Diversity of tremellalean Pertusaria-associated fungi in Norway and the role of secondary metabolites in host specificity

The diversity and host relationships of lichen-inhabiting tremellalean fungi remain understudied, partly due to their asymptomatic yeast phase. This study employs ITS2 metabarcoding to investigate the diversity and colonization patterns of these fungi within 13 Pertusaria lichen species collected in Norway. We discovered a high prevalence, with tremellalean fungi present in 57 % of the examined specimens, representing 12 of the 13 species included, 10 of which were not previously recognized as hosts. Our findings suggest a broad host range for the presumed yeast phase of these fungi, though occurrences of the five most frequent OTUs appeared non-random across different host species. Additionally, negative-binomial general linear models indicated a negative correlation between the presence of tremellalean fungi and the occurrence of specific secondary metabolites, such as thiophaninic acid, fumarprotocetraric acid, and gyrophoric acid, in Pertusaria hosts. These results highlight previously undocumented genetic diversity within tremellalean fungi and suggest that host chemotype significantly influences colonization patterns. Our findings demonstrate a higher frequency of asymptomatic colonization than previously reported, emphasizing the need for further research into their ecological roles and interactions.