Characterization and linkage mapping of R-gene analogous DNA sequences in pea (Pisum sativum L.)

Pea (Pisum sativum L.) sequences that are analogous to the conserved nucleo tide binding site (NBS) domain found in a number of plant disease resistanc e genes (R-genes) were cloned. Using redundant oligonucleotide primers and the polymerase chain reaction (PCR), we amplified nine pea sequences and ch aracterised their sequences. The pea R-gene analog (RGA)-deduced amino acid sequences demonstrated significant sequence similarity with known R-gene s equences lodged in public databases. The genomic locations of eight of the pea RGAs were determined by linkage mapping. The eight RGAs identified ten loci that mapped to six linkage groups. In addition, the genomic organizati on of the RGAs was inferred. Both single-copy and multicopy sequence famili es were present among the RGAs, and the multicopy families occurred most of ten as tightly linked clusters of related sequences. Intraspecific copy num ber variability was observed in three of the RGA sequence families, suggest ing that these sequence families are evolving rapidly. The genomic location s of the pea RGAs were compared with the locations of known pea R-genes and sym genes involved in the pea-rhizobia symbiosis. Two pea RGAs mapped in t he genomic region containing a pea R-gene, Fw, and four pea RGAs mapped in regions of the genome containing sym genes.