Cyanobacteria belonging to the genus Nostoc have the capacity to form symbiotic associations with a wide range of organisms. Diversity, specificity and cellular modifications of the symbiosis between Nostoc and fungi in the formation of lichens were investigated in this thesis.

The use of the tRNA^LeuUAA intron as a genetic marker for the subgeneric identification of Nostoc in complex field material was developed. Lichens belonging to the genera Peltigera and Nephroma show limited variability in their Nostoc symbionts. The in situ symbiont consists of a single strain rather than a community of different Nostocs, and single thalli consistently contained the same symbiont. Patterns in symbiont identity were found in geographically remote populations, and the lichen species, rather than growth locality, was shown to be important for the identity of the Nostoc symbiont. Examination of a P. aphthosa photosymbiodeme revealed that one Nostoc has the capacity to perform the physiological roles found in both bipartite and tripartite lichens. The symbiotic association between bryophytes and Nostoc on the other hand exhibited a much greater variation of Nostoc symbionts.

Evolutionary patterns in the tRNA^LeuUAA intron were analyzed, and it was shown that sequence variation was caused by several processes other than random mutations. Such evolutionary processes in genetic markers are crucial to consider, especially if phylogenetic reconstructions are attempted.

Protein profiles of symbiotic and free-living Nostoc were analyzed using 2-dimensional gel electrophoresis. One of the major proteins in the extracts from freshly isolated symbionts was partially sequenced and shown to contain a fasciclin domain. The corresponding ORF in N. punctiforme was homologous to symbiotically induced genes found in different symbiotic systems.

This thesis gives new perspectives on lichens and provides a platform for further examinations using tools provided by modern biology.

Manuscripts on which thesis is based
Paulsrud P, Lindblad P (1998). Sequence variation of the tRNA^Leu (UAA) intron as a marker for genetic diversity and specificity of symbiotic caynobacteria in some lichens. Applied and Environmental Microbiology 64:310-315. Abstract  Full text

Paulsrud P, Rikkinen J, Lindblad P (1998). Cyanobiont specificity in some Nostoc-containing lichens and in a Peltigera aphthosa photosymbiodeme. New Phytologist 139:517-524. Abstract  Full text

Paulsrud P, Rikkinen J, Lindblad P (2000). Spatial patterns of photobiont diversity in some Nostoc-containing lichens. New Phytologist 146:291-299.

Costa J-L, Paulsrud P, Rikkinen J, Lindblad P (2001). Genetic diversity of Nostoc symbionts endophytically associated with two bryophyte species. Applied and Environmental Microbiology 67:4393-4396. Abstract  Full text

Paulsrud P, Rikkinen J, Lindblad P (2001). Field investigations on cyanobacterial specificity in Peltigera aphthosa. New Phytologist 152:117-123.

Costa J-L, Paulsrud P, Lindblad P (2002). The cyanobacterial tRNA^Leu (UAA) intron: evolutionary patterns in a genetic marker. Molecular Biology and Evolution 19:850-857. Abstract  Full text

Paulsrud P, Lindblad P (2002). Fasciclin domains are present in Nostoc symbionts of lichens. Applied and Environmental Microbiology 68:2036-2039. Abstract  Full text