GrainGenes

II. 20. Inter-allelic recombination in the ml-o locus in barley.

J. Helms Jørgensen and H.P. Jensen, Risø National Laboratory, DK-4000 Roskilde, Denmark.

Many mutagen-induced powdery mildew resistance genes in barley are recessive and functionally allelic in the ml-o (or Reg6) locus. Past experience suggests that they are similar in function because they:

1. Confer the same resistance-spectrum,
2. Condition the same infection type,
3. Affect the primary development of the pathogen at the same stage and with the same intensity,
4. Confer the same pleiotropic effects.

The structural allelism of the following six ml-o genes was studied by recombination.

The six barley lines were selfpollinated and were intercrossed. From 13,400 to 36,900 seedlings from each of the selfed mutants and from F2 of each cross-combination were tested with powdery mildew culture A₆(290).

Among the 120,728 seedlings from the selfings were three susceptible revertants, and among the 349,401 F2 seedlings were 76 susceptible revertants. A unique genetic marker served to eliminate contaminants in the material, and appropriate crosses with the revertants excluded that suppressor genes were present. Therefore, it is safe to assume that the three revertants from the selfings result from back-mutation. Further, since the 76 revertants derived from the F1 plants appeared with a frequency nearly 10 times the revertant frequency from selfings, it is safe to assume that they result predominantly from recombination (and conversion). The recombination frequencies are estimated assuming that only half of the recombinants are susceptible, i.e. those carrying a strand without ml-o genes (e.g., in the genotype +, +/ml-o 1, +). Recombinants carrying a strand with two ml-o genes in coupling (e.g., in the genotype ml-o 1, ml-o 5/ml-0 1, +) are thus assumed to be resistant.

The recombination estimates are shown in Figure 1. It is seen that:

1. The two X-ray induced genes, ml-0 1 and -4, have not recombined.

2. The three EMS induced genes, ml-o 9, -5, and -8, have not recombined.

3. These two groups recombined with each other and with the spontaneous gene ml-0 11.

The absence of recombinants within the first two groups indicates that the genes ml-o 1 and -4, and ml-o 9, -5, and -8 are in the same or in closely linked sites; the upper confidence limit for p = 95% is, however, between 0.008 and 0.015 per cent recombination.

The ml-o locus has apparently three sites. If the three sites are arranged in the most likely order, and the data are pooled, we get the recombination estimates between sites shown at the bottom of Figure 1; i.e., recombination percentages of about 0.01, 0.02, and 0.04. The structural differences between some of the ml-o genes suggest that they may also be functionally different; i.e., that they may be different resistance genes.

Figure 1. Recombination frequency x 10^-4 between six genes in the ml-o locus on barley chromosome 4.

References:

Jørgensen, J. Helms. 1976. Identification of powdery mildew resistant barley mutants and their allelic relationship. In: Barley Genetics III. Verlag Karl Thiemig, München, p. 446-455.

Jørgensen J. Helms 1977. Spectrum of resistance conferred by ml-o powdery mildew resistance genes in barley. Euphytica 26:55-62.

Jørgensen, J. Helms and K. Mortensen. 1977. Primary infection by Erysiphe graminis f. sp. hordei of barley mutants with resistance genes in the ml-o locus. Phytopathology 67:678-685.

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