Scientists in the USA have discovered a remarkable American species of sea slug that can utilise solar power after harvesting the chloroplasts of its algal prey.

Elysia chlorotica, which is found in waters off the east coast of the USA, feeds on a particular algae called Vaucheria litorea in order to harvest its chloroplasts. Whilst this had been known for several years,  no-one understood how it was able to use chloroplasts to harvest the sun’s energy for food.

Last week’s edition of PNAS reported on research conducted by Mary Rumpho and her research team at the University of Maine, who demonstrated that E. chlorotica probably “stole” some of its genes from the algae, in order to make the harvested chloroplasts work!

This is the most remarkable instance of natural ‘genetic engineering’ that I am aware of, and is certainly a significant finding.

In most animals, the digestive process destroys most of the cells and DNA that form part of the food. This is not the case for E. chlorotica, which uses its specially-designed gut to harvest chloroplasts from the algae. The chloroplasts are then used to generate solar-power for the slug.

This effective use of photosynthesis – a process normally found only in plants and some bacteria – means that individual slugs can survive for months without eating. However, E. chlorotica can only harvest chloroplasts from V. litorea.

Chloroplasts are organelles found in plant cells. They are the site of photosynthesis. Containing the green pigment chlorophyll, chloroplasts not only give plants their colour, but permit the sun’s light energy to be turned into chemical energy (food). Animal cells don’t contain chloroplasts, which is why animals have to eat plants or animals to survive.

According to evolutionary theory, chloroplasts started out as separate prokaryotic organisms which were ‘engulfed’ in an endosymbiotic process.  Evidence for this comes from modern-day plant cells. Chloroplasts have their own DNA sequences, separate from the rest of the nuclear DNA of the plant cell they belong to. However, chloroplast DNA (ctDNA) typically encodes a mere 10% of the proteins required for photosynthesis to occur. The remaining 90% of proteins are encoded by the plant’s nuclear DNA.

This raises an important question about E. chlorotica. If the slugs have acquired foreign chloroplasts, how can they use them if they don’t possess the remaining 90% of genes required to make them functional?

Rumpho isolated a gene critical for photosynthesis from the genomic DNA of the sea slug. The gene, called PsbO or “manganese-stabilising protein” has until now only ever been found in plants. Under normal circumstances, animals have no use for such a gene.

Yet E. chlorotica seems to have acquired the gene, thus providing a clue about the answer to the problem. By “stealing” the gene from the algae, the slug is able to maintain the function of the chloroplasts and therefore photosynthesise just like a plant. The assumption is that the slug has also “stolen” the other required genes, but this is yet to be demonstrated.

Rumpho suggests that this is a case of “horizontal gene transfer“, whereby a gene is shared between two unrelated, non-mating organisms; in this case a sea slug and an algae. Critical evidence comes from the results of the sequencing of the PsbO gene. Rumpho et al. found complete homology between the initial 963bp of the PsbO gene in both the algae and the sea slug. Given the evolutionary divergence of algae and molluscs, the evidence of gene “theft” by the slug is strong. The evidence points to the algae as the original source of the gene.

It is not known what the physical process is for horizontal gene transfer in this case, although there are several theories.

I consider this to be the most exciting discovery in biology for 2008. This research not only emphasises how much we still have to learn about the natural world around us, but it plays into the robust debate that currently exists about the process of “genetic engineering”. Here is one more example of gene transfer between completely unrelated organisms, occurring in nature.

Certainly something to ponder.

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More information: Subscribers can access a copy of the original research paper on the PNAS website. Free information is also available on the New Scientist website. Mary Rumpho also has a professional website which contains photos and details of her previous research.

M.E. Rumpho et al. (2008) Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy of Science of the United States of America 105 (46): 17867-17871 [DOI: 0.1073/pnas.0804968105]