Environment and host structure communities of green-algal symbionts in lichens. Trebouxia ITS2 metabarcoding along an altitudinal gradient

Understanding how biotic interactions shape species’ distributions is central to predicting host-symbiont response under climate change. Switches to locally adapted algae have been proposed to be an adaptive strategy of lichen-forming fungi to cope with environmental change. However, it is unclear how entire communities of algal symbionts respond to environmental gradients, and whether they play a role in determining the fungal host’s range limits. Here we used deep-coverage Illumina DNA metabarcoding to track changes in community composition of Trebouxia algae associated with two phylogenetically closely related, but ecologically divergent fungal hosts along a steep altitudinal gradient in the Mediterranean region.Our results indicate that both altitude and host genetic identity are strong predictors of Trebouxia community assembly. The predominantly vertically (and clonally) dispersing fungal host showed stronger altitudinal structuring of its algal communities than the horizontally (sexually) reproducing host. Host ranges were not limited by the lack of compatible algal strains. Our study sheds light on the processes guiding the formation and distribution of specific fungal-algal combinations in the lichen symbiosis. The effect of environmental filtering acting on both symbiotic partners appears to shape the distribution of lichen-forming fungi.