Morphology-based phylogenetic binning to assess a taxonomic challenge: A case study in Graphidaceae (Ascomycota) requires a new generic name for the widespread Leptotrema wightii

Abstract

Molecular phylogenetic analysis presents two challenges when it is transformed into formal classifications: the taxonomic challenge (whether and how to distinguish monophyletic sister clades or how to deal with paraphyletic grades) and the nomenclatural challenge (naming clades, i.e. placing name-giving types accurately on a tree). One approach to the latter is morphology-based phylogenetic binning, which places specimens based on phenotypic features on a molecular tree and assigns uncertainty values to alternative placement options. Here, we use the example of the lichenized fungal genus Leptotrema to demonstrate how morphology-based phylogenetic binning can help to clarify taxonomic and nomenclatural issues when naming phylogenetically defined entities. Leptotrema is known for a common and widespread species, L.wightii, and phylogenetic analyses have been based exclusively on this species, including the recognition of a separate tribe, Leptotremateae. However, the genus name Leptotrema and the tribal name Leptotremateae are based on the name L.zollingeri, which was initially considered to be a synonym of L.wightii, but has recently been shown to represent a distinct species. As L.zollingeri differs considerably in phenotypic features from L.wightii, it can be questioned whether the two are at all related or whether L.zollingeri is actually closer to the genera Myriotrema and Ocellularia in tribe Ocellularieae. The solution to this problem is not trivial, as it affects the correct use of the names Leptotrema and Leptotremateae. Morphology-based phylogenetic binning indeed demonstrated that L.zollingeri clusters with the Myriotrema album group in tribe Ocellularieae with high support. Hence, in contrast with current use, the name Leptotrema becomes available for the M.album group and Leptotremateae becomes a synonym of Ocellularieae. As a consequence, the new names Sanguinotrema and Sanguinotremateae are introduced to accommodate L.wightii and the tribe including this species and the genus Reimnitzia. Although the studied case is specific to lichen fungi, the approach can be used in a much broader context with any kind of taxon or organism.