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DIVERSITY AND SPECIFICITY IN CYANOBACTERIAL SYMBIOSES U. Rasmussen and C. Johansson U.Rasmussen (corresponding author; e-mailullarasmussen@ botan su se) and C Johansson, Department ofBotany,Stockholm University, 106 91 Stockholm, Sweden INTRODUCTION Cyanobacteria constitute one of the largest sub groups of Gram-negative photosynthetic prokary otes and are of great evolutionary antiquity, dating back to the Precambrian (2.8-3.5 billion years ago). They are found worldwide in highly diverse ecosystems, from aquatic (limnic and marine) habi tats to terrestrial systems and from polar to tropical regions of the globe. In terrestrial systems, nitro gen-fixing Nostoc isby far themost common genus and includes cyanobacteria capable of forming symbioses with a broad range of plants and other organisms. Hosts include fungi (Geosiphon species and lichenised fungi), bryophytes (liverworts and hornworts), aquatic ferns (genus Azolla), gym nosperms (cycads) and angiosperms (genus Gunnera) (Bergman et al. 1996). CELL DIFFERENTIATION IN NOSTOC The key to the success of the genus Nostoc, whether in the free-living form or in symbiosis, is most likely its extreme structural and functional plasticity. Influenced by environmental changes, Nostoc undergoes differentiation into several differ ent cell and filament types (P1. I).When grown under nitrogen-limited conditions or in symbiosis, vegetative cells differentiate to form specialised nitrogen-fixing heterocysts at a frequency of ap proximately 5% in the free-living form and up to 80% in symbiosis with hosts such as Gunnera spe cies. In addition, the entire vegetative filament can differentiate into amotile stage, the hormogonium, which serves as a means of dispersal under unfa vourable conditions and constitutes the 'infective unit' during establishment of the symbiosis. Finally, akinetes are differentiated under adverse conditions and function as spores. Akinete differentiation is an unusual stage in symbiotic associations and is known to play a role only in Azolla, in which akinetes function as cyanobacterial inocula for the next sporophyte generation. In addition, the cyanobacteria must change their metabolism from that of a free-living, phototrophic organism to that of a symbiotic, heterotrophic organism in order to be symbiotically competent. The mechanisms in volved in this shift are largely unknown. SPECIFICITY The specificity of the cyanobacterium-host inter action has been investigated by isolating and grow ing the two symbiotic partners separately and then reconstituting the symbiotic association under lab oratory conditions. Nostoc strains isolated from a cycad, the bryophyte Anthoceros and a lichen all infect Gunnera. Similarly, strains isolated from cycads, Gunnera and a lichen infect Anthoceros species. Hence, Nostoc in general exhibits a very low host specificity, with no correlation to the taxonomic status of the host. Furthermore, the host has a low specificity for its cyanobiont. Even certain strains of Calothrix and Chlorogloeopsis can infect both the homwort Phaeoceros and the liverwort Blasia (West and Adams 1997). Moreover, the cyanobiont shows great adap tive flexibility because the same strain has the capacity to infect different host organs. The cyanobacteria are found extracellularly among the fungal hyphae in lichens, in the cavities of bryophyte thalli and Azolla leaves and in the spe cialised roots of cycads, whereas they are found intracellularly in the fungus Geosiphon and in the stem glands of Gunnera. Taken together, these facts indicate that the host is the (more) selective part ner. Cyanobacteria are unique among symbiotic bacteria in their ability to infect a broad range of hosts and organs. GENETIC DIVERSITY The extreme morphological flexibility of Nostoc makes identification and taxonomy based onmorph ology problematic and unreliable. In the recent literature, many symbiotic isolates (from Gunnera, cycads, bryophytes and the fungus Geosiphon) have been referred to as N. punctiforme, but a pheno type-based characterisation has proved inadequate. No genetic analysis has yet been used to identify the taxonomic status of different symbiotic isolates. At present, the taxonomic interrelatedness of the symbiotic Nostoc strains is largely unknown and does not extend below the genus level. More precise genetic information must therefore be ob tained to establish a more reliable classification system. In recent years, molecular methods have contrbuted to amore detailed classification of the genus Nostoc (Table 1). BIOLOGY AND ENVIRONMENT PPOCEENDINGS OF THE ROYAL IRISHACADEMY, VOL 102B, No 1, 53-56 (2002) 0) ROYAL IRISHACAsEMY 53 BIOLOGY AND ENVIRONMENT P1...


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