Comparative genetics of disease resistance within the solanaceae

Genomic positions of phenotypically defined disease resistance genes (R gen es) and R gene homologues were analyzed in three solanaceous crop genera, L ycopersicon (tomato),Solanum (potato), and Capsicum (pepper). R genes occur red at corresponding positions in two or more genomes more frequently than expected by chance; however, in only two cases, both involving Phytophthora spp., did genes at corresponding positions have specificity for closely re lated pathogen taxa. In contrast, resistances to Globodera spp., potato vir us Y, tobacco mosaic virus, and tomato spotted wilt virus were mapped in tw o or more genera and did not occur in corresponding positions. Without. exc eption, pepper homologues of the cloned R genes Sw-5, N, Pto, Prf and I2 we re found in syntenous positions in other- solanaceous genomes and in some c ases also mapped to additional positions near phenotypically defined solana ceous R genes. This detailed analysis and synthesis of all available data f or solanaceous R genes suggests a working hypothesis regarding the evolutio n of R genes. Specifically, while the taxonomic specificity of host R genes may be evolving rapidly, general functions of R alleles (e.g., initiation of resistance response) may be conserved at homologous loci in related plan t genera.