Orienting Linkage Maps on the Chromosomes of Barley 1

CYTOGENETIC studies with barley, Hordeum vulgate, have now progressed to the point where it is possible to assign a linkage group to each of the seven chromosome pairs. The extensive genetic data have been summarized at regular intervals (q. v. Robertson et al., 13) and seven linkage groups established. The chromosome morphology has been established from root-tip mitoses by Tjio and Levan (15) and the five nonsatellite chromosomes numbered from 1 to 5 in the order of their decreasing lengths. Of the two satellite chromosomes, the shorter chromosome with the larger satellite is designated as 6 while chromosome 7 is the longer chromosome with a shorter satellite. Sarvella et al. (14) were able to study meiotic pachytene chromosomes in a radiation induced "short" chromosome strain but it has not been established that this strain 6arries the normal chromatin arrangement. To correlate .the genetic and cytologic information, Burnham et al. (3) isolated a series of homozygous translocations from X-rayed seed of the variety Mars. By intercrossing translocation stocks, and observing meiotic configurations in F1 plants, they assigned temporary letters a to g to the chromosome pending identification in relation to morphology. Utilizing the fact .that in certain cases the karyotypes of homozygous translocations differed sufficiently from normal to permit identification of the chromosome involved, Hagberg and Tjio (4) and Burnham and Hagberg (2) have established the correspondence between the letter designations and the morphological designations. Kramer et al. (10), using translocation-gene linkages and F, meiotic configurations of translocation intercrosses, concluded that two linkage groups were carried by the same chromosome leaving the g chromosome devoid of assigned genes. They suggested the tentative relationships of the five other linkage groups to the letter designations of Burnham. These conclusions were substantiated by Haus (8) using genetic tests, and by Tsuchiya (16) using associations of linkage groups with trisomics. Recently Swomley,a Ramage and Suneson (12), and Burnham (unpublished) have placed genes on .the remaining chromosome thus finally establishing a linkage group for each chromosome, as given by Ramage t al. (11). All the relationships may be summarized as in Table 1. It is now possible to designate the morphological chromosomes, the chromosomes involved in interchanges, the linkage groups, and the trisomic .types with the same Arabic numerals. A major cytogenetic problem remaining is the orientation of each linkage map on its chromosome with an end