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Specifity of Association of Nostocs in Lichens

A generality that seems to be emerging is that there is little specificity governing symbiotic interactions between Nostocs and plants (1,2). A cyanobacterium isolated from Yugoslavian soil can infect a Gunnera from New Zealand. A strain isolated from the coralloid roots of a cycad can form a productive symbiosis with the bryophyte Anthoceros. Per Paulsrud and Peter Lindblad of the University of Uppsala and Jouko Rikkinen of University of Helsinki have made surprising observations (3,4) that casts doubt on this generality.

Paulsrud et al amplified and sequenced the tRNA-Leu region from DNA isolated from Nostocs cyanobionts of bi- and tripartate lichens. The lichen were taken from different sites in Sweden and Finland. The tRNALeu gene from Nostocs contain an intron of highly variable sequence, making it useful for basing phylogenetic inferences. Using the intron sequence as a marker, they found a variety of different Nostoc strains within the lichens collected, but in general, each strain was associated with a single fungal species.

How can one reconcile the apparent specificity of cyanobacterial-fungal associations with the lack of specificity observed with associations with plants? It is difficult to argue that diversity in plant associations was found in lab reconstitutions (where plants were not given a choice of Nostocs) while specificity was found in the field, because some field studies with plants have also demonstrated cyanobiont diversity at different sites (5,6). Paulsrud et al suggested that the difference seen with lichen may result from the ability of a single lichen to fragment and disperse over huge distances, perhaps even globally.

1. Enderlin CS, Meeks JC (1983). Planta 158:157-165
2. Johansson C, Bergman B (1994). New Phytol 126:643-652
3. Paulsrud P, Lindblad P (1998). Appl Environ Microbiol 64:310-315
4. Paulsrud P et al (1998). New Phytol in press
5. Zimmerman WJ, Bergman B (1990). Microb Ecol 19:291-302
6. West NJ, Adams DG (1997). Appl Environ Microbiol 63:4479-4484