Phylogeny of yeasts and related filamentous fungi within Pucciniomycotina determined from multigene sequence analyses

Basidiomycetous yeasts in the Urediniomycetes and Hymenomycetes were examined by sequence analysis in two ribosomal DNA regions: the D1/D2 variable domains at the 5' end of the large subunit rRNA gene (D1/D2) and the internal transcribed spacers (ITS) 1 and 2. Four major lineages were recognized in each class: Microbotryum, Sporidiobolus, Erythrobasidium and Agaricostilbum in the Urediniomycetes; Tremellales, Trichosporonales, Filobasidiales and Cystofilobasidiales in the Hymenomycetes. Bootstrap support for many of the clades within those lineages is weak; however, phylogenetic analysis provides a focal point for in-depth study of biological relationships. Combined sequence analysis of the D1/D2 and ITS regions is recommended for species identification, while species definition requires classical biological information such as life cycles and phenotypic characterization.

Some fungi with pleomorphic life-cycles still bear two names despite more than 20 years of molecular phylogenetics that have shown how to merge the two systems of classification, the asexual “Deuteromycota” and the sexual “Eumycota”. Mycologists have begun to flout nomenclatorial regulations and use just one name for one fungus. The International Code of Botanical Nomenclature (ICBN) must change to accommodate current practice or become irrelevant. The fundamental difference in the size of fungi and plants had a role in the origin of dual nomenclature and continues to hinder the development of an ICBN that fully accommodates microscopic fungi. A nomenclatorial crisis also looms due to environmental sequencing, which suggests that most fungi will have to be named without a physical specimen. Mycology may need to break from the ICBN and create a MycoCode to account for fungi known only from environmental nucleic acid sequence (i.e. ENAS fungi).

Field campaigns in Antarctica, Greenland and the Italian glaciers aiming to explore the biodiversity of these disappearing environments identified several undescribed yeast strains unable to grow at temperature above 20°C and belonging to unknown species. Fourteen of these strains were selected and grouped based on their morphological and physiological characteristics. Sequences of the D1/D2 and ITS regions of the ribosomal RNA demonstrated that the strains belong to unknown species related to Leucosporidium antarcticum. The new genus Glaciozyma is proposed and two new species are described, namely Glaciozyma martinii sp. nov. and Glaciozyma watsonii sp. nov. Additionally, re-classification of Leucosporidium antarcticum as Glaciozyma antarctica is proposed. Strains of Glaciozyma form a monophyletic clade and a well separated lineage within class Microbotryomycetes (Pucciniomycotina, Basidiomycota). The description of Glaciozyma genus and the re-classification of L. antarcticum reduce the polyphyletic nature of the genus Leucosporidium.