Characterization and mapping of resistance to Oidium lycopersicum in two Lycopersicon hirsutum accessions: evidence for close linkage of two Ol-genes on 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 describe (i) the genetics and mapping of resistance to O. lycopersicum in G1.1290, one of the resistant accessions in Lycopersicon hirsutum, (ii) fine mapping of Ol-1 originated from L. hirsutum G1.1560, another resistant accession of L. hirsutum, and (iii) tests of allelism for resistance in G1.1290 and 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 fragment(s) from G1.1290, Ol-3 was found to be associated with several RFLP and SCAR markers on chromosome 6. By using these markers, Ol-3 was mapped between markers TG25/SCAF10 and H9A11 on chromosome 6. Secondly, after testing some F3 lines and their progenies from the cross between L. esculentum cv 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 could not discriminate between Ol-1 and Ol-3, (indirect) evidence suggested that these two genes are not identical. They might instead represent functional genes of a cluster of Ol-homologues.

The Cf-ECP2 gene is linked to, but not part of, the Cf-4/Cf-9 cluster on the short arm of chromosome 1 in tomato.

A gene has been identified in tomato, which confers resistance to Cladosporium fulvum through recognition of the pathogenicity factor ECP2. Segregation analysis of F2 and F3 populations showed monogenic dominant inheritance, as for previously reported Cf resistances. The gene has been designated Cf-ECP2. Using several mapping populations, Cf-ECP2 was accurately mapped on chromosome 1, 7.7 cM proximal to TG236 and 6.0 cM distal to TG184. Although Cf-ECP2 is linked to Cf-4, it is not located in the Hcr9 cluster "Milky Way". Therefore, Cf-ECP2 is the first functional Cf homologue on chromosome 1 that does not belong to this Hcr9 cluster. No recombination events between Cf-ECP2 and CT116 have been observed in three populations tested, representing 282 individuals. The low value for the physical distance per cM around CT116 reported previously and the high probability that Cf-ECP2 is also a member of a Hcr9 cluster will facilitate cloning of the locus.