Theor Appl Genet (1996) 92:858-864 9Springer-Verlag 1996 N. C. Collins 9 N. G. Paltridge 9 C. M. Ford R. H. Symons The u gene for barley yellow dwarf virus resistance maps close to the centromere on the long arm of barley chromosome 3 Received: 25 May 1995 / Accepted: 13 October 1995 Abstract Barley yellow dwarfluteovirus (BYDV) causes serious yield losses in all cereals worldwide. The Yd2 gene from a number of Ethiopian barleys (Hordeum vulgare L.) has been the most effective means of providing resistance against BYDV in cultivated barley. Isolation of the Yd2 gene will enable characterisation of the molecular basis of the Yd2-BYDV interaction. This paper describes the first stage in a project to isolate the gene: the construction of a detailed linkage map of the Yd2 region. The map encom- passes 27.6 centiMorgans (cM) of chromosome 3 and con- tains 19 RFLPs, 2 morphological marker loci, the centro- mere and Yd2. In the mapping population of 106 F 2 indi- viduals, Yd2 perfectly cosegregated with the RFLP loci Xwg889 and XYlp, which were located on the long arm, 0.5 cM from the centromere. The two morphological marker loci, uzu dwarf and white stripe,j, both mapped distal to Yd2. The protein product of the gene at the XYIp locus will provide a convenient assay for the selection of Yd2 during the breeding of BYDV-resistant barley varie- ties. Key words Restriction fragment length polymorphism - Barley yellow dwarf- Resistance 9 Yd2 9 Morphological marker Introduction Barley yellow dwarf luteovirus (BYDV) infects all cereal types and causes significant yield losses worldwide. An- nual yield reductions due to BYDV average 1-3%, al- though losses are tenfold greater in some seasons (Burnett 1984). Symptoms include stunting and leaf yellowing (Os- wald and Houston 1953). BYDV encompasses a number Communicated by G. E. Hart N. C. Collins 9 N. G. Paltridge 9 C. M. Ford 9 R. H. Symons (~) Department of Plant Science, Waite Institute, University of Adelaide, Glen Osmond, S.A. 5064, Australia of virus types within the Luteovirus group, which have been characterised on the basis of aphid vector specificity, serology, ultrastructural symptomology and molecular bi- ology (Rochow 1979; Gill and Chong 1979; Miller et al. 1995). The most effective means of minimising BYDV-in- duced yield loss in barley (Hordeum vulgate L.) has been the Yd2 gene, which has been incorporated into at least seven barley cultivars (Schaller 1984). The Yd2 gene was discovered as a result of a BYDV resistance screen of 6689 barley accessions from a worldwide collection, in which over 100 Ethiopian accessions showed the mildest BYDV symptoms (Schaller et al. 1963). In all of the Ethiopian lines analysed genetically, the resistance was shown to be due to a single locus (Schaller 1964; Rasmussen and Schaller 1959; Damsteegt and Bruehl 1964). Further ge- netic studies by Catherall et al. (1970) demonstrated the existence of a number of Yd2 resistance alleles. In these studies, Yd2 alleles from different Ethiopian accessions ex- pressed different degrees of resistance and dominance when bred into common genetic backgrounds. The effec- tiveness of the Yd2 gene also varies against different types and isolates of BYDV. For example, all BYDV-PAV iso- lates, but not all isolates of BYDV-RPV or BYDV-MAV, appear to be affected by Yd2 (Banks et al. 1992; Skaria et al. 1985; Herrera and Plumb 1989). ELISA studies have shown that gd2 significantly reduces the rate of virus ac- cumulation of affected BYDV types in plant tissue (Banks et al. 1992; Skaria et al. 1985). However, the mechanism by which Yd2 limits BYDV replication remains to be elu- cidated. In our laboratory, we aim to isolate the Yd2 gene from barley in order to characterise the molecular basis of the BYDV-Yd2 interaction. As there is no known way of iden- tifying the gd2 protein product, we adopted a map-based approach to isolating Yd2. Schaller et al. (1964) had al- ready located Yd2 to the long arm of barley chromosome 3 using morphological markers. In this study, we have created a more detailed map of the Yd2 region of barley chromosome 3, as a first step toward isolating the Yd2 gene. The map described here contains 19 restriction fragment