The Life and Death of Elysia Chlorotica

Since the end of university I’ve had some trouble letting go of science as a constant occupation. Fortunately I was given the chance to post here about an old journal club presentation rather than go cold turkey, so here is a short piece on one of the most interesting organisms I’ve read about.

The picture on the left is of a sea slug of the genus Elysia which has evolved to blend in with its surroundings by looking almost exactly like a small leaf. The green colour that makes this camouflage so convincing is produced when the slug feeds on algae and steals their chloroplasts, subsequently storing them throughout its body. The use of chloroplast pigments to produce a leaf colour is common among sea slugs but the subject of this post is exceptional among the rest of its family. While in the majority of sea slugs the chloroplasts stop working within days or weeks of incorporation, those taken up by Elysia chlorotica can remain stable and active within the slug for at least ten months. As a result the slug is a rare example of a photosynthetic animal and it can survive without food for a long time, as long as it has light and carbon dioxide.

E. chlorotica has coevolved with a specific species of algae named Vaucheria litorea, which is a source of food and chloroplasts for the slug. The metamorphosis of E. chlorotica larvae into their slug form takes place only if the larvae are attached to filaments of V. litorea and in the absence of this algal species the larvae will die. Immediately after metamorphosis the slugs feed on the algae to which they are attached and take up their chloroplasts, incorporating them into their branched digestive tract.

As mentioned previously, most species of sea slug lose their chloroplast functions soon after obtaining them from algae. This is expected to be because the majority of genes required for chloroplast survival and function are encoded on the algal nucleus. Since animals do not have genes involved in chloroplast maintenance the question of how E. chlorotica manages to sustain its stolen plastids was investigated by several labs. In a recent study, the chloroplast genome of V. litorea was sequenced to determine whether it had greater genetic autonomy than previously sequenced chloroplasts, however, the chloroplast genome was found to be largely normal and was missing many genes essential for photosynthesis.

One of these missing genes, psbO, encodes a component of photosystem II which is vulnerable to damage during photosynthesis and needs to be regularly resynthesised for chloroplast function. Rumpho et al. were able to amplify the whole of this gene from sea slug DNA using primers based on seqeuence databases. The amplified sequence was shown to correspond exactly to the version of psbO from V. litorea, indicating that lateral gene transfer had taken place between the algae and the slug. The same sequence could be amplified from sea slug eggs which had not yet encountered algae, confirming that the sequence had entered the E. chlorotica germline. The authors behind this study speculate that all of the other genes needed for chloroplast survival and function have also been transferred to the slug germline, explaining the unique ability of E. chlorotica to maintain its chloroplasts for its entire life. So it seems that the sea slugs have stolen both DNA and organelles from algae, which is quite an achievement.

Another interesting property of E. chlorotica is that its generations are separated from each other. Whether slugs are collected in the lab or monitored in their natural environment, the entire adult population undergoes a synchronous death every year after they have laid their eggs. Pierce et al. observed that viral particles could be seen in the cytoplasm and nuclei of slugs just before their mass death and suggested that they may be responsible for the mortalities. The morphology of the viruses and the presence of reverse transcriptase activity suggest that they are retroviruses and, since the particles were found even under controlled laboratory conditions, it is likely that the virus is encoded on the genome of the sea slug. The group who made this observation suggested that the viruses may respond to annual environmental changes and kill the sea slugs in the spring, though this hypothesis has not been tested so far.

Unfortunately, the last paper I found on research into the viruses within the sea slugs was in 1999 and so research into this interesting aspect of the organism may not be ongoing. However I'm hoping to find some more about it in the future.

Rumpho, M. (2000). Solar-Powered Sea Slugs. Mollusc/Algal Chloroplast Symbiosis PLANT PHYSIOLOGY, 123 (1), 29-38 DOI: 10.1104/pp.123.1.29

Pierce, S., Maugel, T., Rumpho, M., Hanten, J., & Mondy, W. (1999). Annual Viral Expression in a Sea Slug Population: Life Cycle Control and Symbiotic Chloroplast Maintenance Biological Bulletin, 197 (1) DOI: 10.2307/1542990

Pierce, S., Massey, S., Hanten, J., & Curtis, N. (2003). Horizontal Transfer of Functional Nuclear Genes between Multicellular Organisms Biological Bulletin, 204 (3) DOI: 10.2307/1543594

Rumpho, M., Worful, J., Lee, J., Kannan, K., Tyler, M., Bhattacharya, D., Moustafa, A., & Manhart, J. (2008). From the Cover: Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica Proceedings of the National Academy of Sciences, 105 (46), 17867-17871 DOI: 10.1073/pnas.0804968105