Picking holes in traditional species delimitations: an integrative taxonomic reassessment of the Parmotrema perforatum group (Parmeliaceae, Ascomycota)

Accurate species delimitation is important as species are a fundamental unit in ecological, evolutionary and conservation biology research. In lichenized fungi, species delimitation has been difficult due to a lack of taxonomically important characteristics and due to the limits of traditional, morphology-based species concepts. In this study we reassess the current taxonomy of the Parmotrema perforatum group, which recognizes six closely related species divided into three species pairs, each pair comprising one apotheciate (sexual) and one sorediate (asexual) species. Each pair is further characterized by a distinct combination of secondary metabolites. It was hypothesized that the three apotheciate species are reproductively isolated sibling species and that each sorediate species evolved once from the chemically identical apotheciate species. In this study, species boundaries were re-examined using an integrative approach incorporating morphological, chemical and molecular sequence data to delimit species boundaries. Phylogenetic trees were inferred from a seven-locus DNA sequence dataset using concatenated gene tree and coalescent-based species-tree inference methods. Furthermore, we employed a multi-species coalescent method to validate candidate species. Micromorphological measurements of conidia were found to be congruent with phylogenetic clusters. Each approach that we applied to the P.perforatum group consistently recovered four of the currently circumscribed species (P.perforatum, P.hypotropum, P.subrigidum and P.louisianae), whereas P.preperforatum and P.hypoleucinum were consistently combined and are thus interpreted as conspecific.