Nematocysts & Spirocysts: The Evolution of Sea Anemone

Scientists discovered a new evolutionary mechanism that allows cnidarians (jellyfish and sea anemones) to adapt to new ecosystems. Scientists discovered that the absence of a single gene can transform a cnidarian's nematocysts (stinger cells) into spirocysts; this transformation allows the cnidarian to colonize in new areas of the ocean. This genetic mutation is believed to have triggered hundreds of evolutionary changes among the cnidarian phylum.

Cnidarians have numerous methods of camouflage. Hence, they are widely studied by scientists and are a large focus of researchers who specialize in how tissues evolve. Much research has been performed on nematocysts. In healthy cnidarians, nematocysts expel poisonous fluid out of harpoons formed on the cell membrane, and in some cases, these stinger cells will shoot sticky material, which helps cnidarians adhere to other surfaces like rocks or sand.

Scientists analyzed the sea anemone nematostella vectensis and their nematocysts. Using CRISPR gene editing, they bred a Sox2 knock-out model of the sea anemone. The Sox2 gene is a key to proper brain development in numerous phyla - including the cnidarian phylum. After deleting the Sox2 gene from the nematostella vectensis test subjects, researchers analyzed the body wall of the KO group under a microscope. In the KO group, the nematocysts often located in the body wall were 98% gone and any nematocysts left had developed crooked harpoons, rendering them damaged and incapacitated. In replace of the nematocysts, in the KO group, were robust spirocysts. Spirocysts have never been seen in the Nematostella vectensisse species. This finding was huge for researchers. It was shocking to see a species develop an organ it doesn't normally produce.

From these findings, researchers have hypothesized that the Sox2 gene not only controls neural growth but also the formation of nematocysts. Some researchers also have concluded that Sox2 gene may inhibit the formation of spirocysts, which would explain why spirocysts formed when the Sox2 gene was deleted in Nematostella vectensisse test subjects.

In opposition, some scientists speculate that spirocysts formed as a means of survival in the absence of Sox2 and that the Sox2 gene may have no connection to the inhibition of spirocysts. The spirocyte cell gives cnidarians the ability to stick to very smooth, slippery surfaces like ice. For example, the Edwardsiella andrillae, which has spirocytes, can bury itself into ice burrows in the Arctic. Perhaps, in the absence of Sox2 the body of the Nematostella vectensisse quickly evolved (created spirocysts) so its body wall could carry out some of the same functions.

Overall, this study gives promising insight into how evolution can occur on small scale in localized areas of certain species. Similar biological transformations have been observed in the nervous systems of other animals like worms and fish (also known as bilaterians). Nematostella vectensisse are from a different branch of the phylogenetic tree than fish and worms, which suggests that this ability to evolutionarily/genetically transform in one part of the body, within hours, predates back about 600 million years ago to both species' common ancestor.

Sources:

Sunagar, K., Columbus-Shenkar, Y.Y., Fridrich, A. et al. Cell type-specific expression profiling unravels the development and evolution of stinging cells in sea anemone. BMC Biol16, 108 (2018). https://doi.org/10.1186/s12915-018-0578-4