IDENTIFICATION OF A QTL DECREASING YIELD IN BARLEY LINKED TO MLO POWDERY MILDEW RESISTANCE

A molecular marker map, including Mlo mildew resistance, of the spring barley cross Derkado (Mlo-resistant) x B83-12/21/5 (Mlo-susceptible) was scanned for yield QTLs to determine whether the association of Mlo resistance with reduced yield was due to linkage or pleiotropy. Over the mapped portion of the genome of the cross, the QTL with the greate st effect upon yield was located within a 22 cM region between mlo and the simple sequence repeat HVM67 on chromosome 4(4H). The association of Mlo resistance with lower yield was therefore due to a repulsion l inkage. Analysis of yield component characters revealed QTL alleles fo r reduced grain number and earlier heading date in the same region, al so associated with Mlo resistance. Genotyping of a range of cultivars and sources of Mlo resistance with the HVM67 simple sequence repeat sh owed that the Derkado HVM67 allele was rare as it was found only in on e other cultivar and four land-races or sources of disease resistance. Grannenlose Zweizeilige, the source, and Salome, the carrier of Mlo r esistance in Derkado, have the same HVM67 genotype, although Salome wa s a mixture of two genotypes, The entire mlo-HVM67 chromosomal segment from Grannenlose Zweizeilige is therefore thought to have been transm itted to Derkado, possibly through joint selection for Mlo resistance and early heading. L92, synonym EP79, was another source of Mlo resist ance with the same HVM67 allele as Derkado but recombination must have occurred during the breeding of Atem as it possesses a different HVM6 7 allele which is present in all the other Mlo sources and cultivars s urveyed.