Disease resistance gene analogs as candidates for QTLs involved in pepper-pathogen interactions

Whereas resistance genes (R-genes) governing qualitative resistance have be en isolated and characterized, the biological roles of genes governing quan titative resistance (quantitave trait loci, QTLs) are still unknown. We hyp othesized that genes at QTLs could share homologies with cloned R-genes. We used a PCR-based approach to isolate R-gene analogs (RGAs) with consensus primers corresponding with conserved domains of cloned R-genes: (i) the nuc leotide binding site (NBS) and hydrophobic domain, and (ii) the kinase doma in. PCR-amplified fragments were sequenced and mapped on a pepper intraspec ific map. NBS-containing sequences of pepper, most similar to the N gene of tobacco, were classified into seven families and all mapped in a unique re gion covering 64 cM on the Noir chromosome. Kinase domain containing sequen ces and cloned R-gene homologs (Pto, Fen, Cf-2) were mapped on four differe nt linkage groups. A QTL involved in partial resistance to cucumber mosaic virus (CMV) with an additive effect was closely linked or allelic to one NB S-type family. QTLs with epistatic effects were also detected at several RG A loci. The colocalizations between NBS-containing sequences and resistance QTLs suggest that the mechanisms of qualitative and quantitative resistanc e may be similar in some cases.