Allelism of Genes in the Ml-a locus

Seven barley lines or varieties, each with a different gene at the Ml-a locus for resistance to Erysiphe graminis were intercrossed. Progeny testing of the F2s using two different fungal isolates per cross provided evidence that there are two or more loci in the Ml-a region. Apparent recombinants were also screened for recombination between the Hor1 and Hor2 loci which are situated either side of the Ml-a locus. The cross between Ricardo and Iso42R (Rupee) yielded one possible recombinant, with Ml-a3 and Ml-a(Rul) in the coupling phase; other recombinants had wild-type genes in the coupling phase. Iso20R, derived from Hordeum spontaneum 'H204', carrying Ml-a6, had an additional gene, in close coupling with Ml-a6, tentatively named Ml-aSp2 or Reglv, causing an intermediate infection type with isolate EmA30. It is suggested that Ml-a(Ar) in Emir and Ml-a(Rul), shown to differ from other Ml-a genes and to be closely linked with the Ml-a locus, be called Ml-a12 or Reglt12 and Ml-a13 or Reglu13 respectively.ADDITIONAL ABSTRACT:Seven barley lines or vars., each with a different gene at the Ml-a locus for resistance to Erysiphe graminis, were intercrossed. Progeny testing of the F2s using 2 different fungal isolates/cross indicated that there are 2 or more loci in the Ml-a region. Apparent recombinants were also screened for recombination between Hor1 and Hor2 loci situated either side of Ml-a.