Field investigations on cyanobacterial specificity in Peltigera aphthosa

•  Possibilities of introducing new cyanobacterial strains into established lichen symbioses were examined by manipulating thalli of Peltigera aphthosa and by using the trnL (UAA) intron to identify specific cyanobacterial strains. •  P. aphthosa has Nostoc symbionts in cephalodia on the upper surface of the thallus. In three different P. aphthosa populations, each consisting of seven thalli, all cephalodia were experimentally removed. The manipulated lichens were then inoculated with known strains of cultured cyanobacteria and left to develop new cephalodia. After a summer in the field the lichens were harvested and the strain identities of cyanobacterial symbionts in 80 newly formed cephalodia were determined. All epiphytic colonies of free-living cyanobacteria found were also analysed. •  Foreign cyanobacteria were not readily incorporated into established P. aphthosa thalli. All newly formed cephalodia contained the same intron sequence, which was identical to that found in the removed cephalodia. At least two inoculated Nostoc strains were able to survive as epiphytic colonies on experimental thalli. Both strains had originally been isolated from bipartite Peltigera species. •  Results indicate that associations between cyanobacteria and lichen-forming fungi can be very specific and stable, which contrasts with the general view that cyanobacterial symbioses are rather unspecific.