Research Station, Agriculture Canada, 195 Dafoe Road, Winnipeg, MB, R3T 2M9
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Accepted for publication 15 October 1999.
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.
© 2000 The American Phytopathological Society