II.17. Locating some genes on barley chromosome 5.
Jens Jensen and J. H. Jørgensen. Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark.
Further investigations have been made on a mutant line, (No. 10), with necrotic spotted leaves. The spots are caused by a single recessive gene (proposed symbol nec1a) located on chromosome 5 (Jensen 1971). In addition the mutant line has a recessive gene, (proposed symbol wst5) giving white streak on the plants in the early developmental stage.
The mutant line was crossed with the variety Nigrinudum, CI 11549, carrying the genes B (black) and JMLnn (powdery mildew resistance) (Hiura 1960) on chromosome 5. Black (B) was classified for in the F2 while the other characters were classified for on F3 seedlings for F2 genotypes. Powdery mildew resistance was classified by inoculating with culture 67.62 of the powdery mildew fungus. The powdery mildew resistance in our material was most probably conferred by gene JMLnn, but to avoid a possible mistake and to indicate the likely identity, we have denoted the resistance gene classified for in our material as MLnn.
The F2 segregation observed was:
No significant x2 deviation from single gene segregation was found for any of the genes.
The table above shows the significance level of x2 for independent inheritance, the recombination percent calculated by the maximum likelihood method, and the crossing-over percent = centimorgans (cM) derived from the Kosambi mapping function (Jensen 1973). From the table it is clear that all four genes are situated on chromosome 5. Apart from an about 5% chance that gene wst5 is placed to the left of B, the most probable gene order is: B, wst5, nec1a, MLnn.
Studies with translocations (Jensen 1971) indicate that nec1a is situated comparatively close to the centromere. Some of our preliminary data show 18% recombination or 19 cM between nec1a and that gene fs2 (fragile stem 2) and the distance between B and fs2 was found to be 45% recombination (equalling 72 cM) by Takahashi et al. (1966). Therefore, gene nec1a is most likely situated to the left of fs2, and as fs2 is situated on the new long arm of chromosome 5 (Tsuchiya 1972), fs2 should be even closer to the centromere than nec1a. Thus the following gene map may be set up (distances in cM).
The nec1a gene has the quality required for a 0 position gene (see Jensen 1973). The fs2 gene, also a candidate as 0 position gene, seems to be nearer the centromere than nec1a, but it is not so easily classified for and is not so viable. - Even if fs2 is nearest the centromere, nec1a may be situated nearer the centre of the gene map. - Therefore, the nec1a is recommended as 0 position gene on chromosome 5 instead of fs2. At present the other known genes on chromosome 5, do not possess the qualities required for a 0 position gene.
References:
Hiura, U. 1960. Studies on disease-resistance in barley. IV. Genetics of the resistance to powdery mildew. Ber. Ohara Inst. Landw. Biol. 11:235-300.
Jensen, J. 1971. Mapping of the 10 mutant genes for necrotic spotting in barley by means of translocations. Barley Genetics II:213-219.
Jensen, J. 1973. Proposal for a new procedure for the preparation of gene maps in barley. Barley Genetics Newsletter 3:
Takahashi, R., J. Hayashi, and U. Hiura. 1966. Inheritance and linkage studies in barley. III. Linkage of the gene for fragile stem-2 and orientation of linkage map on barley chromosome 5. Ber. Ohara Inst. landw. Biol. 13:199-212.
Tsuchiya, T. 1972. Cytogenetics of telotrisomics in barley. Barley Genetics Newsletter 2:93-98.
