MBE Advance Access originally published online on February 12, 2004
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Mol. Biol. Evol. 21(8):1477-1481. 2004
DOI: 10.1093/molbev/msh086
© 2004 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038
Intracellular Spheroid Bodies of Rhopalodia gibba Have Nitrogen-Fixing Apparatus of Cyanobacterial Origin

* Cell Biology, Philipps-University Marburg, Marburg, Germany
Allan Wilson Centre for Molecular Ecology and Evolution, Institute of Molecular BioSciences, Massey University, Palmerston North, New Zealand
E-mail: maier{at}staff.uni-marburg.de.
Nitrogen fixation is not regarded as a eukaryotic invention. The process has only been reported as being carried out by bacteria. These prokaryotes typically interact with their eukaryotic hosts as extracellular and temporary nonobligate nitrogen-fixing symbionts. However, intracellular permanent "spheroid bodies" have been reported within the fresh-water diatom Rhopalodia gibba, and these, too, have been speculated as being able to provide nitrogen to their host diatom. These spheroid bodies have gram-negative characteristics with thylakoids. We demonstrate that they fix nitrogen under light conditions. We also show that phylogenetic analyses of their 16rRNA and nif D genes predict that their genome is closely related to that of Cyanothece sp. ATCC 51.142, a free-living diazotrophic cyanobacterium. We suggest that the intracellular spheroid bodies of Rhopalodia gibba may represent a vertically transmitted, permanent endosymbiotic stage in the transition from a free-living diazotrophic cyanobacterium to a nitrogen-fixing eukaryotic organelle.
Key Words: spheroid body Rhopalodia gibba nitrogen fixation endosymbiont
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