Fine Mapping and BAC Contigs Construction of a Susceptible Gene to Black Spot Disease in Japanese Pear (Pyrus pyrifolia)

Black spot disease, which is caused by Alternaria alternata Japanese pear pathotype, is one of the most harmful diseases in Japanese pear cultivation. Alternaria alternata fungi produce several host-specific toxins (HSTs), and their HSTs have been identified for Japanese pear, strawberry, tangerine, apple, tomato, and rough lemon. The susceptibility to black spot disease derived from Japanese pear 'Kinchaku' is controlled by a single dominant gene, designated Aki, on the linkage group 11. We conducted fine mapping of the gene Aki using 860 F₁ progeny. Novel 45 simple sequence-repeat (SSR) markers were developed from contig sequences located on the chromosome 11 of apple genome, indicating that Aki was located within 0.9 cM genome region between two SSR markers, Mdo.chr11.28 and Mdo.chr11.34. SSR marker Mdo.chr11.30 showed co-segregation with Aki in all the 860 genotypes studied. A bacterial artificial chromosome (BAC) library from the susceptible cultivar ‘Kinchaku’ was constructed with 16 times coverage of its genome. Two BAC clones including three SSR loci (Mdo.chr11.28, Mdo.chr11.30, and Mdo.chr11.34) were isolated to cover the Aki region. The results of sequencing analysis of the BAC clones, physical size of the Aki region between Mdo.chr11.28 and Mdo.chr11.34 was estimated as 150 Kb in Japanese pear ‘Kinchaku’.