Physical mapping of the barley stem rust resistance gene rpg4

The barley stem rust resistance gene rpg4 was physically and genetically lo calized on two overlapping BAC clones covering an estimated 300-kb region o f the long arm of barley chromosome 7(5H). Initially, our target was mapped within a 6.0-cM region between the previously described flanking markers M WG740 and ABG391. This region was then saturated by integrating new markers from several existing barley and rice maps and by using BAC libraries of b arley cv. Morex and rice cv. Nipponbare. Physical/genetic distances in the vicinity of rpg4 were found to be 1.0 Mb/cM, which is lower than the averag e for barley (4 Mb/cM) and lower than that determined by translocation brea kpoint mapping (1.8 Mb/cM). Synteny at high resolution levels has been esta blished between the region of barley chromosome 7(5H) containing the rpg4 l ocus and the subtelomeric region of rice chromosome 3 between markers S1647 4 and E10757. This 1.7-cM segment of the rice genome was covered by two ove rlapping BAC clones, about 250 kb of total length. In barley the markers S1 6474 and E10757 genetically delimit rpg4, lying 0.6 cM distal and 0.4 cM pr oximal to the locus, respectively.