Latest Developments in the Research of Rust Fungi and Their Allies (Pucciniomycotina)

Pucciniomycotina, one of the three subphyla of Basidiomycota, contains a range of microfungi from various habitats and with different lifestyles. In addition to familiar plant pathogenic rusts and anther smuts, the group also contains saprobic and pathogenic yeasts, minute sporocarp-forming fungi, and anamorphic moulds among others. Our knowledge of this group is still improving; over the last 16 years alone, researchers have described 375 new species of Pucciniomycotina, most of which were isolated from less documented areas such as Asia, South America, and Africa. While the majority of these new species belong to the species-rich rust fungi (Pucciniales), exploration in extreme environments such as deep-sea sediments and psychrophilic habitats is uncovering a variety of Pucciniomycotina species, especially yeasts. Molecular phylogenetic studies have greatly improved our understanding of the relationships between these taxa over the last 10 years. As presently circumscribed, the subphylum contains nine classes and 20 orders, the relatedness for most of which was not suspected until recently. Genomic data from members of the subphylum have been scarce but increasing over the last 5 years. We now know, for example, that Pucciniomycotina contains both fungi with the largest known genomes (rust fungi, up to 900 Mb) as well as a fungus with the smallest genome in Basidiomycota (Mixia osmundae, 13 Mb). This chapter discusses these latest developments in Pucciniomycotina research and highlights some challenges still to overcome in order to improve our understanding of this enigmatic group of fungi.