Molecular mapping of the rust resistance gene R ( 4 ) to a large NBS-LRR cluster on linkage group 13 of sunflower.

Rust is a serious fungal disease in the sunflower growing areas worldwide with increasing importance in North America in recent years. Several genes conferring resistance to rust have been identified in sunflower, but few of them have been genetically mapped and linked to molecular markers. The rust resistance gene R ( 4 ) in the germplasm line HA-R3 was derived from an Argentinean open-pollinated variety and is still one of most effective genes. The objectives of this study were to determine the chromosome location of the R ( 4 ) gene and the allelic relationship of R ( 4 ) with the R ( adv ) rust resistance gene. A total of 63 DNA markers previously mapped to linkage group (LG) 13 were used to screen for polymorphisms between two parental lines HA 89 and HA-R3. A genetic map of LG 13 was constructed with 21 markers, resulting in a total map length of 93.8 cM and an average distance of 4.5 cM between markers. Two markers, ZVG61 and ORS581, flanked the R ( 4 ) gene at 2.1 and 0.8 cM, respectively, and were located on the lower end of LG 13 within a large NBS-LRR cluster identified previously. The PCR pattern generated by primer pair ZVG61 was unique in the HA-R3 line, compared to lines HA-R1, HA-R4, and HA-R5, which carry other R ( 4 ) alleles. A SCAR marker linked to the rust resistance gene R ( adv ) mapped to LG 13 at 13.9 cM from the R ( 4 ) locus, indicating that R ( adv ) is not an allele of the R ( 4 ) locus. The markers tightly linked to the R ( 4 ) gene will facilitate gene pyramiding for rust resistance breeding of sunflower.