The Ustilaginomycotina is a subdivision within the division Basidiomycota of the kingdom Fungi. It consists of the classes Ustilaginomycetes and Exobasidiomycetes, and in 2014 the subdivision was reclassified and the two additional classes Malasseziomycetes and Monilielliomycetes added. The name was first published by Doweld in 2001; Bauer and colleagues later published it in 2006 as an isonym. Ustilagomycotina and Agaricomycotina are considered to be sister groups, and they are in turn sister groups to the subdivision Pucciniomycotina. Ustilaginomycotina comprises 115 genera with more than 1700 species. The subdivision is mostly plant parasites on vascular plants, and the distribution of the subdivision is therefore restricted to the distribution of the host. The group is also called the true smut fungi because of the production of teliospores. The name smut is still used as a term since it circumscribes the organization and life cycle of Ustilaginomycotina, but it is not a taxonomic term. Ustilaginomycotina has some of the best known and studied genera of plant parasites like Ustilago and Tilletia and it is also of great economic importance. Morphology Ustilaginomycotina is morphologically a highly diverse group. It consists of two states: anamorphic yeast state and teleomorphic filamentous smut state. These two states in the life cycle can look very different. Defining features The Ustilaginomycotina has different ultrastructural morphologies that defines the subdivision and some of them are also used to delimit the classes in the subdivision. Cellular interaction The cellular interactions are referring to the interaction between the hyphae and the host plant cells. These zones provide a useful ultrastructural character for delimiting the classes in Ustilaginomycotina. The host-interactions zones can either be local interaction zones and then characterizing the Exobasidiomycetes, or enlarged interaction zones characterizing the Ustilaginomycetes. Cellular composition Ustilaginomycotina has a distinctive cell wall composition consistent of mostly glucose and absence of xylose. This character separates the Ustilaginomycotina from Pucciniomycotina and Agaricomycotina. Septal pores The architecture of the septal pores plays an important part in delimiting the subdivision in Basidiomycotina. In contrast to the Pucciniomycotina, the Ustilaginomycotina has a septal pore with a membrane cap or it is poreless. It does not have a dolipore or parenthesome as the Agaricomycotina do. 5S rRNA In 1985 Gottschalk and Blanz did a study about the 5s ribosomal RNA and distinguished two types of structures in the Basidiomycota. These two types was named the type A secondary structure and the type B secondary