II.17 New linkage data with the ml-o locus on barley chromosome 4.
J. Helms Jørgensen and H.P. Jensen, Agricultural Research Department, Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark.
The ml-o locus has a series of non-complementing alleles designated ml-o 1 throuzh ml-o 11 {or Reg 6 a through Reg 6 k) (Jørgensen, 1975) conditioning resistance to the powdery mildew fungus. Linkage data from two-point tests (Jørgensen, 1974) have shown that the ml-o locus is located distally on the long arm of the chromosome-4 linkage map.
In order to locate ml-o more precisely, three crosses were made between marker stocks each possessing one or two of the seven chromosome-4 genes: K (hooded awn), cer-j (eceriferum), min (minute plant), yh (yellow head), br2 (brachytic), Ml-g and ml-o (powdery mildew reaction). The characters conferred by K, cer-j, min and yh were scored on F2 plants in the field; those conferred by the other three on F3 seedlings in the greenhouse. Two powdery mildew cultures were applied, 59.11 which is virulent on Ml-g and avirulent on ml-o, and A-6 (290), which is avirulent on Ml-g and ml-o. The data were analysed as described by Jensen and Jørgensen (1975).
The results from cross no. 1, a four-point test analysed as two threepoint tests, showed that the most likely order of the loci is, respectively, K, Ml-g, br2 and ml-g, br2, ml-o, and - when the data are combined - K, Ml-g, br2, ml-o. In cross no. 2, also a four-point test, gene min failed to show linkage with the other three genes. The data were, therefore, analysed as one three-point test, which suggested the following order of the three loci: K, cer-j, ml-o, and as three two-point tests involving min. Cross no. 3, a threepoint test, indicated that yh is located between br2 and ml-o; the data do not preclude, however, that the order of the three loci is: br2, ml-o, yh.
The present linkage data for the seven genes involved and those from the previous study (Jørgensen, 1974) involving genes f9 (chlorina), K, Ml-g, br2 and ml-o were combined using the maximum likelihood method for estimating linkage maps (cf. Jensen and Jørgensen, 1975). Gene min had to be omitted because it failed to show linkage, and genes yh and cer-j had to be omitted because their linkage relations were inconsistent with the other data. The resulting linkage map for the remaining five loci is as follows (distances in cM):
In summary, the present data have increased the precision of the relative positions of K, Ml-g, br2 and ml-o. They also suggest that yh is located between br2 and ml-o, that cer-j is located between K and ml-o, and that min appears not to be on the long arm of chromosome 4.
References:
Jensen, J. and J. Helms Jørgensen, 1975: The barley chromosome 5 linkage map. II. Extension of the map with four loci. Hereditas 80: 17-26.
Jørgensen, J. Helms, 1974: Location of the ml-o locus on barley chromosome 4. BGN 4: 43-44.
Jørgensen, J. Helms, 1975: Identification of powdery mildew resistant mutants and their allelic relationship In: Proceedings 3rd International Barley Genetics Symposium (in press).
