The genus Buellia remains one of the largest, poorly resolved genera of crustose lichens world-wide. A global revision based on existing data is challenging because of an enormous diversity in the group. In the Galapagos Islands, species of Buellia s.l. are very common, widely distributed and unusually diverse. As a first step towards a more comprehensive revision, we decided to focus on a group easily separated – specimens containing xanthones recognized by their pale yellowish green thalli, displaying a UV+ bright yellow to bright orange fluorescence and/or a C+ orange spot test reaction. The heterogeneous group represents an assemblage of taxa that are not closely, phylogenetically related. Secondary chemistry of the specimens was analyzed with thin-layer chromatography, first using standard solvent system C to confirm the presence of xanthones, then examining specimens further in solvent F, comparing specimen extracts with samples of pure xanthones. Documenting their morphology, anatomy and secondary chemistry, we present here a dichotomous key to distinguish the species. Ultimately these results will contribute to a more comprehensive phylogenetic revision, also employing molecular tools.
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