By J. E. Vanderplank
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Extra info for Disease Resistance in Plants
Consider the host. Here too there is epistasis, by reflection from the pathogen. The effect of the gene Sr6 in wheat is changed by the presence or absence of the gene Sr9e, via the behavior of the pathogen. With epistasis goes stabilizing selection against the accumulation of virulence for the two Sr genes. Stabilizing selection is the opposite of direc tional selection or adaptation. Without opposite selection pressures, P. graminis would adapt itself to a wheat variety with both Sr6 and Sr9e by accumulating associated virulence for these two genes.
This original abundance and subsequent rarity have come about despite the fact that Australian wheat breeders have not used gene Sr7b. Virulence for gene Sr7b changed to avirulence in the continuous absence of gene Sr7b from Australian wheat cultivars. The change came about as an indirect adaptation to the wheat breeders' using genes Sr6, 9b, 15, and 17. 44 Chapter 5. Adaptation of the Pathogen to the Host: Wheat Stem Rust in Australia Indirect adaptation results from the ABC-XYZ grouping of genes.
Flor (1958) used an F x hybrid of Melampsora lini heterozygous for avirulence at four loci and found significant differences in mutation rates. At one locus two natural mutants were found in 200,000 uredospores; at another, one natural mutant in 600,000 uredospores; and at the other two, no mutants in 300,000 and 900,000 uredospores, respectively. Luig (1979) studied mutation in Puccinia graminis tritici in natural condi tions and also with a mutagen (ethyl methanesulfonate). At the one extreme, avirulence for the resistance genes Sr5, Sri5, Sr21, and Sr9e in wheat was found to have a very high spontaneous mutation rate to virulence, as well as a high rate after treatment with the mutagen.