Researchers have presented a controversial new theory suggesting that transmissible cancers could eventually evolve into their own multicellular species. They also say that a peculiar group of marine parasites may serve as living proof of the theory.
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Until fairly recently, parasites called myxosporeans were thought to be offshoots of the eukaryotic line that is not plant, animal or fungi. However, in 1995, researchers Mark Siddall and colleagues posited that myxosporeans are actually unusual types of cnidarians – a group that includes corals and jellyfish. Since then, genetic research has generated evidence to support that idea.
However, this theory does not explain how the parasites acquired such peculiar traits; they have some of the smallest known animal genomes and lack genes that are thought to be essential for multicellular life. Scientists do not yet understand how or why a multicellular organism would have discarded these seemingly essential genes.
The Scandals hypothesis
As reported in the journal Biology Direct, Alexander Panchin at the Russian Academy of Sciences and colleagues have described a wild, but technically plausible theory that would explain it.
Early this year, the team proposed that when myxosporeans branched off from their cnidarian kin, they did not do so as independent organisms, but as tumors.
While acknowledging that the possibility of animals being derived from cancer is far-fetched, Panchin and colleagues have referred to them as Scandals, which stands for “Speciated by cancer development animals.”
Initially, the researchers considered Scandals as no more than a thought experiment. While Panchin was writing about transmissible cancer, he found that his colleagues were finding genes needed for complex tissues in certain simple parasites.
Further discussion led to what Panchin refers to as the “fantastic” idea – that the simple animals could be of cancerous origin. Next, the team decided to analyze all of the data, which led them to propose a new hypothesis.
Firstly, a Scandal would start off as a cancer. More specifically, it would begin as a transmissible cancer so that it would not die once its host did.
Secondly, it would need to be able to spread to other species and thirdly, it would eventually evolve multicellularity. All three stages seem unlikely, but Panchin and colleagues explain how the hypothesis is actually feasible.
Starting off as a transmissible cancer
A Scandal starting off as a transmissible cancer is the easiest stage to justify, since scientists already know that, whilst rare, it does happen. Devil facial tumor disease (DFTD), for example, is known to be transmitted between Tasmanian devils when they bite each other.Transmissible cancers have also been found in molluscs, so there is no reason to assume they would not also arise in cnidarians.
The cancer spreading between species
The spread of cancer between species seems more far-fetched, yet it is still not unheard of, says Athena Aktipis who specializes in cancer evolution at Arizona State University.
As one example, Aktipis refers to a man with HIV who was found to be infected with tumor cells from a tapeworm. Such worm cancers have been seen many times previously in young people with weakened immune systems. If this can happen, then why not consider whether cancers could become parasites in other species, says Aktipis.
I think that the field has been way too cautious about talking about when cancer becomes its own species, or its own kind of organism.”
Panchin thinks that researchers have seen too many examples of transmissible tumors such as DFTD, which are parasites.
Possibly the most unlikely part of the hypothesis is that the cancerous parasite evolves multicellularity. Although simple animals, Myxosporeans are multicellular, so if they did originate from a transmissible tumor, that tumor would have needed to evolve distinct cell types.
Multicellularity is thought to have evolved in eukaryotes 25 times or more. However, in animals specifically, it is thought to have arisen only once, at the base of our lineage.
Some multicellular eukaryotes are known to have reverted back to being unicellular organisms, but no animals are yet known to have done so. In keeping with the Scandal hypothesis, there are no known cases of an organism gaining multicellularity, losing it and then gaining it again, as would be in keeping with the Scandal hypothesis.
“We understand that this is a very improbable scenario,” says Panchin.
However, improbability is not the same as impossibility
I think it’s certainly possible that clusters of cancer cells that are transmissible could evolve to have something like a life cycle. There’s nothing special about the evolutionary process that says you can only evolve a life cycle if you are a branch of the evolutionary tree that didn’t derive from [a part of] another organism.”
In an effort to acquire more substantial evidence for their theory, Panchin and colleagues compared the genomes of various simple species that were mostly parasitic with the genomes of five myxosporeans, three single-celled creatures, and 29 other animals.
They searched for indicators of a cancerous origin by looking for the absence of genes that are frequently lost when cells become malignant.
The scientists had expected that parasites other than Myxosporeans would be the most likely Scandal candidates, but instead, they found that it was only the Myxosporeans that had lost key tumor-suppressing genes.
After further investigation, they found that the Myxosporeans had lost so many genes related to apoptosis that they probably cannot trigger the pathway at all.
This is unusual: “Even if you look at very simplified parasites which are animals, we don’t see this degree of lack of cancer-related genes,” says Panchin.
Researchers say the theory should at least be considered
Aktipis thinks that the researchers have presented some interesting reasons why the possibility that some parasites may have evolved from transmissible cancers should at least be considered.
Adrian Baez-Ortega, a bioinformatician at the University of Cambridge also thinks the hypothesis should be researched further.
“There is almost nothing evolution cannot do,” he says. However, rather than concentrating on specific genes that may be missing, researchers should scan candidate species for the various genomic changes that arise in cancer, he suggests.“If a cancer were to become a long-lived species, all these modifications would be preserved in its genome.”
Panchin says that even if further research fails to support that myxosporeans did evolve from cancers, Scandals could still exist:
We are hoping that maybe some zoologists who have been investigating some other peculiar kind of animal at some point will say, ‘Probably those guys are wrong about Myxosporea, but this [animal], he’s obviously a cancer.’”
Wilcox, C. ‘Wild theory’: Can aggressive cancers evolve into new species? August 28, 2019. Genetic Literacy Project. geneticliteracyproject.org
Panchin, A. Y. et al. (2019). From tumors to species: a SCANDAL hypothesis. Biology Direct. biologydirect.biomedcentral.com/articles/10.1186/s13062-019-0233-1