Abstract
The ml-o locus in barley confers resistance to all known races of the fungus Erysiphe graminis f.sp. hordei. Since the molecular mechanisms underlying ml-o-mediated resistance are currently undefined, experiments have been initiated to isolate the gene by means of its map position. A collection of backcross lines containing ml-o alleles derived from six barley genotypes allowed us to identify a set of DNA markers very tightly linked to the resistance locus. These markers span an unexpectedly small segment of 8.6 centimorgans on chromosome 4 that includes the Ml-o locus. Two of the markers cosegregate with the resistance locus on the basis of 44 homozygous resistant plants identified within a segregating F2 population derived from an intravarietal cross. Colinearity of the resistance-linked markers was confirmed in an F2 mapping population derived from a wide cross between Hordeum vulgare subsp. vulgare and Hordeum vulgare subsp. spontaneum. The two markers cosegregating with the resistance locus in the former cross define in the latter cross an interval of 2.4 centimorgans within which Ml-o is most probably situated. The set of linked markers opens up the possibility of carrying out a bidirectional chromosomal walk or jump to the gene.