What is a Species? Tasmanian Devil Transmissible Tumors

An uninfected Tasmanian Devil. The infected ones look so pathetic I just couldn't post a picture of one of them.
An uninfected Tasmanian Devil. The infected ones look so pathetic I just couldn’t post a picture of one of them.

Can cancer become an infectious disease?  Usually with think of cancer as limited to the individual in whom the cancer formed.  But what if cancerous cells could infect other individuals.

Here is a link to a story on the sad case of what happens when cancer “learns” to infect more than one individual:  BBC News – Cull ‘cannot save’ Tasmanian devil. Tasmanian facial tumor disease is passed from on Tasmanian devil to another. Usually we think of communicable diseases as being caused by bacteria or viruses but in this case the communicable disease is caused by cells of a cancerous tumor.  When Tasmanian Devils bite each other during disputes they transmit the cells of the tumor to one another which then grow to the point that it interferes with their ability to see or eat causing their death.

This cancer has been the subject of quite a bit of study because the cancer cells seem to go unnoticed by the infected animal.  There are many questions about how the cancer cells get around the immune system of the new host.  This cancer is thought to have originated in one Tasmanian devil around 1996 and since spread from one devil to another.  Today, as much as 80% of the wild population of Tasmanian devils are infected and will almost certainly die from this invasive cancer.

Transmissible tumors and our definition of life

The interesting question raised by this strange case of transmissible tumors is what is life? or more specifically what is a species?  These tumors originated as a single tumor in one Tasmanian devil almost 20 years ago.  Today these cells have changed the number of chromosomes and undergone numerous genomic rearrangements.  They definitely do not behave like tissues of a Tasmanian devil.  In many ways they are a unique organism playing by their own rules.  The cells are still mammalian with respect to their genomes but with so many differences compared to their host they can hardly be considered the same as the host from which they were created.

Most cancerous exhibit some genetic changes that in many ways makes a cancer a rogue cell line that no longer is acting in the interest of the body but rather as a parasite in the body.   However, the life of a cancer cell line dies when the host dies.   In this unusual case the cells of this Tasmanian devil cancer have “learned” to escape one body and move to another host and survive there.  This lineage of cells is not all that different from a parasitic virus or bacteria that is passed from one host to another to continue its existence and the latter are considered separate species/life forms.  In fact, one prominent theory for the origin of viruses considers viruses to be escaped pieces of a genome.  We all have pieces of our genome called transposable elements which are pieces of our DNA that can replicate themselves and move to another chromosome. A virus can be viewed as just a replicated piece of DNA that packages itself in such a way that it can not only move around the genome in a single cell but can escape from a cell and the whole organisms and find a new host organism.

In the case of Tasmanian devils we have an idea of when this cell line originated (1996 or a bit earlier). So far it seems unlikely that these tumor cells can invade other species of marsupials and so this new cell type, or new organism, may only survive as long as Tasmanian devils survive as their hosts.  This is similar to the viruses which cannot kill all their hosts or they will go extinct.  Many parasitic organisms are in the same predicament, they need a host to complete their life cycle by they must allow the hosts to survive long enough to reproduce or risk their own extinction.

There are two other examples of a communicable tumor.  The best know is that is a venereal tumor in dogs that is common in feral dogs in South America but is found even among domestic dogs in NA. I have written about these tumors before: Canine Transmissible Tumors: seeking immortality by becoming a parasite. This cancer has been transferred from dog to dog for much longer than the one that infects Tasmanian Devils.  Over that time it has divided into a number of cell lineages with unique mutations and behaviors.  The other example is from a hamster that gets tumors passed from one to another through the bite of a mosquito.

I think it is not unreasonable to suggest that in at least two of these cases we have witnessed the origin of a new parasitic organism that over time may adapt to infect other related organisms allowing them to persist as a unique line of cells just like a bacteria or virus in a variety of hosts.  As it is passed on, these cells will lose many of the genes they no longer need owing to their parasitic nature. The cells will become less and less identifiable with respect to the organism from which they were derived. Far in the future, it may be that someone investigating these parasitic organism may not be able to recognize this organism as once having been a Tasmanian devil. These parasites will appear to have no ancestor but rather be a unique branch of animal life.  In many ways they already appear to be a unique life form, it is only our ability to peer deep into the genomes of these cells that allows us to identify the genetic origins of these parasites.

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