Convergent Evolution of Disease Resistance Gene Specificity in Two Flowering Plant FamiliesW

Plant disease resistance ( R ) genes that mediate recognition of the same pathogen determinant sometimes can be found in distantly related plant families. This observation implies that some R gene alleles may have been conserved throughout the diversification of land plants. To address this question, we have compared R genes from Glycine max (soybean), Rpg1-b , and Arabidopsis thaliana , RPM1 , that mediate recognition of the same type III effector protein from Pseudomonas syringae , AvrB. RPM1 has been cloned previously, and here, we describe the isolation of Rpg1-b . Although RPM1 and Rpg1-b both belong to the coiled-coil nucleotide binding site (NBS) Leu-rich repeat (LRR) class of R genes, they share only limited sequence similarity outside the conserved domains characteristic of this class. Phylogenetic analyses of A. thaliana and legume NBS-LRR sequences demonstrate that Rpg1-b and RPM1 are not orthologous. We conclude that convergent evolution, rather than the conservation of an ancient specificity, is responsible for the generation of these AvrB-specific genes.