Endopeptidase polymorphism and linkage of the Ep-D1c null allele with the Lrl9 leaf-rust-resistance gene in hexaploid wheat

The aim of this study was to test whether the null allele Ep-D1c of the endopeptidase Ep-D1 can be used as a marker for the Lrl9 leaf rust resistance gene. The frequency of Ep-D1c was determined in 1134 winter wheat, spring wheat and spelt breeding lines and varieties. Only eight lines were found to carry Ep-D1c. Six of these lines originated from crosses with RL6040, the gene donor for Lrl9. The other two lines were leaf-rust susceptible in the seedling stage and therefore did not carry Lr19. The genetic distance between Ep-D1c and Lr19 was determined in a reciprocal cross between the lines FAP75184 (Ep-D1c, Lr19) and FAP75106 (Ep-D1a, leaf-rust susceptible in the seedling stage). Out of 840 F₂ seedlings screened, 162 were homozygous for Ep-D1c. From 150 of these F₂ plants, F₃ seedlings were screened for segregation for leaf-rust resistance with isolates avirulent on Lr19. Only one F₂ plant produced susceptible F₃ progeny indicating a recombination event between Ep-D1c and Lr19. From these data, a genetic distance of 0.33 ± 0.33cM between Ep-D1c and Lr19 was calculated. The results show that Ep-D1c is a useful marker for a practical breeding programme allowing the rapid identification of plants homozygous for Lrl9.