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
Brian J.
Green,
Wei-Ye
Li,
James R.
Manhart,
Theodore C.
Fox,
Elizabeth J.
Summer,
Robert A.
Kennedy,2
Sidney K.
Pierce,3 and
Mary E.
Rumpho4*
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.
1
This work was supported in part by the American
Chemical Society Herman Frasch Foundation (grant no. 343-HF92 in
Agricultural Chemistry to M.E.R.), by the National Science Foundation
(grant nos. IBN-9505416 to S.K.P. and M.E.R. and IBN-9808904 to
M.E.R. and J.R.M.), by the Texas A&M University Interdisciplinary
Research Initiative (grants to M.E.R. and J.R.M.), and by the Texas
Agricultural Experiment Station.
2
Present address: Department of Biology, University of
Maine, Orono, ME 04469.
3
Present address: Department of Biology, University of
South Florida, Tampa, FL 33620.
4
Present address: Department of Biochemistry,
Microbiology and Molecular Biology, 5735 Hitchner Hall, Room 273, University of Maine, Orono, ME 04469-5735.
*
Corresponding author; e-mail m-rumpho{at}tamu.edu; fax
979-845-0627.
© 2000 American Society of Plant Physiologists
