structure of the 5S rRNA. This is a useful tool for delimiting between the subdivisions. The Ustilaginomycetina has the type B secondary structure and they share this character with the Agaricomycotina, and it separates them from the Pucciniomycotina which has the type A secondary structure. Classification Historic classification In 1847 Tulasne and Tulasne divided the so-called smut fungi into two groups called Ustilaginacceae and Tilletiaceae. Traditionally morphological characters of the basidia was used for the delimitation of the smut fungi group, but after a thorough investigation of the ultrastructural characters the group was revised. With the use of electron microscopy, Bauer et al. 1997 found two separates lines of the smut fungi, namely the Microbotryales (which is now moved to Puzziniomycotina) and the Ustilaginomycetes. Within the Ustilaginomycetes they identified three lineages: the Entorrhizomycetidea, Ustilaginomycetidae and the Exobasidiomycetidae. Modern classification Ustilaginomycotina consists of both anamorphic yeast species and teleomorphic filamentous smuts. These two states can look very different and is not always easy to connect. The two different states has therefore produced different names on the same species. With the use of both morphological characters and molecular data, the subdivision is now reclassified and many of the species has been renamed by the 'One Fungus = One Name' principle.' By looking at ultrastructural morphological structures like host-parasite interactions, the septal pore apparatus and molecular sequence analysis a new classification was proposed. Here Ustilaginomycotina consisted of the three classes Ustilaginomycetes, Exobasidiomycetes and Entorrhizomycetes. But, the presence of Entorrhizomycetes was questioned and are now considered as incertae sedis in the Basidiomycota. In 2014 a new multiple gene sequence analysis showed that two additional classes, forming two independent deep lineages should be recognized as classes in the subdivision: Malasseziomycetes and Monilielliomycetes. Ecology Distribution Since Ustilaginomycotina is mostly plant parasites, the group is restricted to the host species of vascular plants, and mainly on angiosperms and monocots. This encompasses a geographical distribution in both tropical, temperate and arctic regions. Most species are highly host-specific and this may be a product of coevolution with different angiosperm lineages. This is supported by studies that shows that some monophyletic lineages in the Ustilaginomycotina are restricted to monophyletic lineages in the angiosperms. But not all taxa in Ustilaginomycotina are host-specific, some have a broad host range and others have also made a host jump to other vascular plants and not only monocots in the angiosperms. Role in the environment