Insights into intrathalline genetic diversity of the cosmopolitan lichen symbiotic green alga Trebouxia decolorans Ahmadjian using microsatellite markers

• Microsatellites reveal high intrathalline algal diversity in two lichen-forming fungi (X. parietina, A. ciliaris).
• High genetic differentiation among thalli suggests that algal diversity is generated de novo via mutation.
• Thalli of Xanthoria parietina harbor more diverse algal pools, suggesting additional environmental uptake of photobionts.

Trebouxia decolorans is a widespread and common symbiotic green alga that is found in association with different species of lichen-forming fungi. By applying T. decolorans-specific microsatellite markers, we investigated the within-thallus diversity of T. decolorans in thalli of Xanthoria parietina and Anaptychia ciliaris. We found several algal strains in most of the thalli of both hosts. High genetic differentiation among thalli suggests that algal diversity is generated de novo via mutation in both fungal hosts. Rarefied allelic richness of the algae was higher in thalli of X. parietina. Our results indicate that in X. parietina intrathalline algal diversity is additionally created by environmental uptake of algae either at the start of the symbiotic association or during the lifetime of the thallus. This study indicates that promiscuous host-symbiont associations in lichen symbioses with Trebouxia spp. may be more common than currently recognized.

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