Species Delimitation in Taxonomically Difficult Lichen-forming Fungi: an Example from Morphologically and Chemically Diverse Xanthoparmelia (Parmeliaceae) in North America

Mounting evidence suggests many morphology-based species circumscriptions in lichenized ascomycetes misrepresent fungal diversity. The lichenized ascomycete genus Xanthoparmelia includes over 800 described species displaying a considerable range of morphological and chemical variation. Species circumscriptions in this genus have traditionally been based on thallus morphology, medullary chemistry, and the presence or absence of sexual or asexual reproductive structures. Notwithstanding concerted effort on the part of taxonomists to arrive at a natural classification, modern taxonomic concepts for the most part remain unclear. Here we assess the evolution of characters traditionally regarded as taxonomically important by reconstructing a phylogenetic hypothesis based on sequence data from four nuclear ribosomal markers as well as fragments from two protein-coding nuclear loci. A total of 414 individuals were tested, representing 19 currently accepted species. Most sampled species, as currently circumscribed, were recovered as polyphyletic, suggesting that major diagnostic characters have evolved in a homoplasious manner. The vagrant growth form, distinct medullary chemistries, and production of vegetative diaspores appear to have evolved independently multiple times. Application of a population assignment test resulted in the recognition of 21 species-level genetic clusters, each of which was supported by a comparison of genetic distances as well as a Bayesian species delimitation method calculating probabilities associated with speciation events. Inferred clusters are largely incongruent with traditionally circumscribed species due to the prevalence of cryptic diversity and, in some cases, high levels of intraspecific morphological and chemical variation. These results call for a major taxonomic revision of Xanthoparmelia species in western North America.