PHYSIOLOGICAL AND MOLECULAR ASPECTS OF ORGANELLE SEQUESTRATION IN THE CILIATE MYRIONECTA RUBRA
Johnson, Matthew David
Stoecker, Diane K
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The phototrophic ciliate Myrionecta rubra was studied in order to understand the role of organelle retention from its prey, Geminigera cryophila (Cryptophyceae). Sequencing of SSU rRNA genes of M. rubra, and a close relative Mesodinium pulex, revealed short (<1600>bp) sequences with high substitution rates. Phylogenetic analysis yielded a basal placement within the ciliates. Application of a fluorescence in situ hybridization probe revealed the genes were expressed in the cytoplasm and nucleoli of the ciliates. Phylogenetic analysis of the chloroplast nucleomorph SSU rRNA gene in M. rubra confirmed their origin to be from G. cryophila. Feeding on G. cryophila was shown to increase growth, which remained constant for 4 weeks of starvation and then declined slowly over time. Chlorophyll a (chl a) synthesis and plastid division decreased faster than growth, resulting in declines in cell pigment over time. During starvation photosynthetic efficiency declined slightly, while overall photosynthesis became uncoupled from growth, resulting in decreased growth efficiencies. While plastids in M. rubra or G. cryophila have equal efficiency at saturating irradiance, they are more efficient in G. cryophila in low light and have a greater overall quantum yield of photochemistry. Lower C-specific photosynthetic rates and chl a:C ratios resulted in lower growth compared to G. cryophila, which was reflected in increased partitioning of C to lipid fractions and lower protein production in M. rubra. A novel process, retention of prey nuclei (PN), was found to occur in M. rubra. G. cryophila nuclei are retained in a transcriptionally active state for >20 days, and remain in the cytoplasm for at least 30 days, apparently with no net division. Cell and plastid division are maximum during PN retention, and decline after PN loss. The PN genes LHCC10 and GAPDH were expressed at levels greater than those observed in G. cryophila. Expression of a plastid (psbA) and nucleomorph (cbbX) gene was also confirmed. M. rubra is a unique phototroph, capable of regulating and dividing certain G. cryophila organelles, and reaching maximum growth potential when PN are present. Feeding on G. cryophila appears to be most important for obtaining PN.