Discovery and mapping of two new rust resistance genes, R17 and R18, in sunflower using genotyping by sequencing.

KEY MESSAGE: Discovery of two rust resistance genes, R17 and R18, from the sunflower lines introduced from South Africa and genetic mapping of them to sunflower chromosome 13. Rust, caused by the fungus Puccinia helianthi Schw., is one of the most serious diseases of sunflower in the world. The rapid changes that occur in the virulence characteristics of pathogen populations present a continuous threat to the effectiveness of existing rust-resistant hybrids. Thus, there is a continued need for the characterization of genetically diverse sources of rust resistance. In this study, we report to identify two new rust resistance genes, R17 and R18, from the sunflower lines, KP193 and KP199, introduced from South Africa. The inheritance of rust resistance was investigated in both lines using two mapping populations developed by crossing the resistant plants selected from KP193 and KP199 with a common susceptible parent HA 89. The F2 populations were first genotyped using genotyping by sequencing for mapping of the rust genes and further saturated with markers in the target region. Molecular mapping positioned the two genes at the lower end of sunflower chromosome 13 within a large gene cluster. Two co-segregating SNP markers, SFW01497 and SFW08875, were distal to R17 at a 1.9 cM genetic distance, and a cluster of five co-segregating SNPs was proximal to R17 at 0.7 cM. R18 co-segregated with the SNP marker SFW04317 and was proximal to two cosegregating SNPs, SFW01497 and SFW05453, at 1.9 cM. These maps provide markers for stacking R17 or R18 with other broadly effective rust resistance genes to extend the durability of rust resistance. The relationship of the six rust resistance genes in the cluster was discussed.