Mapping Ol-4, a gene conferring resistance to Oidium neolycopersici and originating from Lycopersicon peruvianum LA2172, requires multi-allelic, single-locus markers

Lycopersicon peruvianum LA2172 is completely resistant to Oidium neolycopersici, the causal agent of tomato powdery mildew. Despite the large genetic distance between the cultivated tomato and L. peruvianum, fertile F-1 hybrids of L. esculentum cv. Moneymaker x L. peruvianum LA2172 were produced, and a pseudo-F-2 population was generated by mating F-1 half-sibs. The disease tests on the pseudo-F-2 population and two BC1 families showed that the resistance in LA2172 is governed by one dominant gene, designated as Ol-4. In the pseudo-F-2 population, distorted segregation was observed, and multi-allelic, single-locus markers were used to display different marker-allele configurations per locus. Parameters for both distortion and linkage between genetic loci were determined by maximum likelihood estimation, and the necessity of using multi-allelic, single-locus markers was illustrated. Finally, a genetic linkage map of chromosome 6 around the Ol-4 locus was constructed by using the pseudo-F-2 population.