BGN 12: Attempts to locate powdery mildew resistance gene Ml-(La) to a barley chromosome BARLEY GENETICS NEWSLETTER, VOL. 12, II. RESEARCH NOTES
Jensen et al., pp. 65-68

II. 23. Attempts to locate powdery mildew resistance gene Ml-(La) to a barley chromosome.

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

One of the important donors of powdery mildew resistance for northwest European spring barley (Jørgensen 1982) is the so-called Laevigatum-resistance. The resistance is conditioned by one apparently dominant gene tentatively designated Ml-(La) conferring an infection type 2-3nc or 3nc (moderately susceptible) on seedlings (Torp et al. 1978). In the present report we describe our attempts to locate gene Ml-(La) to chromosome using five high-lysine genes and several translocations as markers.

The five high-lysine mutants, Risø Nos. 13, 16, 17, 527, and 1508, induced in the variety 'Bomi' that has gene Ml-(La) were crossed with translocation lines induced in the variety 'Bonus'. The crosses were made for locating the high-lysine genes in the five mutants. In order to make the linkage tests efficient, only shrivelled F2 seeds (seeds homozygous for a high-lysine gene are shrivelled) were sown in the field. All the high-lysine mutants have each one recessive high-lysine gene; the genes in mutants 13 and 527 are on chromosome 6, those in mutant 16 and 17 are on chromosome 1, and that in mutant 1508 is on chromosome 7 (Jensen 1979).

A number of cross combinations from the above F2 material were used for locating gene Ml-(La). The F2 plants were progeny tested as F3 seedlings with powdery mildew culture CR3 that is avirulent on the 'Bomi' mutants and virulent on the 'Bonus' translocation lines.

Some of the F2 plants listed in Table 1 were used to check the inheritance of the resistance. Those underlined were not used because a possible linkage between Ml-(La) and one of the high-lysine genes, either directly or indirectly via a translocation, may cause a deviation from a Mendelian segregation for Ml-(La). Therefore, only the not-underlined 1225 F2 plants in Table 1 were used. Assuming that the resistance is conferred by one gene these F2 plants will segregate in a 1:2:1 ratio of homozygous resistant, heterozygotes, and homozygous susceptible, respectively. the observed segregation was 347:595:283. A X2 of 7.69 significant at the 5 per cent level indicated some deviation from the expected ratio. The deviation is probably caused by difficulties in scoring the F seedlings because the two infection types, 2-3 or 3 and 4, are fairly close to each other, and because gene Ml-(La) may be incompletely dominant. However, considering this, we regard the observed segregation as a strong indication for the 'Laevigatum'-resistance in 'Bomi' to be conferred by a single gene.

Table 1. The cross combinations and the number of F2 plants analyzed for reaction to powdery mildew.

X2 tests for independent inheritance of gene Ml-(La) and the five high-lysine genes on chromosomes 1, 6, and 7, respectively, did not suggest any linkage.

The joint F2 segregation for gene Ml-(La) and for partial sterility vs. fertility due to the translocations are shown in Table 2. The data do not suggest any linkage between Ml-(La) and the 13 translocation breakpoints, not even when the data per chromosome are combined.

Table 2. The joint F2 segregation for partial sterility vs. fertility and for powdery mildew resistance gene Ml-(La) (A vs. a).

If the translocation breakpoints are in or near a centromere, and when the actual number of F2 plants is considered, it can be estimated that gene Ml-(La) probably is located more than about 30 centimorgans from a centromere. However, 1) all the translocation breakpoints are not at or near a centromere, and 2) the underlined numbers of F2 plants in Table 1 showed linkage between the breakpoints and the high-lysine genes. Both these factors contribute to increasing the length on the linkage map on which gene Ml-(La) is probably not located. A conservative estimate suggests that gene Ml-(La) is probably located more than about 50 centimorgans away on either side of the centromere.

References:
Jensen, J. 1979. Chromosomal location of one dominant and four recessive high-lysine genes in barley mutants. In: Seed Protein Improvement in Cereals and Grain Legumes. I. IAEA, Vienna, 1979, p. 89-96.

Jørgensen, J. Helms. Durability of barley powdery mildew resistance genes in Denmark 1963-1980. In: "Durable Resistance in Crops". Proc. NAT0 Symp., Italy, 30 Sept. - 11 Oct. 1981 (in press).

Torp, J., H. P. Jensen and J. Helms Jørgensen. 1978. Powdery mildew resistance genes in 106 northwest European spring barley varieties. Kgl. Vet. - og Landbohøhsk. Arsskrift 1978, 75-102.

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