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CELL BIOLOGY & MOLECULAR GENETICS

Genetic Mapping of the Or₅ Gene for Resistance to Orobanche Race E in Sunflower

^a Dep. of Crop and Soil Sci., Oregon State Univ., Corvallis, OR 97331-3002, USA
^b Pioneer Hi-Bred Int., Avda. Reino Unido s/n, Edificio Aditec 2a Planta, 41012 Sevilla, Spain
^c Pioneer Hi-Bred Int., Belediye Binasi Kat 3, Ahmetbey-Kirklareli, Turkey
^d Pioneer Hi-Bred Int., 18285 County Road 96, Woodland, CA 95695-9340, USA

^* Corresponding author (steven.j.knapp{at}orst.edu)

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 Or₅, a gene for resistance to Race E. The Or₅ 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 Or₅ locus mapped to the end of LG3 distal to the SSR marker loci (the closest SSR marker locus was 6.2 cM downstream of Or₅. The terminal and perhaps telomeric location of Or₅ on LG3 sheds light on difficulties, past and present, of identifying flanking DNA markers tightly linked to Or₅.

Abbreviations: AFLP, amplified fragment length polymorphism • BSA, bulked segregant analysis • cM, centimorgan • CMS, cytoplasmic-genic male sterility • LOD, likelihood odds • MAS, marker-assisted selection • PCR, polymerase chain reaction • RAPD, random amplified polymorphic DNA • RIL, recombinant inbred line • RFLP, restriction fragment length polymorphism • SCAR, sequence characterized amplified region • SSR, simple sequence repeat