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Evolutionary divergence in the nif h.d.k. gene region among nine symbiotic anabaena azollae and between anabaena azollae and some free living heterocystous cyanobacteria



Evolutionary divergence in the nif h.d.k. gene region among nine symbiotic anabaena azollae and between anabaena azollae and some free living heterocystous cyanobacteria



Symbiosis 3(2): 159-178



Using cloned nif DNA from the free-living Anabaena sp. PCC7120, we examined restriction sites in the nifK, nifD and nifH region of Anabaena azollae extracted from 9 Azolla species representing 4 Euazolla and 5 Rhizosperma. Most of the restriction sites in the nifK and nifD genes were conserved among the endosymbiotic members of both Azolla sections. The restriction fragments which hybridized to the nifH probe were identical in the 5 endosymbionts of Rhizosperma, but differed from those observed in the 4 A. azollae extracted from Euazolla species. From restriction site variation in the nifDH region, we established that all symbiotic Anabaena derive from a common ancestral Anabaena azollae and belong to two slightly divergent evolutionary lines. No hybridization was found between DNA from the endosymbionts and a probe carrying a 1.8 kb fragment of the region separating nifK and nifDH in vegetative cells of Anabaena sp. PCC7120, whereas the same probe hybridized to the DNA extracted from seven free-living Anabaena-Nostoc strains.

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