Parmotrema xanthinum
Parmotrema xanthinum is a species of corticolous, foliose lichen in the family Parmeliaceae. It has a pale yellowish to greenish-grey body that attaches loosely to its bark surface. First described in 1884, it was the subject of recent taxonomic studies that showed it to be the same species as the previously distinct Parmotrema madagascariaceum. The lichen can be identified by its broad, across, with small finger-like projections on its surface and hair-like projections along its edges. While particularly common in southeastern North America, including coastal and mountainous regions, it is widely distributed in subtropical and temperate areas worldwide. The species can be chemically distinguished by the presence of specific secondary metabolites including usnic acid and protolichinestrinic acid in its tissues.
Taxonomy
The lichen was first named and scientifically described by the Swiss lichenologist Johannes Müller Argoviensis, who called it Parmelia proboscidea var. xanthina. It gained full status as a species in Mason Hale's 1974 circumscription of the genus Parmotrema.Parmotrema xanthinum has historically been distinguished from its close relative Parmotrema madagascariaceum primarily based on differences in chemotaxonomy. Both species are characterised by the presence of margins, isidia all over the thallus surface, and the production of usnic acid in the cortex, along with fatty acids of the protolichenisterinic acid group in the medulla. The main differentiator has been the presence of gyrophoric acid in P. madagascariaceum. More recent studies incorporating biogeography, chemistry, morphology, and molecular phylogenetics have led to the treatment of these two taxa as conspecific, meaning they are now considered the same species.
Molecular phylogenetics analyses of nuclear ribosomal internal transcribed spacer sequence data do not differentiate Parmotrema madagascariaceum and P. xanthinum as separate entities. The sequence divergence observed is typical of variation within a single lichen species, further supporting the conspecific treatment of these taxa.