Jensen and Jørgensen, pp. 29-31

II.6. Gene mapping on chromosome 5.

Jens Jensen and J. Helms Jørgensen. Danish Atomic Energy Commission, Research Establishment Risø, DK-4000 Roskilde, Denmark.

A few years ago we began to work on the improvement of the gene map of chromosome 5. Our results are still incomplete but some preliminary data are given below The best way in which to overcome a possible environmental and genetical effect on the recombination frequency when mapping genes is by using threepoint tests and since threepoint tests are not commonly used in barley the results of two threepoint tests are reported here:

Common to both crosses is the parent "erectoides-b", a dense spike mutant of the variety Gold, which has the barley leaf rust resistance gene Pa4. The other parent in cross 1 is the variety Kwan, which has the powdery mildew resistance gene Mla. The other parent in cross 2 is the variety Algerian, which has the powdery mildew resistance gene Mla.

The classification for erectoides was made on field grown F2 plants by measuring the internode length of their spikes, whereas the classification for resistance to powdery mildew and leaf rust was made on F3 seedlings in the greenhouse.

The F2 segregations obtained were as follows:

Assuming equal recombination frequency on the female and on the male side the following maximum-likelihood estimates of the recombination frequencies have been found.

Each of the two crosses establish a definite gene order. Genes Mlk and Mla are located between genes ert-b and Pa4. The combined data from the two crosses suggest the gene order to be: ert-b, Mla, Mlk, Pa4.

The data indicate interference. In cross 1 the coincidence coefficient was estimated as 0.71 + 0.15 and in cross 2 as 0.56 + 0.16 by the maximum likelihood method. This suggests that crossing over in one chromosome segment is reducing the crossing over frequency in the neighboring segment. However, in another threepoint test involving a different part of chromosome 5 no interference was found.

From the two crosses described above and from several other crosses, multiple marker gene lines will be picked up and made available when retested and multiplied.

The erectoides gene ert-b is not always a good marker. In our material there was no clear cut difference between the erectoides and the wild type phenotypes, but a certain degree of overlapping. Genes Pa4, Mlk and Mla were useful markers in our material because we had the appropriate pathogen cultures and the greenhouse facilities for the disease screening. In contrast to many other marker genes, disease resistance genes do not reduce the vitality of the plant. Therefore, they can be used in multiple marker stocks without difficulties.

The analysis of data from a threepoint test is more complicated than the analysis of "normal" F2 data. The combined estimation of the three recombination frequencies by the maximum-likelihood method requires extensive calculations so that a computer is considered a necessity.

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