Differential patterns of evolution and distribution of the symbiotic behaviour in nostocacean cyanobacteria

Many cyanobacteria commonly identified as belonging to the genus Nostoc 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 Nostoc 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 Gunnera, Azolla, Cycas, Dioon, Encephalarthos, Macrozamia, and Anthoceros) with free-living Nostoc isolates originating from different habitats. After preliminary clustering with ARDRA (Amplified Ribosomal DNA Restriction Analysis), phylogeny was reconstructed on the basis of 16S rRNA gene sequences and compared with morphological characterization, obtaining several supported clusters. Two main Nostoc clusters harboured almost all cyanobionts of Gunnera, Anthoceros and of several cycads, together with free-living strains of the species N. muscorum, N. calcicola, N. edaphicum, N. ellipsosporum, and strains related to N. commune. We suggest that the high occurrence of symbiotic strains within these clusters is explained by the intensive hormogonia production that was observed in many of the studied strains. However, no evidence for discrimination between symbiotic and free-living strains either by molecular or morphological approaches could be found. Sequences of Azolla cyanobiont filaments taken directly from leaf cavities clustered tightly together in the vicinity of the planktic cyanobacterium Cylindrospermopsis raciborskii, of benthic Anabaena cylindrica 133 and A. oscillarioides HINDAK 1984/43, with high bootstrap values. The phylogenetic analysis showed that two distinct patterns of evolution of the symbiotic behaviour might exist for the nostocacean cyanobacteria, one leading to symbioses of Nostoc species with a wide variety of plants, the other leading to the association of a unique cyanobacterial type with the water fern Azolla.