Joint Dispersal Does Not Imply Maintenance of Partnerships in Lichen Symbioses

Dispersal of symbiotic partners by joint propagules is considered as an efficient strategy to maintain successful associations and to circumvent low symbiont availability. Joint dispersal is widespread in diverse symbioses and a particularly common reproductive mode in lichens. We were interested in the implications of joint symbiont dispersal on population genetic structure and investigated patterns of symbiont association in populations of two closely related lichen species in the genus Physconia, with similar range of compatible algal partners. One of the lichen species is characterized by joint dispersal of both symbionts, whereas the other species propagates by meiotic fungal spores alone. The latter species must re-establish the symbiotic stage with appropriate algae sampled from the environment. Both fungal species have specialized on photobionts representing a monophyletic lineage of the algal genus Trebouxia. The results indicate no correlated association of symbiont genotypes in the species with joint symbiont dispersal. We rather show that algal gene diversity in populations of lichenized fungi with different propagation strategies is not necessarily different. The association with algae that differ from the co-dispersed genotypes during the vegetative development of the thalli is the most likely explanation for the observed pattern. Maintenance of symbiotic associations is an option but not a strict consequence of joint symbiont dispersal in lichens.