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Inheritance of Carboxin Resistance in a European Field Isolate of Ustilago nuda

Two carboxin-resistant field isolates of Ustilago nuda from Europe were crossed with a carboxin-sensitive field isolate from North America. Meiotic tetrads isolated from germinating F₁ teliospores of one of the hybrids were tested for carboxin resistance and mating type. Carboxin resistance was shown to be controlled by a single gene (CBX1R), because a 1:1 segregation of carboxin resistance was observed in all 27 tetrads. Tetrad analysis indicated that the loci for carboxin resistance (Cbx1) and mating type (MAT1) segregate independently but may be located on the same chromosome. Tetrad analysis was not possible with the F₁ hybrid of he other field isolate, and its resistance cannot yet be attributed to CBX1R. Carboxin resistance was qualitatively dominant to sensitivity in vitro, as demonstrated by triad analysis of germinating F₁ teliospores. Quantitative in planta infection percents supported the conclusion that CBX1R is dominant, although incompletely, in the F₁ hybrid of one of the field isolates. Also, fewer than expected carboxin-sensitive F₂ individuals were observed in planta. However, inoculations of host plants with U. nuda have resulted in similar, unexpected variation in the past.