Characterization of a potential eukaryote to eukaryote horizontal gene transfer
Steve Massey, University of South Florida

Sacoglossan sea slugs participate in a type of cell-level symbiosis, termed kleptoplasty, whereby chloroplasts obtained from their algal food source are taken up by, and maintained inside, specialized gut cells. The two species of slugs we are studying, Elysia chlorotica and Elysia crispata, can maintain their chloroplasts in a functional state for several months. Under bright light the symbiotic chloroplasts photosynthesize, fixing carbon and evolving oxygen in the slug cell as if they were still in the algae. We are investigating the symbiosis at the protein and genetic levels, in order to understand the adaptations that allow the slugs to maintain chloroplast function for such extended periods of time. In particular, we have demonstrated that synthesis of chloroplast proteins continues for the duration of the symbiosis. Some of proteins synthesized in the slug chloroplasts appear to be photosynthetic proteins that are universally encoded by nuclear genes in algae and plants. Our pharmacological evidence indicates that indeed these proteins are synthesized in the slug cell from nuclear genes. So we are currently testing the slug for the presence of algal nuclear genes encoding photosynthetic proteins.

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Further references

Pierce, S.K., Massey, S.E., Hanten, J.J., Curtis, N.E. 2003. Horizontal transfer of functional nuclear genes between multicellular organisms. Biol Bull 204, 237-240.

Hanten, J.J., Pierce, S.K. 2001. Synthesis of several light-harvesting complex I polypeptides is blocked by cycloheximide in symbiotic chloroplasts in the sea slug, Elysia chlorotica (Gould): a case for horizontal gene transfer between alga and animal? Biol Bull 201, 34-44.

Green, B.J., Li, W.Y., Manhart, J.R., Fox, T.C., Summer, E.J., Kennedy, R.A., Pierce, S.K., Rumpho, M.E. 2000. Mollusc-algal chloroplast endosymbiosis. Photosynthesis, thylakoid protein maintenance, and chloroplast gene expression continue for many months in the absence of the algal nucleus. Plant Physiol 124, 331-342.

Mujer, C.V., Andrews, D.L., Manhart, J.R., Pierce, S.K., Rumpho, M.E. 1996. Chloroplast genes are expressed during intracellular symbiotic association of Vaucheria litorea plastids with the sea slug Elysia chlorotica. PNAS USA 93, 12333-12338.

Pierce, S., Biron, R., Rumpho, M. 1999. Endosymbiotic chloroplasts in molluscan cells contain proteins synthesized after plastid capture. J Exp Biol 199, 2323-2330.

Rumpho, M.E., Summer, E.J., Manhart, J.R. 2000. Solar-powered sea slugs. Mollusc/algal chloroplast symbiosis. Plant Physiol 123, 29-38.

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