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In Situ Plastid and Mitochondrial DNA Determination: Implication of the Observed Minimal Plastid Genome Number

Mark J. Maguire, Lynda J. Goff and Annette W. Coleman
American Journal of Botany
Vol. 82, No. 12 (Dec., 1995), pp. 1496-1506
Stable URL: http://www.jstor.org/stable/2446177
Page Count: 11
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
In Situ Plastid and Mitochondrial DNA Determination: Implication of the Observed Minimal Plastid Genome Number
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Abstract

The total loss of plastid DNA has never been reported for any alga or plant cell line, with the sole exception of the protozoan Euglena, yet plastid distribution at mitosis is apparently stochastric (Birky and Skavaril, Journal of Theoretical Biology, vol 106, pp 441-447, 1984) and accidental loss might be expected. It is not obvious how stem cells of photosynthetic eukaryotes avoid this problem. The chrysophyte alga Ochromonas danica, described as having but one or two plastids, can proliferate indefinitely without the benefit of photosynthesis. Under such conditions its plastid genome copy number per cell might drop to the absolute minimum compatible with maintaining its inheritance. In situ quantitation of Ochromonas plastid DNA in both photosynthetic and enriched mixotrophic growth, and in heterotrophic growth in prolonged darkness, suggests that plastids are capable of very wide variation (7 to >200 genomes/plastid) in their DNA content, and likewise, cells can vary from one to >8 plastids per cell, with total genomes numbers from 7 to >1,000 per cell. Among many growth conditions tested, the smallest plastids were found in rapidly dividing cells grown in the dark, many of which contained but one plastid The inability to find plastids with fewer than seven plastid genome equivalents of DNA, even in these rapidly multiplying cells grown in total darkness for months, suggests that multiple copies of the plastid genome may be very carefully maintained, even in the prolonged absence of photosynthesis. This implies that multiple copies are important for reasons other than photosynthetic capability; two possibilities are the biosynthetic steps necessary for eukaryote cell survival known to occur solely within a plastid, and/or the potential that multiple plastid genome copies provide to escape the effects of Muller's ratchet.

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