Mapping strategy for resistance genes against Cladosporium fulvum on the short arm of Chromosome 1 of tomato: Cf-ECP5 near the Hcr9 Milky Way cluster

In the past, numerous Lycopersicon accessions have been described that harb or resistance genes to Cladosporium fulvum (Cf genes). Several Cf genes hav e been isolated, like Cf-4, Cf-4A and Cf-9, which are present on the short arm of Chromosome 1, and Cf-2 and Cf-5, which reside on Chromosome 6. To id entify Cf genes linked to the Hcr9 cluster "Milky Way" on the short arm of Chromosome 1, we test-crossed 66 resistant Lycopersicon accessions to the n ear-isogenic line Moneymaker-Cf4, and the F(1)s were crossed to the suscept ible tomato cultivar Moneymaker. Putative linkage between an unknown Cf gen e and Cf-4 was concluded based on small-scale allelic tests from an under-r epresentation of susceptible genotypes in the progenies of 24 plants after inoculation with race 0 of C. fulvum. In this way, of the 21 resistant line s tested, 10 harbored a Cf gene that was linked to the Hcr9 Milky Way clust er. Moreover, one of the lines harboring a Cf gene closely linked to Cf-4 s pecifically recognizes the extracellular protein ECP5 of C. fulvum and was designated Cf-ECP5. Using a testcross population of 338 plants, we mapped C f-ECP5 more accurately at 4 cM proximal to the Hcr9 Milky Way locus. This r eport shows that the method of small-scale allelic tests provides a useful tool to rapidly screen for Cf genes on the short arm of Chromosome 1. Furth er analysis of these Cf genes will elucidate the complex genetic organizati on of Cf genes on Chromosome 1 of tomato.