The team eventually realized that the leukemia is a contagious cancer. Unlike almost every other tumor, which begins in an individual and then dies with it, the clam cancer cells are immortal, independent parasites that can move through the water from one host to another. So far, only eight such cancers have been discovered—two in Tasmanian devils, one in dogs, one in the clams, and four more in other kinds of shellfish.
Steamer’s role in all of this is unclear. It’s still possible that the gene’s cut-and-paste antics were the original trigger that created the contagious leukemia in the first place. But cancer aside, Goff and Metzger have found that Steamer has an extraordinary story of its own.
They found Steamer-like genes in the DNA of many species of shellfish—clams, oysters, mussels, cockles, and more. Bizarrely, some of these genes were incredibly similar, even when their hosts were only distantly related. For example, Atlantic razor clams and soft-shell clams have been evolving separately for between 300 and 500 million years, and many of their genes are only 65 percent identical. By contrast, their copies of Steamer are around 97 percent identical.
Another weird pattern: When the team created a family tree of the various species in which Steamer genes appear, and put it next to Steamer’s own genealogy, the two look completely different. This discrepancy means that Steamer isn’t just inherited in the usual vertical way, from parent to offspring. It also moves horizontally between individuals, and even between species. “These events are occurring in evolutionary time,” says Goff. “It could be one event every 10,000 years, or 100,000 years. But once that seeding has occurred, that single gene can easily expand within the new species.”
There are other examples of similar jumping genes, but “the scale documented here is unprecedented,” says Nancy Craig from Johns Hopkins University. Aside from shellfish, Goff’s team also discovered Steamer-like genes in animals from six other major groups (phyla), including corals, sponges, worms, and sea urchins. They’re even present in fish like zebrafish, carp, and salmon, and seem to have jumped from one fish genome to another at least five times in the past. In fact, the Steamer gene in zebrafish is almost identical to the original one that was first identified in the soft-shell clams. “It’s astounding,” says Goff. “We don’t have any bright insights into how it could have happened.”
Here’s a clue: Steamer genes only seem to jump between aquatic animals. It’s not in birds or mammals, insects or spiders. This suggests that it’s moving through the water—but how?
One possibility is that it’s acting like a virus. Steamer belongs to the same large group of retrotransposons that gave rise to retroviruses like HIV. That transition happened when some of these genes gained the ability to enclose themselves in an envelope of proteins, allowing them to travel outside their host cells and enter new ones. But Steamer genes haven’t made that leap yet. They can’t make their own envelopes, so it’s anyone’s guess how they might first escape from their host cells.