A third example emerged last year. Along North America’s east coast, soft-shell clams were dying from a strange type of leukemia. Michael Metzger and Stephen Goff, scientists from Columbia University, studied these cancers and found that they were all genetically identical to each other, but genetically distinct from their hosts. That’s the same pattern seen in the Tasmanian devil and dog tumors—a clear sign that these cancers arrive in their hosts, rather than originating from them. They drift through the sea, these selfish shellfish cells, traveling from one cancer-ridden clam to another.
Intrigued, Metzger and Goff polled their marine biologist colleagues and learned that many other shellfish species are afflicted by similar rapidly spreading leukemias. They collected cockles and golden carpet shell clams from the coast of Spain, and mussels from the coast of Vancouver. In all three cases, they found the same signature pattern: a genetic match between all the tumors, and a mismatch between each tumor and its respective host.
“Prior to this, we believed that transmissible cancers were bizarre flukes of nature that happened due to a set of unfortunate coincidences in very unlucky species,” says Elizabeth Murchison, a University of Cambridge cancer researcher who studies the Tasmanian devil tumor. Instead, they are “probably relatively common, at least some bivalves, and the processes whereby cancers become transmissible are not as rare as we previously thought.”
Indeed, Metzger and Goff found that cockles have given risen to two strains of contagious cancer. Their tumors belonged to two distinctive lineages, each of which seems to have independently arisen from a different healthy cell. That explains why the same disease presents in two distinct ways, characterized by cells that look different under the microscope. “People noted that, said, ‘Isn’t that odd?’ and moved on,” says Goff. “This explains the mystery.”
There’s precedent for a dual origin. Earlier this year, Murchison showed that the Tasmanian devil’s contagious tumor also arose twice. “We absolutely couldn’t believe it,” she told me at the time. “It’s the last thing I could have possibly imagined.”
It might now be the second-to-last thing. A bigger surprise came when Metzger and Goff studied the golden carpet shells. Their tumors were not just genetically distinct from their hosts, but wildly so, with matches as low as 78 percent for certain critical genes. “They weren’t even close,” says Goff. “We then realized they were a near perfect match to the cells of another species, the pullet shell.” The cells must have originated there before jumping into the golden carpets.
Oddly, the pullet shells themselves show no signs of the cancer. They may have given rise to it, but they no longer suffer from it. Why? “One could imagine that the species-of-origin is now resistant to the tumor,” says Goff, “but we don’t know that.”