Plant Physiol, September 2000, Vol. 124, pp. 331-342

Mollusc-Algal Chloroplast Endosymbiosis. Photosynthesis, Thylakoid Protein Maintenance, and Chloroplast Gene Expression Continue for Many Months in the Absence of the Algal Nucleus¹

Program in Molecular and Environmental Plant Sciences (B.J.G., W.-Y.L., T.C.F., R.A.K., M.E.R.), Department of Horticultural Sciences (B.J.G., W.-Y.L., T.C.F., E.J.S., M.E.R.), and Department of Biology (J.R.M., R.A.K.), Texas A&M University, College Station, Texas 77843; and Department of Biology, University of Maryland, College Park, Maryland 20742 (S.K.P.)

Early in its life cycle, the marine mollusc Elysia chlorotica Gould forms an intracellular endosymbiotic association with chloroplasts of the chromophytic alga Vaucheria litorea C. Agardh. As a result, the dark green sea slug can be sustained in culture solely by photoautotrophic CO₂ fixation for at least 9 months if provided with only light and a source of CO₂. Here we demonstrate that the sea slug symbiont chloroplasts maintain photosynthetic oxygen evolution and electron transport activity through photosystems I and II for several months in the absence of any external algal food supply. This activity is correlated to the maintenance of functional levels of chloroplast-encoded photosystem proteins, due in part at least to de novo protein synthesis of chloroplast proteins in the sea slug. Levels of at least one putative algal nuclear encoded protein, a light-harvesting complex protein homolog, were also maintained throughout the 9-month culture period. The chloroplast genome of V. litorea was found to be 119.1 kb, similar to that of other chromophytic algae. Southern analysis and polymerase chain reaction did not detect an algal nuclear genome in the slug, in agreement with earlier microscopic observations. Therefore, the maintenance of photosynthetic activity in the captured chloroplasts is regulated solely by the algal chloroplast and animal nuclear genomes.