Kleptoplast Molluscs

Kleptoplasty is a symbiotic relationship between an algae and a host organism. The algae ingested and digested by the host and the chlorophyll containing plastids retain their photosynthetic capabilities with the host. Most of these hosts are single cell organisms such as Dinoflagellates, Ciliates, and Foraminifera. There is at least one multicellular animal that also practices kleptoplasty—the mollusc sea slugs in the clade Sacoglossa.

Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica.
Rumpho ME, Worful JM, Lee J, Kannan K, Tyler MS, Bhattacharya D, Moustafa A, Manhart JR.

Elysia chlorotica

The sea slug Elysia chlorotica acquires plastids by ingestion of

its algal food source Vaucheria litorea. Organelles are sequestered in the mollusc’s digestive epithelium, where they photosynthesize for months in the absence of algal nucleocytoplasm. This is perplexing because plastid metabolism depends on the nuclear genome for >90% of the needed proteins. Two possible explanations for the persistence of photosynthesis in the sea slug are (i) the ability of V. litorea plastids to retain genetic autonomy and/or (ii) more likely, the mollusc provides the essential plastid proteins. Under the latter scenario, genes supporting photosynthesis have been acquired by the animal via horizontal gene transfer and the encoded proteins are retargeted to the plastid. We sequenced the plastid genome and confirmed that it lacks the full complement of genes required for photosynthesis. In support of the second scenario, we demonstrated that a nuclear gene of oxygenic photosynthesis, psbO, is expressed in the sea slug and has integrated into the germline. The source of psbO in the sea slug is V. litorea because this sequence is identical from the predator and prey genomes. Evidence that the transferred gene has integrated into sea slug nuclear DNA comes from the finding of a highly diverged psbO 3′ flanking sequence in the algal and mollusc nuclear homologues and gene absence from the mitochondrial genome of E. chlorotica. We demonstrate that foreign organelle retention generates metabolic novelty (“green animals”) and is explained by anastomosis of distinct branches of the tree of life driven by predation and horizontal gene transfer.

In an article in The New Scientist, Ms. Rumpho talks about the study on Elysia chlorotica,  a green sea slug, with a gelatinous leaf-shaped body, that lives along the Atlantic seaboard of the US.

She has known for some time that E. chlorotica acquires chloroplasts – the green cellular objects that allow plant cells to convert sunlight into energy – from the algae it eats, and stores them in the cells that line its gut.

Young E. chlorotica fed with algae for two weeks, could survive for the rest of their year-long lives without eating, Rumpho found in earlier work….

They then turned their attention to the sea slug’s own DNA and found one of the vital algal genes was present. Its sequence was identical to the algal version, indicating that the slug had probably stolen the gene from its food.

“We do not know how this is possible and can only postulate on it,” says Rumpho, who says that the phenomenon of stealing is known as kleptoplasty.

One possibility is that, as the algae are processed in the sea slug’s gut, the gene is taken into its cells as along with the chloroplasts. The genes are then incorporated into the sea slug’s own DNA, allowing the animal to produce the necessary proteins for the stolen chloroplasts to continue working.

Another explanation is that a virus found in the sea slug carries the DNA from the algal cells to the sea slug’s cells. However, Rumpho says her team does not have any evidence for this yet.

In another surprising development, the researchers found the algal gene in E. chlorotica’s sex cells, meaning the ability to maintain functional chloroplasts could be passed to the next generation.

The researchers believe many more photosynthesis genes are acquired by E. chlorotica from their food, but still need to understand how the plant genes are activated inside sea-slug cells.

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Rebecca Johnson gives a presentation on her research into these animals.

It has been speculated that host cells ingesting other organisms is the origin of  mitochondria within cells. Perhaps what we have here is an example of something that happened those billions of years ago.

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