Stinging Cells Hold Clues to Biodiversity

The cnidocytes – or stinging cells – characteristic of sea anemones, hydras, corals and jellyfish, and which make us take care of our feet when we wade in the ocean, are also an excellent model for understanding the emergence of new cell types, according to new Cornell research.

In new research published in the Proceedings of the National Academy of Sciences on May 2, Leslie Babonis, assistant professor of ecology and evolutionary biology in the College of Arts and Sciences, showed that these stinging cells evolved by reassigning a neuron inherited from a cnidarian pre-ancestor.

“These surprising results demonstrate how new genes acquire new functions to drive the evolution of biodiversity,” Babonis said. “They suggest that the co-option of ancestral cell types was an important source of new cellular functions during early animal evolution.”

Understanding how specialized cell types, such as stinging cells, is one of the main challenges in evolutionary biology, Babonis said. For nearly a century it has been known that cnidocytes have developed from a pool of stem cells which also give rise to neurons (brain cells), but until now no one knew how these stem cells decide to make a neuron or a cnidocyte. Understanding this process in living cnidarians can reveal clues to the current evolution of cnidocytes, Babonis said.

Cnidocytes (“cnidos is Greek for “stinging nettle”), common to species of the diverse phylum Cnidaria, can throw a poisonous barb or drop or allow cnidarians to stun prey or deter invaders. Cnidarians are the only animals that have cnidocytes, but a lot of animals have neurons, Babonis said, so she and her colleagues at the University of Florida’s Whitney Lab for Marine Bioscience studied cnidarians — especially sea anemones — to understand how a neuron could be reprogrammed to create a new cell.

“One of the unique characteristics of cnidocytes is that they all have an explosive organelle (a small pocket inside the cell) that contains the harpoon that shoots out to sting you,” Babonis said. “These harpoons are made of a protein that is also only found in cnidarians, so cnidocytes seem to be one of the clearest examples of how the origin of a new gene (which codes for a unique protein) could drive the evolution of a new type of cell.”

Using functional genomics from the starlet sea anemone, Nematostella vectensis, the researchers showed that cnidocytes grow by turning off the expression of a neuropeptide, RFamide, in a subset of developing neurons and transforming these cells into cnidocytes. Additionally, the researchers showed that a single cnidarian-specific regulatory gene is responsible for both deactivating neural function in these cells and activating cnidocyte-specific traits.

Neurons and cnidocytes are similar in shape, Babonis said; both are secretory cells capable of ejecting something out of the cell. Neurons secrete neuropeptides, proteins that quickly communicate information to other cells. Cnidocytes secrete poisonous harpoons.

“There’s a single gene that acts like a switch – when it’s on you get a cnidocyte, when it’s off you get a neuron,” Babonis said. “It’s a pretty simple logic to check the identity of cells.”

This is the first study to show that this logic is in place in a cnidarian, Babonis said, so this feature was likely to regulate how cells became different from each other in early multicellular animals.

Babonis and his lab plan future studies to determine how prevalent this genetic on/off switch is in creating new cell types in animals. One project, for example, will investigate whether a similar mechanism is behind the new skeleton-secreting cells in corals.

This research was supported by the National Science Foundation and NASA.

Reference: Babonis LS, Enjolras C, Ryan JF, Martindale MQ. Novel regulatory gene promotes novel cell fate by suppressing ancestral sea anemone fate Nematostella vectensis. PNAS. 2022;119(19):e2113701119. do I: 10.1073/pnas.2113701119

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