Genetic mapping of the [Or.sub.5] gene for resistance to Orobanche Race E in sunflower

Orobanche cumana Wallr. (= O. cernua Loefl., broomrape), a weedy parasitic plant, is a serious pest of cultivated sunflower (Helianthus annuus L.). Breeding for resistance has been crucial for protecting sunflowers from broomrape damage, a challenging task because new races of the pathogen continually emerge and ultimately defeat known resistance genes. Despite several attempts to identify DNA markers tightly linked to Orobanche resistance genes, the closest reported thus far is 5.6 centimorgans (cM) downstream of [Or.sub.5], a gene for resistance to Race E. The [Or.sub.5] locus was placed on the simple sequence repeat (SSR) map of sunflower by genotyping and phenotyping 262 recombinant inbred lines (RILs) from a cross between elite inbred lines (PHC x PHD) segregating for resistance to Orobanche Race E. Polymerase chain reaction (PCR) multiplexes were used to screen 78 SSR marker loci, strategically positioned throughout the genome, for polymorphisms between resistant and susceptible bulks of PHC x PHD RILs. The bulks were polymorphic for three of five Linkage Group 3 (LG3) SSR marker loci amplified by the PCR multiplexes. The RILs were phenotyped for resistance to Race E and genotyped for 13 SSR markers from the upper end of LG3. The [Or.sub.5] locus mapped to the end of LG3 distal to the SSR marker loci (the closest SSR marker locus was 6.2 cM downstream of [Or.sub.5]. The terminal and perhaps telomeric location of [Or.sub.5] on LG3 sheds light on difficulties, past and present, of identifying flanking DNA markers tightly linked to [Or.sub.5].