Department of Plant Pathology, University of Arizona, Tucson 85721-0036, U.S.A.
The avirulence genes that have been identified in Magna-porthe grisea exhibit varying degrees of stability in infection assays. AVR1-MARA is considered one of the stable avirulence genes. In an effort to understand this stability, we analyzed the AVR1-MARA locus by physical mapping and chromosome walking. By walking toward AVR1-MARA from restriction fragment length polymorphism markers on both sides of the locus, we isolated sequences that are inseparable from AVR1-MARA, but we were unable to clone the complete locus. In contrast, the virulent locus avr1-MARA was isolated easily. A detailed comparative map of the two loci was constructed that identifies two deletions in the virulent locus, suggesting that virulence may be due to partial or complete deletion of the gene. Physical mapping also revealed that one progeny strain from a cross between avirulent and virulent parents appears to have spontaneously mutated to virulence.