High selectivity in symbiotic associations of lichenized ascomycetes and cyanobacteria

The selectivity of mycobionts and cyanobionts in lichen symbioses were examined. We analyzed symbiotic cyanobionts, collected from different sample sites, and compared them to free-living cyanobacteria Nostoc. Cyanobionts were obtained from lichens assigned to the genera Pseudocyphellaria and Sticta, in particular. Multiple gene loci were screened and direct optimization was used in the phylogenetic analyses. We show that many lichen fungi are strongly selective towards their cyanobionts. Lichenized ascomycetes seem to be able to identify and choose a specific strain, species or a species group of Nostoc with which to associate. The present analyses also suggest that some of the Nostoc taxa may be specialized in symbiotic life with only lichenized ascomycetes. Despite the selectivity observed in fungi, there appears to be no coevolution between the partners. We have also discussed the problems of using the tRNA(Leu) intron as a marker in phylogenetic analyses