Lichen host phylogeny
Phylogenetic analysis based on ITS sequence data revealed that all the samples we gathered belong to the genera Parmelia and Peltigera (Supplementary Fig. S1). The Parmelia samples collected in Turkey represented a single species, P. submontana, while the Parmelia samples from Korea were identified as P. praesquarrosa. Our Peltigera samples were shown to encompass a total of four distinct species. The Peltigera species from Turkey included Peltigera aff. neocanina, Peltigera aff. membranacea, P. degenii, and P. praetextata. In contrast, all Peltigera samples from Korea were identified as P. degenii. In summary, Parmelia samples from Turkey and Korea each represented a single species, while the Peltigera samples collected in Turkey showed greater species diversity compared to those from Korea.
Taxonomic diversity of lichen mycobiome
In order to assess the overall diversity of the fungal community within lichens, we observed the taxonomic composition (Fig. 1a). At the phylum level, except for unclassified fungi, members of Ascomycota (40%) ere the most abundant, with fungi belonging to Basidiomycota (24%) following. At the class level, Dothideomycetes (21%) were the dominant group, followed by Tremellomycetes (20%) and Eurotiomycetes (8%). We investigated whether the taxonomic composition of the lichen mycobiome varied based on host type at the order level (Fig. 1b). There were no significant differences in diversity observed among the four host types: Korean Parmelia and Peltigera, and Turkish Parmelia and Peltigera. When compared across countries, the proportion of Pleosporales was higher in the Turkish lichen mycobiome (8%) than Korean’s (3%), regardless of host species.
To understand the factors influencing the clustering of the lichen mycobiome, we observed a hierarchical heatmap of the top 100 abundant ASVs based on host lichen genus and geographical differences (Fig. 1c). It was not clear which of these two factors had a greater impact on the clustering. This suggests that additional analysis is needed to clearly identify the factors shaping the fungal community. To summarize, it can be concluded that the lichen mycobiome, in terms of taxonomic diversity, does not exhibit significant differentiation based on host phylogeny or geographical distance, and it appears to share similar features.
Influential factors in shaping the lichen mycobiome and alpha diversity of the mycobiome
To explore which factor, either the host genus or geographical distance, has a more significant impact on the lichen mycobiome, we conducted a two-dimensional ordination through canonical correspondence analysis (Fig. 2a). It appears that the fungal community is more influenced by location similarity. However, we believed that further statistical testing was necessary and thus performed a variation partitioning analysis (Fig. 2b). While the size of the residual component was substantial (0.97), it conclusively revealed that the lichen mycobiome is more strongly influenced by location (0.02) than by the host (0.01).
We also examined how alpha diversity of the lichen mycobiome varies according to host type. Nestedness is a term used when one group exhibits a subset-like pattern within another group, and it is useful for assessing overall alpha diversity. As shown in the figure, regardless of location, Peltigera mycobiome has a more nested structure compared to fungal community inside Parmelia (Fig. 2c). For Chao1, representing species richness, there were no significant differences between Parmelia and Peltigera mycobiomes across locations (Mann-Whitney U test, p > 0.05, Fig. 2d). Similarly, Shannon index, which signifies species diversity, did not exhibit significant differences based on location (Mann-Whitney U test, p > 0.05). Additionally, there were no notable distinctions in Chao1 and Shannon index when comparing lichen mycobiomes based on their host genera Parmelia and Peltigera (Mann-Whitney U test, p > 0.05, Supplementary Fig. S2a). In summary, the lichen mycobiome is more strongly influenced by location differences than by the host genus, and Peltigera mycobiome appears to have a more nested structure than that of Parmelia.
Core members of the lichen mycobiome
Generally, the term ‘core microbiome’ refers to a set of members that are commonly found within different hosts or samples. In other words, the core mycobiome signifies the fungal strains that appear consistently, regardless of the habitat. We aimed to identify the core mycobiomes within the two genera, Parmelia and Peltigera, utilizing two measures: relative abundance and prevalence (Fig. 3a). As the cutoff values for both measures increased, the number of core members (core size) decreased. When we set the relative abundance cutoff to 0.1%, we discovered 12 core members for Parmelia and 14 core members for Peltigera (Fig. 3b). Interestingly, among the core mycobiomes of the two different lichen hosts, there were several common fungal strains, such as Cutaneotrichosporon debeurmannianum, Chaetothyriales, and Dothideomycetes (Fig. 3c). Among them, the abundance of C. debeurmannianum was relatively high (Fig. 3d). In summary, we have identified the core mycobiomes of Parmelia and Peltigera, and these two core mycobiomes share common fungal strains.
Host or locational specialist in lichen mycobiome
Linear discriminant analysis (LDA) is a useful tool in the field of microbial community analysis for identifying microbial taxa with statistically significant differences in abundance. In our study, we employed LDA to identify host genus or location-specific specialists. We focused on data that were identified at the genus level and had LDA scores of 3.0 or higher, excluding unclassified data such as unclassified family data. In the Parmelia mycobiome, we observed the frequent presence of eleven taxa, including Didymella fabae and Gyrographa gyrocarpa in lichens from Turkey (Fig. 4a). Conversely, in Korean Parmelia, we found that only Septobasidium carestianum was a specialist. In the Peltigera mycobiome, we confirmed the significant abundance of sixteen taxonomic groups, including Herpotrichia junperi and Mycosphaerella tassina, in Turkish samples (Fig. 4b). Additionally, thirteen taxonomic groups, including Monochaetia junipericola and Cyphellophora, were significantly abundant in Korean Peltigera.
We also investigated lichen mycobiome specialists at the host genus level (Supplementary Fig. S2b). Among Parmelia specialists, three taxonomic groups, including Gyrographa gyrocarpa, were identified, while in Peltigera, 32 unique taxonomic groups, including Phlebia, were significantly more abundant as specialists. In summary, our comparison between Turkish and Korean lichens revealed a higher number of specialists in Turkish lichen communities, and when comparing lichen genera, Peltigera exhibited a greater number of specialists than Parmelia.