Chromosome location, DNA markers and rust resistance of the sunflower gene R5

Sunflower rust, caused by the fungus Puccinia helianthi Schwein., was not a serious problem for many decades because of successful deployment of effective resistance genes in commercial sunflower (Helianthus annuus L.) hybrids in North America. In the 1980s and early 1990s, however, a shift in virulence of the rust race population in North America rendered most of the commercial hybrids susceptible to new virulent races. A germplasm line, HA-R2, carrying the rust resistance gene R (5) was released as a multi-race rust-resistant line in 1985 but has not been widely used in commercial hybrid production. R (5) remains effective against the prevalent rust races of sunflower in North America. This gene was previously reported to be associated with two simple sequence repeat (SSR) markers, ORS316 and ORS630, which were mapped to linkage group (LG) 13 of sunflower. However, out of the 63 markers of LG13 screened in the present study, only 18, including ORS316 and ORS630, were polymorphic. These markers, which covered all of LG 13, were assayed in 94 individual F2 progenies derived from the cross of HA 89 with HA-R2. All failed to detect any locus in LG13 associated with the gene R (5) . Subsequently, a bulked segregant analysis was employed with an additional 510 SSR markers selected from the remaining 16 LGs of the sunflower genome. This analysis demonstrated that the LG2 markers showed association with rust resistance. Genotyping of the 94 F2 individuals with 23 polymorphic SSR markers from LG2 confirmed the R (5) location on LG2, flanked by two SSR markers, ORS1197-2 and ORS653a, at 3.3 and 1.8 cM of genetic distance, respectively. The markers for R (5) developed in this study will provide a useful tool for speeding up deployment of the R (5) gene in commercial sunflower hybrid production.