Nucleotide composition bias of rDNA sequences as a source of phylogenetic artifacts in Basidiomycota—a case of a new lineage of a uredinicolous Ramularia-like anamorph with affinities to Ustilaginomycotina

A Ramularia-like hyphomycete was discovered on uredinia of Phakopsora ampelopsidis on leaves of wild Ampelopsis brevipedunculata and cultivated Parthenocissus tricuspidata in several cities in Taiwan. The micromorphology of this fungicolous fungus is similar to that of species of the genus Ramularia which are mostly plant parasites but some species grow on rust fungi. Analyses of SSU and LSU rDNA and RPB2 gene sequence data and of ultrastructural features observed by scanning and transmission electron microscopy revealed that the fungus represents a novel lineage within the Ustilaginomycotina. The name Quasiramularia phakopsoricola is proposed for this new species in a new genus, a new family, and a new order. Sequence data obtained for ITS1, 5.8S, and ITS2 rDNA, however, did not match to any known fungal lineage. Bioinformatics analyses showed that these sequence data are extremely GC poor, but most probably do not represent a pseudogene. Extreme deviations of GC content can be observed in several lineages of Basidiomycota. Such variations affect the results of phylogenetic analyses and are an important source of artifacts. © 2021, German Mycological Society and Springer-Verlag GmbH Germany, part of Springer Nature.