Genetic mapping of genes for susceptibility to black spot disease in Japanese pears.

Black spot disease, which is caused by the Japanese pear pathotype of Alternaria alternata (Fr.) Keissler, is one of the most harmful diseases in Japanese pear cultivation. We identified the exact positions and linkage groups (LGs) of the genes for susceptibility to black spot in the Japanese pear (Pyrus pyrifolia Nakai) cultivars 'Osa Nijisseiki' (gene Ani) and 'Nansui' (gene Ana). Segregation of susceptibility and resistance fitted the expected ratio of 1:1 in progeny of 'Nansui' but showed a slight distortion in progeny of 'Osa Nijisseiki'. We mapped the genes for susceptibility to black spot in both populations using a genome scanning approach. The simple sequence repeat (SSR) markers CH04h02 and CH03d02 showed tight linkage to Ani and Ana. Although Ani and Ana are derived from different sources, both genes are located at the top region of LG 11. Information about the positions of the susceptibility genes and the molecular markers linked to them will be useful for marker-assisted selection in pear breeding programs.

Genetic mapping of the pear scab resistance gene Vnk of Japanese pear cultivar Kinchaku.

Pear scab (caused by Venturia nashicola) is one of the most harmful diseases of pears, especially Japanese and Chinese pear species. The molecular identification and early selection of resistant plants could greatly improve pear breeding. We have identified the position of the scab resistance gene, designated Vnk in an indigenous Japanese pear cultivar Kinchaku, within the pear genome by using simple sequence repeat (SSR) markers derived from pear and apple. The position of Vnk was identified in the central region of linkage group 1 of Kinchaku. Several amplified fragment length polymorphism (AFLP) markers linked to Vnk were obtained by bulked segregant analysis. Among them, the AFLP marker closest to Vnk was converted into a sequence tagged site (STS) marker. Four random amplified polymorphic DNA (RAPD) markers previously found to be loosely associated with Vnk (Iketani et al. 2001) were successfully converted into STS markers. Six markers (one SSR Hi02c07 and five STSs converted from AFLP and RAPD) showed tight linkages to Vnk, being mapped with distances ranging from 2.4 to 12.4 cM. The SSR CH-Vf2, which was isolated from a BAC clone of the contig containing the apple scab gene Vf, was mapped at the bottom of linkage group 1 in Kinchaku, suggesting that the Vnk and Vf loci are located in different genomic regions of the same homologous linkage group.