A Fish Cancer Became Its Own Animal
Black tumours spreading through a border lake are not repeatedly forming inside fish. Genomes show the cancer cells themselves are moving between hosts.
The Discovery
A melanoma in North American catfish has crossed a biological line. It no longer belongs to the fish in which it began. The cancer is now a clonal lineage that passes between animals, carrying the genome of its original host into each new body.
In a Nature study published in the journal's 3 September issue, researchers compared normal and tumour tissue from brown bullheads in Lake Memphremagog, which straddles Vermont and Quebec. Tumours from genetically different fish were more closely related to one another than to the fish carrying them.
The parasite is not carrying the cancer. The cancer is the parasite.
What Makes It Strange
Most cancers arise when one animal's own cells accumulate mutations. Even when a virus triggers tumours in many hosts, each tumour still matches its host. These melanomas share large numbers of variants across unrelated fish, the signature expected when living cancer cells engraft directly into new animals.
- Raised black lesions affected 23–37% of sampled bullheads from 2014 to 2017.
- Some melanomas invaded muscle or spread to internal organs.
- The team found no microorganism unique to diseased fish that could explain transmission.
- Genomic evidence identifies the lineage as the first known naturally transmissible cancer in a fish and in freshwater.
Only a few other animal groups are known to host natural transmissible cancers: dogs, Tasmanian devils and several bivalves. In each case, a cell lineage escaped the lifespan of its founder by becoming contagious.
The Implications
The finding changes the investigation around Lake Memphremagog. Pollution and water quality remain important, especially because the lake supplies drinking water, but contaminated runoff alone cannot explain why tumours carry a shared foreign genome.
It also gives researchers another natural experiment in immune evasion. For foreign cells to survive, the lineage must enter a new fish, avoid rejection and keep dividing. How it moves between bullheads is still unresolved. Contact with damaged skin is one possibility, but the study does not establish a route.
What Happens Next
The lineage can now be tracked across lakes, generations and tumour stages. Researchers can test when it arose, whether some fish resist it and whether its genome is adapting to new hosts.
Cancer usually dies with its owner. In these catfish, one tumour found a way to inherit the future.