RECEPTOR-LIKE GENES IN THE MAJOR RESISTANCE LOCUS OF LETTUCE ARE SUBJECT TO DIVERGENT SELECTION

Disease resistance genes in plants are often found in complex multigen e families. The largest known cluster of disease resistance specificit ies in lettuce contains the RGC2 family of genes. We compared the sequ ences of nine full-length genomic copies of RGC2 representing the dive rsity in the cluster to determine the structure of genes within this f amily and to examine the evolution of its members. The transcribed reg ions range from at least 7.0 to 13.1 kb, and the cDNAs contain deduced open reading frames of similar to 5.5 kb. The predicted RGC2 proteins contain a nucleotide binding site and irregular leucine-rich repeats (LRRs) that are characteristic of resistance genes cloned from other s pecies. Unique features of the RGC2 gene products include a bipartite LRR region with >40 repeals. At least eight members of this family are transcribed. The level of sequence diversity between family members v aried in different regions of the gene. The ratio of nonsynonymous (K- a) to synonymous (K-s) nucleotide substitutions was lowest in the regi on encoding the nucleotide binding site, which is the presumed effecto r domain of the protein. The LRR-encoding region showed an alternating pattern of conservation and hypervariability. This alternating patter n of variation was also found in all comparisons within families of re sistance genes cloned from other species. The K-a/K-s ratios indicate that diversifying selection has resulted in increased variation at the se codons. The patterns of variation support the predicted structure o f LRR regions with solvent-exposed hypervariable residues that are pot entially involved in binding pathogen-derived ligands.