Discussion of Phylogenetic Relationships
Note: The classification of Ustilaginomycotina is being revised. The tree at the top of this page reflects the current classification, whereas the following tree and the text on this page reflect a prior classification.
Since Tulasne and Tulasne (1847) the smut fungi are traditionally divided into the phragmobasidiate Ustilaginaceae and the holobasidiate Tilletiaceae, which are sometimes treated as separate orders (Kreisel 1969, Oberwinkler 1987). Difficulties in their classification have been discussed, e.g. by Durán (1973) and Vánky (1987), neither of whom listed higher taxa in this group. Based predominantly on host-parasite interactions and septal pore apparatus, a radical change in the systematics of smut fungi has been proposed by Bauer et al. (1997). As a result, on the one hand the Microbotryales were excluded from the Ustilaginomycetes and on the other the Exobasidiales s. l., Graphiolales and Microstromatales were included in this group. Cellular interaction (Bauer et al. 1997) and cell wall carbohydrate composition (Prillinger et al. 1993) indicate that the class Ustilaginomycetes in this composition is monophyletic, and phylogenetic analyses of rDNA sequences (Swann and Taylor 1993, Berres et al. 1995, Begerow et al. 1997) are consistent with this hypothesis.
Ultrastructural and rDNA sequence analyses provide evidence for the existence of three major groups within the Ustilaginomycetes (Bauer et al. 1997, Begerow et al. 1997). The basal dichotomy is between the Entorrhizomycetidae and the branch uniting the Ustilaginomycetidae and Exobasidiomycetidae (Bauer et al. 1997, Begerow et al. 1997). In contrast with the Ustilaginomycetidae and Exobasidiomycetidae, the septal pores of the Entorrhizomycetidae are not enclosed by tripartite membrane caps. The Ustilaginomycetidae form enlarged interaction zones, whereas the Exobasidiomycetidae form local interaction zones.
The Ustilaginomycetes represents the sister group of the Hymenomycetes. Type B secondary structure of the 5S rRNA and glucose as major cell wall carbohydrate component are shared by both classes (Gottschalk and Blanz 1985, Prillinger et al. 1993).
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