GrainGenes

II.14 Indications for a necessary revision of the barley karyogramme.

G. Künzel and H. Nicoloff, Zentralinstitut für Genetik und Kulturpflanzenforschung der Akademie der Wissenschaften der DDR, Gatersleben (German Democratic Republic).

First exact measurements and descriptions of the somatic chromosomes of barley were published by Tjio and Hagberg (1951). The presently accepted designation of the chromosomes by Arabic numerals is mainly based upon these studies; numbers 1 to 5 designate the non-satellite chromosomes arranged in order of decreasing total length, 6 the chromosome with the larger satellite and 7 the chromosome with the smaller satellite.

Extensive studies of translocations in barley by C. R. Burnham and later on also A. Hagberg resulted in the establishment of the first tester set in the variety 'Mars' and a second one in the variety 'Bonus'. Both sets were brought into relation, and the letter designation of chromosomes originally used for the 'Mars' chromosome set was transformed into Arabic numerals. In addition to this, the correspondence to the morphologically characterized chromosomes of Tjio and Hagberg was ascertained by the analysis of lines homozygous for translocations sufficiently different from normal ones as to permit identification of the chromosomes involved. On the basis of these standardized translocation tester sets, translocations have been used to associate the seven linkage groups of barley with the morphologically identified chromosomes, and to identify the additional chromosomes of primary trisomics. Trisomics have also been used for associating linkage groups with the corresponding chromosomes. From these studies linkage group 1 is expected to be associated with the longest of the barley chromosomes, and groups 2 to 5 with chromosomes 2 to 5 in view of the gradual decrease in their length, resp.

First indication that chromosome 1 might not be the longest chromosome of the standard karyotype was recently presented by Tuleen (1973). He also reported estimates of the lengths of chromosomes 2 and 3 based upon karyotype analysis of multiple translocations in which the translocated chromosomes proved distinguishable from each other and from the remaining chromosome pairs. Some results with our own translocation lines recently gave quite similar indications.

The lines homozygous for translocations were first analyzed as to the chromosomes involved in the interchange, by means of a translocation tester set kindly placed at our disposal by A. Hagberg. The set consists of the lines T 1-5f, T 1-6e, T 2-3a, T 2-4c, and T 5-7b. Furthermore, translocation lines involving the same two interchanged chromosomes were intercrossed to confirm the results obtained with the tester set. For measurements of somatic chromosomes, root tips treated with 0.0125% colchicine in a saturated solution of 1-bromonaphtalene for about 2 hrs. at 24°C were fixed in Clarke, hydrolyzed and stained in Schiff's solution. Root tips were squashed for microscopic examination and well spread metaphases inspected. The chromosomes were measured from enlarged photographs. In order to eliminate deviations due to different contraction of individual metaphases, each of the chromosome arms was expressed in percent of the total of the lengths of all chromosomes measured in the same cell. Furthermore, the length of the long arm of chromosome 6 was used as a standard for comparing our measurements with those reported by Tjio and Hagberg (1951) and Tuleen (1973). For this purpose, the relative lengths were multiplied by a constant, so that the length of the long arm of chromosome 6 would be 62.1 units, i.e. the same estimate as Hagberg's.

As is well known, only three of the seven chromosome pairs - 5, 6 and 7 - are easily recognizable in somatic metaphases. Moreover, only small differences in the total lengths and in the arm ratios exist between chromosomes 1 to 4, so that considerable difficulties arise in determining the pair relationships even in good metaphases. One possibility to overcome these difficulties is the use of translocation lines in which all of the chromosomes are easily interdistinguishable. This method was applied by Tuleen (1973) and we also have used it. In a stock homozygous for the translocations T 26 (2-7) and T 14 (3-4), chromosome 1 can easily be distinguished from all other chromosomes. The relative mean arm lengths of chromosome 1 (Table 1) are based upon measurements of 20 metaphases from the mentioned line homozygous for two translocations.

Table 1. The relative arm lengths of the chromosomes of barley.

For estimating the relative arm lengths of chromosome 2, translocations were used in which one of the two chromosomes involved is chromosome 2 and the other either chromosome 5, 6, or 7, i.e., a chromosome easily recognizable in the standard karyotype. Out of these translocations only those were considered in which both chromosomes involved could reliably be distinguished from all others.

This, e.g., is the case with our translocation line T 515 (2-5). The relative mean lengths of the individual arms of the translocated chromosomes (2⁵ and 5²) were determined by measurements of 5 metaphases. As to the ratio between the respective chromosome arms, the relative lengths of both translocated chromosomes were summed up; from this sum the relative length of chromosome 5, known from standard karyotype analyses was subtracted. In this way, estimates of the relative arm lengths of chromosome 2 were obtained. In addition, four other translocations involving chromosome 2 and chromosome 6 or 7, resp., were also used for length estimations of chromosome 2. The mean values in Table l are based on the estimates obtained from these five translocation lines. In a similar way, i.e., by use of four translocations involving chromosomes 3 and 7, the relative arm lengths for chromosome 3 were estimated. For chromosome 4 five translocations involving chromosomes 5 or 7 as the second one were used, while the arm lengths of chromosomes 5, 6, and 7 were ascertained from the standard karyotype.

Our results point into the same direction as those reported by Tuleen (1973). Therefore, chromosome 1 appears in fact not to be the longest chromosome of the barley karyotype. Further work is in progress to construct karyotypes suitable for direct measurements of chromosomes 2 to 4.

The results will also serve as an essential basis and a startpoint to studies in which standard and reconstructed karyotypes of barley will be used for analysing the specific distribution patterns of chemically induced chromosome breaks. They will be carried out by the second author at the Institute of Genetics and Plant Breeding Sofia (Bulgaria).

Acknowledgement:

The senior author is indebted to Prof. A. Hagberg for providing the translocation tester set of barley.

References:

Tjio, J. H., and A. Hagberg. 1951. Cytological studies of some X-ray mutants of barley. Anales Estac. Exp. Aula Dei 2: 149-167.

Tuleen, N. A. 1973. Karyotype analysis of multiple translocation stocks of barley. Can. J. Genet. Cytol. 15: 267-273.

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