IDENTIFICATION AND MAPPING OF A LEAF RUST RESISTANCE GENE IN BARLEY LINE Q21861

Barley line Q21861 possesses an incompletely dominant gene (RphQ) for resistance to leaf rust caused by Puccinia hordei. To investigate the allelic and linkage relations between RphQ and other known Rph genes, F-2 populations from crosses between Q21861 and donors of Rph1 to Rph1 4 (except for Rph8) were evaluated for leaf rust reaction at the seedl ing stage. Results indicate that RphQ is either allelic with or closel y Linked to the Rph2 locus. A doubled haploid population derived from a cross between Q21861 and SM89010 (a leaf rust susceptible line) was used for molecular mapping of the resistance locus. Bulked segregant a nalysis was used to identify markers linked to RphQ, using random ampl ified polymorphic DNAs (RAPDs), restriction fragment length polymorphi sms (RFLPs), and sequence tagged sites (STSs). Of 600 decamer primers screened, amplified fragments generated by 9 primers were found to be linked to the RphQ locus; however, only 4 of them were within 10 cM of the target. The RphQ locus was mapped to the centromeric region of ch romosome 7, with a linkage distance of 3.5 cM from the RFLP marker CDO 749. Rm2, an RFLP clone from the ribosomal RNA intergenic spacer regio n, was found to be very closely linked with RphQ, based on bulked segr egant analysis. An STS marker, ITS1, derived from Rm2, was also closel y linked (1.6 cM) to RphQ.