The Nostoc Symbiont of Lichens : Diversity, Specificity and Cellular Modifications

Abstract: 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 tRNALeuUAA 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 strainn 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 tRNALeuUAA 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 pr for further exaovides a platform for further examiniations using tools provided by modern biology.