DIARTHONIS Clem. (1909)

MycoBank number: MB 1500; Index Fungorum number: IF 1500, Facesoffungi number: FoF

The genus as currently circumscribed is monotypic, so the description of D. spadicea below constitutes that of the species.

Phylogenetic trees constructed by Frisch et al. (2014) showed that Arthonia spadicea fell outside of the main Arthonia clade, with indications that it (along with Arthothelium norvegicum) formed a sister group with the Arthoniaceae as a whole. The branch was referred to as the Coniangium clade by Thiyagaraja et al. (2020), based on the long-forgotten genus Coniangium Fr. (1821). However, the type is C. vulgare Fr., which is a synonym of Arthonia vinosa. According to work by Frisch et al. (2018), A. vinosa is not congeneric with A. spadicea, and it belongs within a clade that includes A. didyma within the Arthoniaceae sensu stricto. However, the generic name Diarthonis Clem. (1909) is available for use; it was introduced (with a minimal but nomenclaturally acceptable diagnosis) for the single species D. lurida (Ach.) Clem., basionym Arthonia lurida Ach. (1803). That name was formally rejected following acceptance of a nomenclatural proposal by Hawksworth & Sherwood (1981) to protect the later name Arthonia spadicea Leight. (1854). The Code states that names using a rejected name as a basionym are similarly rejected, but that generic names with the rejected name as type are allowable. Therefore, Diarthonis can be used for a genus including A. spadicea, and that epithet can be retained in the newly recognized genus. The formal combination needed is given at the end of this publication.

Frisch et al. (2014) and Thiyagaraja et al. (2020) reported that DNA of D. spadicea clustered in a distinct clade with that of Arthothelium norvegicum, and that name may need to be transferred as a second species of Diarthonis. However, the phylogenetic branch lengths are quite long and more data are required to confirm the relationship.