Many cyanobacteria commonly identified as belonging to the genus are well-known cyanobionts (symbionts) of a wide variety of plants and fungi. They form symbioses with bryophytes, pteridophytes, gymnosperms and angiosperms that are considerably different in the type of reciprocal interaction between the host and the cyanobiont. The phylogenetic and taxonomic relationships among cyanobionts isolated from different hosts and strains isolated from free-living conditions are still not well understood. We compared phylogeny and morphology of symbiotic cyanobacteria originating from different host plants (genera , , , , , and ) with free-living isolates originating from different habitats. After preliminary clustering with ARDRA (amplified rDNA restriction analysis), phylogeny was reconstructed on the basis of 16S rRNA gene sequences and compared with morphological characterization, obtaining several supported clusters. Two main clusters harboured almost all cyanobionts of , and of several cycads, together with free-living strains of the species , , , and strains related to . We suggest that the frequent occurrence of symbiotic strains within these clusters is explained by the intensive hormogonia production that was observed in many of the strains studied. However, no evidence for discrimination between symbiotic and free-living strains, either by molecular or morphological approaches, could be found. Sequences of cyanobiont filaments, taken directly from leaf cavities, clustered tightly with sequences from the planktic cyanobacterium , from the benthic 133 and from HINDAK 1984/43, with high bootstrap values. The phylogenetic analysis showed that two distinct patterns of evolution of symbiotic behaviour might exist for the nostocacean cyanobacteria, one leading to symbioses of species with a wide variety of plants, the other leading to the association of a unique cyanobacterial type with the water fern .


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vol. , part 3, pp. 553–564

Amplified rDNA restriction analysis (ARDRA) of symbiotic and free-living cyanobacteria performed on the 16S rRNA gene+ITS region. [PDF](21 KB)

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