Microbiome change by symbiotic invasion in lichens

Summary Lichens are obligate symbioses between fungi and green algae or cyanobacteria. Most lichens resynthesize their symbiotic thalli from propagules, but some develop within the structures of already existing lichen symbioses. D iploschistes muscorum starts as a parasite infecting the lichen C ladonia symphycarpa and gradually develops an independent D iploschistes lichen thallus. Here we studied how this process influences lichen‐associated microbiomes and photobionts by sampling four transitional stages, at sites in S weden and G ermany, and characterizing their microbial communities using high‐throughput 16S rRNA gene and photobiont‐specific ITS rDNA sequencing, and fluorescence in situ hybridization. A gradual microbiome shift occurred during the transition, but fractions of C ladonia ‐associated bacteria were retained during the process of symbiotic reorganization. Consistent changes observed across sites included a notable decrease in the relative abundance of A lphaproteobacteria with a concomitant increase in B etaproteobacteria. A rmatimonadia, S partobacteria and A cidobacteria also decreased during the infection of C ladonia by D iploschistes . The lichens differed in photobiont specificity. C ladonia symphycarpa was associated with the same algal species at all sites, but D iploschistes muscorum had a flexible strategy with different photobiont combinations at each site. This symbiotic invasion system suggests that partners can be reorganized and selected for maintaining potential roles rather than depending on particular species.