High-Resolution Mapping of the Hor1/Mla/Hor2 Region on Chromosome 5S in Barley. Richard A. DeScenzo. (1) USDA-Agricultural Research Service, Field Crops Research. (2) Department of Plant Pathology; Roger P. Wise (1,2,3), and Mamatha Mahadevappa (2,3). (1) USDA-Agricultural Research Service, Field Crops Research; (2) Department of Plant Pathology, and (3) Interdepartmental Genetics Program, Iowa State University, Ames 50011 U.S.A. MPMI 7:657-665. Accepted 25 May 1994. Copyright 1994 The American Phytopathological Society.

A high-resolution mapping population consisting of 270 individual lines, each representing an independent recombination event between the Horl and Hor2 loci, was used to construct a high-density restriction fragment length polymorphism (RFLP) map of the Horl/Mla/Hor2 region. To identify informative markers for screening the recom-binant population, the Franger (containing Mla6 and Mlal4) and Rupee (containing Mlal3) parental lines were screened for RFLPs using 12 restriction endonucleases and 35 cDNA and genomic clones, that had been shown to map to barley chromosome 5 or wheat group 1. As a result, 61 probe/enzyme combinations were chosen to screen the recombinant population. Analysis of the RFLP mapping data indicated nine of the 21 polymorphic probes mapped within the Horl/Hor2 interval. Map distance between Horl and Hor2 was calculated to be approximately 8.1 cM. Nine of the remaining probes mapped outside of the Horl/Hor2 interval and proximal to Hor1. The remaining three probes mapped to separate linkage groups. Sixteen of the recombinant lines had regions of DNA that were heterozygous at one or more RFLP loci within the Horl/Hor2 interval. Data from the high-resolution RFLP map of the Horl/Hor2 region was integrated with previous recombination data which positioned the Mla6, Mlal3, and Mlal4, resistance alleles, in relation to Horl and Hor2. Five of the probes hybridized to multiple sites. BCD249, a cDNA-derived barley clone, hybridized to two sequences 0.3 cM apart, indicating a duplication event within the Hor1/Hor2 interval. One of these sites, Xbcd249.2, was positioned within 1.6 cM from the Mla alleles.