II.19. Location of the ml-o locus on barley chromosome 4.
J. Helms Jørgensen, Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark.
The ml-o locus has a series of recessive, spontaneous, and induced alleles conferring resistance to the powdery mildew fungus (Erysiphe graminis f. sp. hordei). It failed to show a statistically significant linkage with three powdery mildew resistance loci on chromosome 5 and with the powdery mildew resistance locus Ml-g (Jørgensen, 1971a), which is on the long arm of chromosome 4. However, crosses with trisomics showed that ml-o was on chromosome 4, and it was suggested that ml-o was on the short arm of this chromosome (Jørgensen, 1971b).
In order to locate ml-o more precisely, six crosses were made between the five lines having the five genes f9 (chlorina), K (hooded awn), br2 (brachytic), Ml-g, and ml-o. By scoring F2 plants for K and F2 plant progenies for f9 br2 Ml-g, and ml-o, the following recombination percentages were obtained.
These observed recombination percentages (1) were transformed into map distances (given in centimorgans, cM) (2) by the Kosambi mapping function. Then estimates of all the map distances between the five loci were calculated by a maximum liklihood method (Jensen and Jørgensen, 1974) which utilized jointly the information from all six crosses. The map distances thus estimated are given above (3) together with the distances transformed into recombination percentages (4).
The reverse order of the loci br 2 and Ml-g is also likely according to the data, but less likely than that given above. The data show that the ml-o locus is located distally on the long arm of the chromosome 4 linkage map (in BGN Vol. 3, 1973).
References:
Jensen, J., and J. Helms Jørgensen. 1974. The barley chromosome 5 linkage map. I. Literature survey and map estimation procedure. - In preparation.
Jørgensen, J. Helms. 1971a. Studies on induced and spontaneous mildew resistance genes at the ml-o locus in barley (Abstract). Hereditas 69:298.
Jørgensen, J. Helms. 1971b. Comparison of induced mutant genes with spontaneous genes in barley conditioning resistance to powdery mildew. In: Mutation Breeding for Disease Resistance (Proc. Panel, Vienna, 1970), IAEA, Vienna: 117-124.
