IV.2 Coordinator's report: Chromosome 5.
Jens Jensen, Agricultural Research Department, Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark.
Allelism of the bra-c1 (bracteatum) gene with the trd gene was reported by Tsuchiya (1974).
The linkage relations between several genes on chromosome 5 were investigated by Wiberg (1974) in a comprehensive study of powdery mildew. The F2 or F3 population for the gene combinations: Ml-at with Ml-a, Ml-p, at, B, and trd; Ml-a with Ml-p, at, B, and trd; and Ml-p with at, B, and trd were not large enough to demonstrate the known cases of linkage with statistical significance, and the recombination percentages were - unfortunately - not given. Hence, these extensive studies do not provide data that can be used to improve the linkage map. In several varieties with a resistance gene at the Ml-a locus, Wiberg found an additional Ml- gene, which in most cases showed a recombination with the Ml-a locus of 6 to 8 percent; in other cases the additional Ml-gene was farther away from Ml-a. Further, Wiberg presented evidence indicating that the Ml-a4 gene, which is assigned to the Ml-a locus, probably belongs to the Ml-k locus.
In the past year I have found no other published linkage data between loci on chromosome 5. However, Dr. R. T. Ramage has kindly provided me with linkage data from an unpublished three-point test, showing that the order of the gene involved is trd, B, and at. The recombination percentages obtained were: trd, B 14.4 + 1.1, B, at 33.7 + 1.8, and trd, at 44.0 + 2.0. The segregations expected at these recombination percentages differed significantly from those observed. The deviating segregations had not apparently influenced the recombination estimates significantly since they fit the map.
By adding the linkage data from Ramage to those available when preparing the last coordinator's report (BGN 4: 102-106), the present linkage map for chromosome 5 appears as shown in Figure 1. The map was estimated by the procedure described by Jensen (1974) and Jensen and Jørgensen (1975a, 1975b). In estimating the map 50 of the 61 available linkage data are used; the remaining 11 data had to be omitted due to their inconsistency with the other data. The main changes on the map are the moving of trd and ea-k to below B, and the inclusion of the f7 (chlorina) gene. As some of the linkage data used for the map estimation may be unreliable, the order of some of the loci - most likely Ml-at, ml-d, Ml-p and f7 - may be incorrect. Further, the order of some closely linked loci may be doubtful; this is seen from the relatively large standard deviation of the estimates of the intervening distances.
References:
Jensen, J. 1974. Genetic linkage map of barley chromosome 5. I. A procedure to estimate linkage maps applied to literature data. BGN 4: 40-42.
Jensen, J. and J6rgensen, J. Helms. 1975a. The barley chromosome 5 linkage map. I. Literature survey and map estimation procedure. Hereditas (in press).
Jensen, J. and Jørgensen, J. Helms. 1975b. The barley chromosome 5 linkage map. II. Extension of the map with four loci. Ibid. (in press).
Tsuchiya, T. 1974. Further results of allelism testing in barley. BGN 4: 82-85.
Weiberg, A. 1974. Genetical studies of spontaneous sources of resistance to powdery mildew in barley. Hereditas 77: 89-148.
