Field investigations on cyanobacterial specificity in Peltigera aphthosa

Possibilities of introducing new cyanobacterial strains into established li chen symbioses were examined by manipulating thalli of Peltigera aphthosa a nd by using the trnL (UAA) intron to identify specific cyanobacterial strai ns. P. aphthosa has Nostoc symbionts in cephalodia on the upper surface of the thallus. In three different P. aphthosa populations, each consisting of sev en thalli, all cephalodia were experimentally removed. The manipulated lich ens were then inoculated with known strains of cultured cyanobacteria and l eft to develop new cephalodia. After a summer in the field the lichens were harvested and the strain identities of cyanobacterial symbionts in 80 newl y formed cephalodia were determined. All epiphytic colonies of free-living cyanobacteria found were also analysed. Foreign cyanobacteria were not readily incorporated into established P. aph thosa thalli. All newly formed cephalodia contained the same intron sequenc e, which was identical to that found in the removed cephalodia. At least tw o inoculated Nostoc strains were able to survive as epiphytic colonies on e xperimental thalli. Both strains had originally been isolated from bipartit e Peltigera species. Results indicate that associations between cyanobacteria and lichen-forming fungi can be very specific and stable, which contrasts with the general vi ew that cyanobacterial symbioses are rather unspecific.