Cloning and linkage mapping of resistance gene homologues in apple.

Apple ( Malus x domestica Borkh.) sequences sharing homology with known resistance genes were cloned using a PCR-based approach with degenerate oligonucleotide primers designed on conserved regions of the nucleotide-binding site (NBS). Sequence analysis of the amplified fragments indicated the presence of at least 27 families of NBS-containing genes in apple, each composed of several very similar or nearly identical sequences. The NBS-leucine-rich repeat homologues appeared to include members of the two major groups that have been described in dicot plants: one possessing a toll-interleukin receptor element and one lacking such a domain. Genetic mapping of the cloned sequences was achieved through the development of CAPS and SSCP markers using a segregating population of a cross between the two apple cultivars Fiesta and Discovery. Several of the apple resistance gene homologues mapped in the vicinity, or at least on the same linkage group, of known loci controlling resistance to various pathogens. The utility of resistance gene-homologue sequences as molecular markers for breeding purposes and for gene cloning is discussed.