THE MAJOR RESISTANCE GENE-CLUSTER IN LETTUCE IS HIGHLY DUPLICATED ANDSPANS SEVERAL MEGABASES

At least 10 Dm genes conferring resistance to the oomycete downy milde w fungus Bremia lactucae map to the major resistance cluster in lettuc e. We investigated the structure of this cluster in the lettuce cultiv ar Diana, which contains Dm3. A deletion breakpoint map of the chromos omal region flanking Dm3 was saturated with a variety of molecular mar kers. Several of these markers are components of a family of resistanc e gene candidates (RGC2) that encode a nucleotide binding site and a l eucine-rich repeat region. These motifs are characteristic of plant di sease resistance genes. Bacterial artificial chromosome clones were id entified by using duplicated restriction fragment length polymorphism markers from the region, including the nucleotide binding site-encodin g region of RGC2. Twenty-two distinct members of the RGC2 family were characterized from the bacterial artificial chromosomes; at least two additional family members exist. The RGC2 family is highly divergent; the nucleotide identity was as low as 53% between the most distantly r elated copies. These RGC2 genes span at least 3.5 Mb. Eighteen members were mapped on the deletion breakpoint map. A comparison between the phylogenetic and physical relationships of these sequences demonstrate d that closely related copies are physically separated from one anothe r and indicated that complex rearrangements have shaped this region. A nalysis of low-copy genomic sequences detected no genes, including RGC 2, in the Dm3 region, other than sequences related to retrotransposons and transposable elements. The related but divergent family of RGC2 g enes may act as a resource for the generation of new resistance phonot ypes through infrequent recombination or unequal crossing over.