Studies on recombination between allele in the ml-o locus of barley and on pleiotropic effects of the alleles

Five independently arisen genes, which are functionally allelic in the m1-o locus and conditioning resistance to the powdery mildew fungus, were tested for structural allelism by recombination. Two heteroallelic crosses produced susceptible recombinants in F2 with a frequency of 8.6 and 2.0 x 10-4, respectively, showing that m1-o 1 is structurally non-allelic to m1-o 5 and to m1-o 9. Homozygous resistant populations derived from crossing different m1-o resistant barleys with susceptible varieties were exposed to two successive cycles of selection against necrotic leaf spotting. Field experiments with selected F5 lines and their parents showed that the m1-o resistant parents differed in severity of spotting, and that the spotting can be reduced or eliminated by crossing and subsequent selection. It appears that a reduction in the necrotic leaf spotting is accopanied by an increase in grain yield. Two new experiments designed to further elucidate the interallelic recombination in m1-o and the pleiotropic effects are briefly described. (author)