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Evolution: Can New Species Evolve From Cancers? Maybe. Here’s How.

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Researchers agree it’s a long shot, but transmissible cancers could theoretically evolve into independent species. Certain weird parasites might be living proof.

Quanta Magazine said:
Aggressive cancers can spread so fiercely that they seem less like tissues gone wrong and more like invasive parasites looking to consume and then break free of their host. If a wild theory recently floated in Biology Direct is correct, something like that might indeed happen on rare occasions: Cancers that learn how to roam between hosts may gradually evolve into their own multicellular species. Researchers are now scrutinizing a peculiar group of marine parasites called myxosporeans to see whether they might be the first known example.

Even among microscopic parasites, myxosporeans are enigmatic. They were first discovered nearly two centuries ago, and more than 2,000 species are recognized today. Their complex life cycles make study particularly difficult: It wasn’t until the 1980s that scientists realized the ones found in fish were the same species as those found in worms, and not completely different classes of parasite. And while most parasites are content merely to snuggle into their animal host’s tissues, myxosporeans often take up residence inside a host’s own cells.

Until fairly recently, myxosporeans were considered to be protists, offshoots of the eukaryotic line that are neither plants, animals nor fungi. In 1995, however, Mark Siddall, then at the Virginia Institute of Marine Science, and his colleagues argued that myxosporeans are weird members of the cnidarians, the group that includes jellyfish and corals. Since then, genetic studies have bolstered that position.

But their location on the tree of life doesn’t explain how myxosporeans ended up with such strange traits. Myxosporeans boast some of the smallest known animal genomes. The genome of Kudoa iwatai, for example, is estimated to be a mere 22.5 megabases, considerably smaller than that of any other cnidarian genome. It’s less than one-twentieth the size of the genome of Polypodium hydriforme, a closely related cnidarian parasite.

Moreover, their genomes have not just been catastrophically reduced. They specifically lack certain genes thought to be essential for multicellular life. It’s not clear how or why a complex multicellular creature discarded these seemingly necessary genes along with huge chunks of its DNA.

Yet Alexander Panchin, a senior researcher at the Russian Academy of Sciences, and his colleagues have an intriguing if controversial hypothesis to explain it. Early this year, they proposed that myxosporeans initially branched off from their cnidarian kin not as independent animals but as tumors.

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