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Variation at the b Mating Type Locus of Ustilago maydis

First author: USDA Forest Service, North Central Forest Experiment Station, Forestry Sciences Laboratory, 5985 Highway K, Rhinelander, WI 54501-9128; second author: Department of Plant Pathology, University of Minnesota, St. Paul 55108; third, fourth, and fifth author: Department of Plant Biology, University of Minnesota, St. Paul 55108; and second, fourth, and fifth author: Center for Community Genetics, University of Minnesota, St. Paul

Population level diversity at the Ustilago maydis b mating type locus was determined in samples from four Minnesota locations using a combination of plate mating techniques and a polymerase chain reaction (PCR)-based assay. The PCR method allows rapid identification of b types from samples of natural populations and utilizes the hypervariable regions of the b locus that determine mating type specificity. Results demonstrated high levels of b diversity within populations, with one population yielding 17 of the total 18 b types found in the study. Pairwise G_ST values were in the range of 0.02 to 0.05, and common b mating types were found across broad geographic distances. These data demonstrated that very low levels of differentiation among U. maydis populations occur with respect to b locus variation. Consistent with frequency-dependent selection models, b types were represented at approximately equal frequencies within the entire Minnesota population. However, neutral evolutionary models for patterns of geographic distribution and variation at b cannot be entirely excluded. The importance to agricultural practices of understanding population genetic processes is discussed.

Additional keywords: barcode, basidiomycete, corn smut, DNA fingerprint, mating system.