reasonable case for describing CTVT as a new species of Canis. A very recently evolved, parasitic species. They are evolving on a path independent of dogs.

True, they are dependent on dogs as hosts, but some other parasitic organisms are dependent on a single host and they don’t loose their species status for that. In the case of the transmissible tumor of clams, it move among different species and even genera of hosts.

I think the main reason these transmissible tumors haven’t been named as species is that the people dealing with them are medical people, not taxonomists. A botanist would name something like this as a new species in a heartbeat.

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It’s a fun philosophical topic! We’re accustomed to closely related species looking almost identical to one another and to their ancestor. This certainly isn’t the case here. I think that’s the likely reason that a taxonomist hasn’t described it as a species. It just doesn’t fit the model of how new species form.

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In so many ways, evolution doesn’t work the ways we assume it does.

But they hardly breed at all, at least for now we don’t call cancer metastases its offspring, even in asexual species there is an exchange of genetic information, here it only can potentially do it with the host dog.

I think this is one of those things we just really haven’t managed to fit into the existing taxonomic system.
It would be ridiculous to consider a transmissible canine tumor a “dog”, yet it is made of tissue from the canine host. Some of these things may be caused by as-yet-unknown viruses, in which case they’re more akin to galls - a different organism hijacking the host’s tissues for its own means.

We certainly ought to have a clear way of documenting it, since it is contagious and spreads, but it’s difficult to know where to place it.

Another one I was thinking of recently is prions, especially after listening to this podcast episode about Chronic Wasting Disease in deer. It’s not really an organism, per say, but it’s insanely contagious, difficult to control, and could be a human health hazard too.

Perhaps for iNaturalist purposes these things could just be grafted straight to “Life” until some classification is figured out.

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Yes, the transmissible tumors are asexual. We botanists are used to dealing with asexual species. When I pointed out that they don’t interbreed with dogs, I was just pointing out that they don’t appear to be members of the species Canis familiaris any more.

that’s interesting. i hadn’t read about anybody suggesting this kind of origin or this way of thinking about these kinds of tumors before (as akin to galls). we know that viruses can cause cancer, and it does look like there are examples where cancer cells retain part of the DNA from a virus.

but we also know that regular cells can contain bits of viral DNA, and in these cases, we don’t think of these regular cells as being akin to galls.

so then even if a transmissible tumor was caused by a virus, and it had incorporated part of the viral code into its own genetic code, i don’t see why we would prioritize the viral identity above the identity of the tumor.

that’s interesting. does that mean you think of these as potentially an entirely different domain or kingdom? or are you just saying that we just don’t know what they are? (i think if we were to try to take them out of their source organism’s genus or family, it might make sense to drop them into Protista, since that seems to be a mix of Eukaryotes we don’t really know what to do with.)

transmissible tumors strike me as very different from prions both in terms of what they are and how they could be potentially classified. i’m not sure we can draw many lessons from prions to inform the discussion about transmissible tumors.

i think the more important thing than being asexual is that they can be thought of as unicellular organisms, right?

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Their mutations are pretty specific and imo not enough to be called anything but Canis, yes, their genetic material expresses itself in an unusual way, but that is normal for tissues too, they all look weird until summed together and that is a mutated tissue.
Remembering about human cancer, there’re cases of surgeons getting cancer through cuts, who can say that cancer on human genitals has 0 chance of being transmitted naturally?

I don’t know that the label “unicellular” fits well with transmissible tumors, but they do seem to lack specialized cells, so they aren’t like most multicellular organisms, either.

So far, human cancers don’t seem to be very successful new species, except the strain from Henrietta Lacks’ cancer. But perhaps that one and certainly the clam tumor do seem to be functioning as species.

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Since it hasn’t been mentioned yet, some people here may also appreciate to learn of the SCANDAL hypothesis – the suggestion that class Myxosporea (Cnidaria) may have originated as a transmissible cnidarian cancer, and subsequently underwent speciation like any other group of organisms.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6343361/


thanks. that’s an interesting paper.

it may be worth putting the paper in a little context:

Even Panchin [the lead author of the paper] and his colleagues aren’t going all-in on the hypothesis that myxosporeans are Scandals. “I think that’s fair to say it’s probably not true,” he said. It’s just that, with the work they’ve done so far, they can’t rule it out. “We’ve been trying to refute it with the means that we have.”

(the same sentiment is expressed in the original paper, but it’s a little clearer for laypeople in plain English.)

but i guess the key takeaway is that life and evolutionary mechanisms are complex, and it’s possible that transmissible tumors could be the source of new species. so even if they haven’t provided evidence for the mechanism exactly, they’ve at least gotten people thinking more about this kind of thing and have provided ideas for methods to look for possible cases of this kind of speciation.

as a side note, i kind of hate that the name of their hypothesis reflects the fact that they knew it would be controversial:

Panchin knows the idea of cancer-derived animals sounds far-fetched — so much so that, in the paper, he and his co-authors refer to them as Scandals (an acronym for “s peciated by can cer d evelopment animals ”).

(in my opinion, it’s a weak acronym, and it discounts the potential for this kind of mechanism in non-animal organisms.)

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