Comparative genetics of nucleotide binding site-leucine rich repeat resistance gene homologues in the genomes of two dicotyledons: Tomato and Arabidopsis

The presence of a single resistance (R) gene allele carl determine plant di sease resistance. The protein products of such genes may act as receptors t hat specifically interact with pathogen-derived factors. Most functionally defined R-genes are of the nucleotide binding site-leucine rich repeat (NBS -LRR) supergene family and are present as large multigene families. The spe cificity of R-gene interactions together with the robustness of plant-patho gen interactions raises the question of their gene number and diversity in the genome. Genomic sequences from tomato showing significant homology to g enes conferring race-specific resistance to pathogens were identified by sy stematically "scanning" the genome using a variety of primer pairs based on ubiquitous NBS motifs. Over 70 sequences were isolated and 10% are putativ e pseudogenes. Mapping of the amplified sequences on the tomato genetic map revealed their organization as mixed clusters of R-gene homologues that sh owed in many cases linkage to genetically characterized tomato resistance l oci. Interspecific examination within Lycopersicon showed the existence of a null allele. Consideration of the tomato and potato comparative genetic m aps unveiled conserved syntenic positions of R-gene homologues. Phylogeneti c clustering of R-gene homologues within tomato and other Solanaceae family members was observed but not with R-gene homologues from Arabidopsis thali ana. Our data indicate remarkably rapid evolution of R-gene homologues duri ng diversification of plant families.