Anyone who has been stung by a jellyfish might think they know more than enough about cnidocytes, the cells that deliver the sting. But Leslie Babonis has found that these cells have much more to tell, including insight into a simple evolutionary mechanism that may enable jellyfish, sea anemones, corals, and their relatives—collectively known as cnidarians—to quickly adapt to new environments. Babonis has found that a single genetic switch can transform cells that sting into cells that cling, enabling the animal to colonize new surfaces. The work suggests this kind of switch may have helped power evolutionary transformations for hundreds of millions of years.
The finding, reported last week at the virtual annual meeting of the Society for Integrative and Comparative Biology, is “an impressive biological discovery … [providing] powerful observations about how new cell types evolve and arrive in novel places” says Kristen Koenig, an evolutionary developmental (evodevo) biologist at Harvard University. Researchers have long known that some single genetic switches can cause one body part to develop as another in more complex organisms. But Babonis’s finding suggests this simple mechanism for evolutionary change dates back to some of the earliest animals.
Cnidocytes have multiple guises, making them a tempting focus for biologists studying how new kinds of tissues evolved. The stinger cells (also called nematocytes) of jellyfish shoot tiny, toxin-laden harpoons, whereas some cnidocytes in sea anemones and corals shoot out threads of sticky material that help the animals anchor themselves in substrates like mud. “The cnidocytes are a great model system to study the evolution and diversification of novel cell types,” revealing principles that can be applied to other animals, says Billie Swalla, an evodevo researcher at the University of Washington, Seattle.
Babonis, an evodevo biologist at Cornell University, looked at the starlet sea anemone Nematostella vectensis, a 5-centimeter, translucent creature common in Atlantic Ocean estuaries. She and her colleagues used the gene-editing tool CRISPR to knock out the anemone’s Sox2 gene, known to be important in neural development in many animals.
After disabling Sox2, the researchers closely examined the body wall and other parts of the anemone under a light microscope. Nematocytes in the tentacle tips developed crooked harpoons, Babonis found. The nematocytes normally found in the body wall were missing, replaced by fat cnidocytes called robust spirocytes, known for their stickiness. Those cells had never been seen in this species.
“You don’t usually expect organisms to be capable of producing organs that they don’t normally develop,” says Nicole Webster, an evolutionary biologist at Clark University. “It’s like a human growing a prehensile tail.”
It seems the Sox2 gene controls a switch. When it’s working, stinger cells form. When it’s not, spirocytes form, but apparently only in the body wall. (No one has yet studied whether Sox2 is important for spirocyte formation in other anemones or cnidarians.) By transforming cells in just one part of the body, such developmental switching may have equipped early cnidarians to explore new habitats, Babonis suggested. For example, some sea anemones—equipped with plenty of robust spirocytes—can burrow into ice; one, Edwardsiella andrillae, has colonized the underside of the Antarctic ice shelf.
The study suggests “how the evolution of novel traits can exist on a small scale—different cell types—but can lead to larger consequences,” Webster says.
Researchers have seen similar switches, called homeotic transformations, in the nervous systems of worms, flies, and fish, known as bilaterians because they have symmetrical left and right sides. Sea anemones are from a different branch of the animal tree; they have only two layers of cells and radial rather than bilateral symmetry. Thus the finding suggests that this kind of powerful evolutionary mechanism—a simple genetic switch—dates back at least 600 million years to the common ancestor of bilaterians and cnidarians, “before [the] sophisticated morphology of worms and flies came on the scene,” Babonis says.
Marymeagan Daly, an integrative biologist at Ohio State University, Columbus, who studies the Antarctic sea anemones, cautions that the experimentally transformed cnidocytes need to be more closely examined to be sure they are bona fide spirocytes; Babonis agrees and notes other open questions, such as whether spirocytes or nematocytes evolved first and thus, whether burrowing or predation was cnidocytes’ earliest function. Still, Daly says, the work “shows that big changes can be triggered by small differences in [gene] expression.”