Plant & Animal Genome IX Conference Plant & Animal Genome IX Conference
Workshop: Compositae
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Resistance to downy mildew, one of the most serious diseases of sunflower (Helianthus annuus L.), is linked to resistance gene candidates (RGCs) of the TIR-NBS-LRR subclass on LG8 of the Advanta RFLP map. Genes conditioning resistance to various races of the pathogen have been found in cultivated lines and in wild sunflower species. Our aim was to investigate the genomic organization of RGCs implicated in downy mildew resistance in 11 diverse inbred lines, six partially inbred lines (HA335-HA340) constructed by introgression of resistance to race 2 from three wild sunflower species into HA89, and individuals from the six wild donor populations. We used PCR primers designed to amplify intron 1 and a segment of the NBS region of a targeted TIR-NBS-LRR subgroup previously localized to LG8. GeneScan analysis revealed 25 unique RGC amplicons among the 23 DNA samples. The deduced size of intron 1 ranged from 98 to 460 nucleotides. Inbred line DNAs amplified 10 of the 25 RGCs, and comprised four haplotype groups. Susceptible parent HA89 DNA amplified only three RGCs. The same set of 14 RGCs was amplified from HA335-HA339 DNAs, suggesting co-introgression. In contrast, wild samples amplified a diverse set of 12-17 RGCs. Six RGCs were mapped to a small segment of LG8 in HA370 X HA372 F2 progeny segregating for resistance to race 1. These data suggest that a large cluster of TIR-NBS-LRR genes reside on sunflower LG8, and that domesticated inbred lines contain only a subset of the RGC genetic diversity available in wild sunflower species.