Characterization and mapping of resistance to Oidium lycopersicum in two Lycopersicon hirsutum accessions: evidence for close linkage of two Ol-geneson chromosome 6 of tomato

The cultivated tomato is susceptible to powdery mildew (Oidium lycopersicum ). Several accessions of wild species are resistant. In this study we descr ibe (i) the genetics and mapping of resistance to O. lycopersicum 2 in G1.1 290, one of the resistant accessions in Lycopersicon hirsutum, (ii) fine ma pping of Ol-1 originated from L. hirsutum G1.1560, another resistant access ion of L. hirsutum, and (iii) tests of allelism for resistance in G1.1290 a nd G1.1560. Initially, it is demonstrated that the resistance in G1.1290 to O. lycopersicum is controlled by an incompletely dominant gene, designated Ol-3. By using an advanced breeding line (ABL) containing introgression fr agment(s) from G1.1290, Ol-3 was found to be associated with several RFLP a nd SCAR markers on chromosome 6. By using these markers, Ol-3 was mapped be tween markers TG25/SCAF10 and H9A11 on chromosome 6. Secondly, after testin g some F-3 lines and their progenies from the cross between L, esculentum c v Moneymaker and L. hirsutum G1.1560, we provided more evidence for the map position of Ol-1 to lie between SCAF10 and H9A11, indicating that Ol-1 and Ol-3 are in the same chromosome region. Thirdly, although allelism tests c ould not discriminate between Ol-1 and Ol-3, (indirect) evidence suggested that these two genes are not identical. They might instead represent functi onal genes of a cluster of Ol-homologues.