2 UNNECESSARY POWDERY MILDEW RESISTANCE GENES IN A SYNTHETIC RYE POPULATION ARE NEUTRAL ON FITNESS

A synthetic winter rye population was produced with two race-specific powdery mildew resistance genes, one dominant (Rm1) and the other (rm2 ) recessive, each at a frequency of about 0.50. The population was adv anced by open-pollination in an isolated plot under mildew-free condit ions for eight years. Samples of generations Syn-0 through Syn-7 were inoculated in the laboratory with two mildew isolates, one avirulent t o either resistance gene, the other virulent to Rm1 and avirulent to r m2, to discriminate resistant and susceptible phenotypes. From the pro portions of resistant plants, frequencies of Rm1 and rm2 were calculat ed and the fitness of carriers of resistance alleles was estimated in relation to carriers of susceptibility alleles at the two loci using c ontinuous models and linear regression analyses. Frequencies of the tw o resistance genes oscillated only weakly over the eight generations. Coefficients of selection against Rm1(-) and rm2rm2 genotypes were -0. 04 and -0.02, respectively, and not significantly different from zero. Thus the two resistance genes were selectively neutral. It is conclud ed that pyramiding of major powdery mildew resistance genes in rye var ieties should not reduce their yield potential in the absence of milde w.