GENETIC CHARACTERIZATION OF THE MAJOR RESISTANCE GENE CLUSTER IN LETTUCE (Lactuca sativa)

¹ Department of Vegetable Crops, University of California, Davis, CA 95616 USA
² Institute of Evolution, Haifa University, Haifa, Mt. Carmel 31905, Israel

Clustering of disease resistance genes is a recurring theme in many plant genomes. In lettuce, the majority of genes conferring resistance to lettuce downy mildew (Dm genes) cluster into three major linkage groups. To investigate the stability and evolution of these resistance specificities, we have been studying the genetic and physical organization of these clustered regions, with particular emphasis on the largest cluster, which contains at least 10 downy mildew resistance genes, including Dm1 and Dm3. Several approaches are being taken to investigate the evolutionary mechanisms generating resistance specificities. We have been examining three different populations for the occurrence of spontaneous mutations at various Dm loci. Extensive complementation experiments and molecular analyses have been conducted. All losses of Dm activity studied mapped to single Dm genes. A majority of Dm3 mutants exhibited large deletions of genetic material. Current data indicate that unequal crossing-over is occurring, but is not the primary evolutionary force generating new variation. In addition to the examination of spontaneous mutations, we have been investigating the genetic variation present in a variety of natural populations, cultivars and species. Both microsatellites and fingerprinting methods have been used to establish resistance gene haplotypes in Lactuca species. Phylogenetic analyses of the diversity of resistance gene haplotypes have shown that polymorphisms are ancient, predating the separation of species.