HOST SPECIFICITY AND PHYLOGENETIC RELATIONSHIPS AMONG TILLETIA SPECIES INFECTING WHEAT AND OTHER COOL SEASON GRASSES
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The 140 species of Tilletia (Ustilaginomycotina, Basidiomycota) are biotrophic pathogens that cause bunt and smut diseases of grasses (Poaceae). Among the most economically important species are the wheat bunt pathogens, T. caries, T. contraversa, and T. laevis. Most smut fungi infect relatively few hosts, but the host range of T. contraversa includes 65 species in 17 grass genera. The objective of this study was to incorporate sequence data from ITS rDNA, eukaryotic translation elongation factor 1-alpha (EF1-alpha), and the second largest subunit of RNA polymerase II (RPB2) with three anonymous loci (A13, A16 and P18) to elucidate relationships and test the host range of the wheat bunt fungi and related Tilletia species infecting cool-season grasses in the Pacific Northwestern U.S. Sorus shape and teliospore size of 60 dwarf and common bunt isolates analyzed with k-means clustering revealed two groups, one largely corresponding to common bunt and the second to dwarf bunt. Network analysis based on three anonymous loci and RBP2 region revealed three clades, but none of the clades contained isolates of all three species. Phylogenetic analyses of the three anonymous loci, EF1-alpha, ITS rDNA and RPB2 were used to test the conspecific status of Eurasian and North American isolates of T. contraversa infecting species of nine grass genera. Phylogenetic trees were reconstructed independently for RPB2/ITS/EF1-alpha and A16/P18/ITS/EF1-alpha loci. NeighborNet networks were estimated from A16/P18/ITS/EF1-alpha loci and compared with the NeighborNet networks constructed with "gene-jackknifing" method. All phylogenetic trees and networks suggested a wide host range for T. contraversa. The analyses failed to support dwarf bunt pathogen as genetically distinct from the common bunt pathogens, but Eurasian isolates of T. contraversa on Elymus repens and Thinopyrum intermedium were consistently placed in a clade distinct from the majority of T. contraversa isolates, suggesting that substructure exists among isolates in this group. In contrast to the broad host range supported for T. contraversa, phylogenetic analysis of Tilletia species on cool-season grasses based on ITS, EF1-alpha and RPB2 revealed well-supported clades corresponding to host. A new species, T. puccinelliae, was proposed for a bunt infecting Puccinellia distans (alkaligrass) in Washington.