II. 8. Two segmental tetrasomics that constitute chromosome 5 of barley.
M.D. Sheedy and R.T. Ramage. Plant Science Department, University of Arizona, Tucson, Arizona 85721, U.S.A.
Various kinds of trisomics have been used effectively in associating genes with the chromosomes and chromosome segments upon which they are carried (Tsuchiya, 1983). If a gene under investigation is on the extra chromosome, it will show a trisomic F2 ratio, whereas if it is on one of the other chromosomes, it will show a disomic ratio. The exact trisomic ratio is dependent upon the F1 genotype, the type of chromosome segregation and the transmission rate of the extra chromosome. There is usually reduced transmission of the extra chromosome through both the eggs and the pollen and especially through the pollen. Consequently, the trisomic parent is almost always used as the female in crosses. In using trisomics to assign genes to chromosomes, a considerable amount of cytology is required to identify both female parents and F1 plants.
Tetrasomics can greatly facilitate the use of trisomics in assigning genes to chromosomes; they serve as a source of trisomic individuals. Tetrasomics can be used as either the male or female parent in crosses and, in either case, all the F1's should be trisomic. The amount of cytology needed to identfy trisomic F1 plants would be greatly reduced. Male transmission of the extra chromosome in F1 trisomic plants will cause additional distortion of F2 ratios making it possible to determine trisomic segregation with smaller numbers of F2 plants.
We have established two segmental tetrasomic lines that, between them, should represent all of chromosome 5 and an undetermined amount of chromosome 7. Any gene located on chromosome 5 should show a trisomic ratio with one or the other of these two tetrasomics. The extra chromosomes in the two lines originated from the two pairs of interdependent chromosomes described by Wiebe (1976).
Wiebe (1976) and Wiebe et al. (1974) described 8-paired barley lines that were derived from a balanced tertiary trisomic that had an extra chromosome that came from the translocation T5-7a. In one 8-paired line, a translocation between the extra chromosome and the normal chromosome 5 apparently occurred resulting in two pairs of interdependent chromosomes. The karyotypes of the chromosomes involved in the evolution of these 8-paired barleys were reviewed by Sheedy and Ramage (1984).
Ries (1982) studied the cytological and breeding behavior of the 8-paired barleys that contained the two pairs of interdependent chromosomes. In the F2 of crosses between 8-paired plants and normal 7-paired diploids, he observed about 3 percent trisomics for one or the other of the fragment chromosomes.
We are using the two kinds of trisomics in an attempt to identify that part of chromosome 5 carried by each fragment chromosome. Selfing the fragment trisomics has resulted in lines with an extra pair of fragment chromosomes, i.e., tetrasomic lines. The two fragment chromosomes differ in total length and centromere position and can be readily distinguished. We have obtained several plants of each of the two kinds of segmental tetrasomics. Both kinds are reasonably stable and appear to be relatively vigorous and fertile.
References:
Ries, M.N. 1982. Heteropoloidy and chromosome interdependence in barley; cytological and breeding behavior of an eight paired line. Ph.D. Dissertation, University of Arizona 70 pp.
Sheedy, M.D. and R.T. Ramage. 1984. Karyotypes of chromosomes involved in the evolution of 8-paired barleys. Barley Newsletter 27:42.
Tsuchiya, T. 1983. Aneuploids and chromosome mapping in barley. In: Cytogenetics of Crop Plants, MacMillan, India pp. 251-281.
Wiebe, G.A. 1976. Eight-paired barley. Barley Genetics III:242-246.
Wiebe, G.A., R.T. Ramage and R.F. Eslick. 1974. Eight-paired barley
lines. BGN 4:93-95.