Fine mapping of two major QTLs conferring resistance to powdery mildew in tomato

Tomato (Solanum lycopersicum) is the most cultivated crop in the Solanaceae family and is a host for Oidium neolycopersici, the cause agent of powdery mildew disease. In wild species of tomato, genes (Ol-1–Ol-6) for monogenic resistance have been identified. Moreover, three quantitative resistance loci (QRLs), namely Ol-qtl1, Ol-qtl2 and Ol-qtl3, have been mapped in Solanum neorickii G1.1601. In this work, we developed several advanced backcross populations in order to fine-map these Ol-qtls. Resistant lines harboring individual Ol-qtl were produced and used in recombinant screening. Ten recombinants were identified in chromosomal regions carrying Ol-qtl1s. The recombinant individuals were used to produce recombinant families (RFs). By screening these RFs with molecular markers and testing them with O. neolycopersici, we could localize Ol-qtl1 in a region of about 2.3 Mbp on the long arm of chromosome 6 and Ol-qtl2 in a region of 2.5 Mbp on the short arm of chromosome 12. On the other hand, the presence of Ol-qtl3 locus was not confirmed in this study. The fine-mapping results further demonstrated the co-localization between Ol-qtls and genes for monogenic resistance; the Ol-qtl1 interval contains the Ol-1 gene and the Ol-qtl2 interval harbors the Lv gene that confers monogenic resistance to Leveillula taurica, another species of tomato powdery mildew.