BGN 10: Allelism of genes in the Ml-a locus BARLEY GENETICS NEWSLETTER, VOL. 10, II. RESEARCH NOTES
Giese et al., pp. 22-24

II. 9. Allelism of genes in the Ml-a locus.

H. Giese, H. P. Jensen and J. Helms Jørgensen, Agricultural Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark. "R"

A large number of powdery mildew resistance genes have been assigned to the Ml-a locus on the short arm of chromosome 5 in barley.

The following seven barley varieties/lines each possessing one resistance gene in the Ml-a locus were selected and intercrossed.

The allelism of the seven genes stated above were examined by progeny testing the individual F2 plants. A full classification of all nine F2 genotypes was obtained by progeny testing each cross combination twice using two different powdery mildew isolates (cf. Table 1).

Table 1. Results from tests of allelism on the Ml-a locus.

Apparent recombinant F2 plants were retested with the appropriate powdery mildew isolates. In addition they were screened for their hordein banding patterns; the loci Horl and Hor2 are located on either side of the Ml-a locus (cf. Jensen 1980), and thus recombination between two Ml-a genes should also give recombination between the two hordein loci. Most of the barley varieties/lines used have different genes in the Horl and Hor2 loci producing banding patterns sufficiently different to detect recombination. As a final test individual apparent recombinant F3 plantlets are grown to maturity and F4 plants will be powdery mildew tested.

In four crosses possible recombinant plants were obtained (Table 1). The recombination percentages were estimated by the method of maximum likelihood. For crosses in which no recombinant plants were obtained, the upper limit of recombination at the 5% significance level was calculated. One possible recombinant plant in the cross 'Ricardo' x Iso 20R has not yet been verified in the F3 retest.

The cross between 'Ricardo' and Iso 42R provided one possible recombinant plant with the genes Ml-a3 and Ml-a(Rul) in coupling phase. All other recombinant plants found had wild type genes in coupling phase.

The barley line Iso 20R, derived from H. spontaneum line H 204 with the gene Ml-a6, was found to have an additional gene producing an intermediate infection type with the isolate Em A30. This gene is in close coupling to Ml-a6 and has been tentatively named Ml-a(Sp2) or Reglv.

The resistance spectra conditioned by the genes Ml-a to Ml-all were determined with 11 powdery mildew isolates. All spectra of resistance were distinctly different. The genes Ml-a(Ar) and Ml-a(Ru1) are thus different from any of the previously named Ml-a genes. The tests of allelism have shown that they are in or very close linked to the Ml-a locus and it is suggested that they be called Ml-a12 or Reglt12 and Ml-a13 or Reglu13, respectively.

The results from the present studies provide evidence for the presence of two or more loci in the Ml-a region. These findings are in accordance with suggestions by Favret (1969) and Moseman and Jørgensen (1972) that the Ml-a locus probably consists of several closely linked sites or loci.

References:

Favret, E. A. 1969. The host-pathogen system and its genetic relationships. Barley Genetics II. (Proc. 2nd Int. Barley Genet. Symp., Pullman, Washington), p. 457-471.

Jensen, J. 1980. Coordinator's report: Chromosome 5. Barley Genet. Newsletter 10:

Moseman, J. and J. H. Jørgensen. 1973. Differentiations of resistance genes at the Ml-a locus in six pairs of isogenic barley lines. Euphytica 22:189-196.

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