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A Unique Trait Associated with Increased Diversification in a Hyperdiverse Family of Tropical Lichen–Forming Fungi

Premise of research. With more than 2000 described species, Graphidaceae represents one of the largest families of lichenized fungi. The underlying causes for such a high level of diversity have not yet been studied in detail. Two possible drivers of high diversity in this group include its predominantly tropical habitat, a major factor for high species richness in other groups of organisms, and the presence of columella, a unique trait within this family. Methodology. We employed a combination of comparative methods to investigate possible factors that are associated with the hyperdiverse nature of the family, including modeling evolutionary diversity using stepwise Akaike information criterion (MEDUSA) to identify clades with an increased diversification rate and binary state speciation and extinction (BiSSE) analysis to determine whether diversification rates change in association with selected character states. Pivotal results. BiSSE analysis revealed a significantly higher diversification rate in the lineages with presence of a columella, while there was no significant difference in diversification rates between tropical and nontropical lineages. A further analysis with MEDUSA also identified two or three diversification rate shifts, one of which occurred in the Ocellularia clade, with a large number of species with a columella. Conclusions. The results suggest a stronger role of the columella, compared to tropical habitat, in driving high levels of diversification in the family. The possible ecomorphological importance of columella structures is discussed.