II. 1. Further evidence on "nucleolar dominance" in barley translocation lines.
M. Anastassova-Kristeva, Institute of Morphology, Academy of Sciences, Sofia, Bulgaria; R. Rieger, G. Künzel, Zentralinstitut fur Genetik und Kulturpflanzenforschung der Akademie der Wissenschaften der DDR, 43Z5 Gatersleben; H. Nicoloff, Institute of Genetics, Academy of Sciences, Sofia, Bulgaria; and A. Hagberg, The Swedish Seed Association, S-26800 Svaløf, Sweden. "R"
The standard barley karyotype (2n = 2x = 14) contains two SAT-chromosome pairs: Chromosome 6 with a larger satellite and "stronger" NOR (nucleolus organizing region) and chromosome 7 with a smaller satellite and "weaker" NOR. After mitosis these two pairs of SAT-chromosomes form 4 primary nucleoli which during interphase follow a definite pattern of nucleolar fusion (Anastassova-Kristeva et al. 1977). Since the formation of nucleoli is due to the synthetic activity of rRNA cistrons (rDNA) localized in the secondary constrictions of the SAT-chromosomes, it becomes possible to look for the synthetic activity (transcription) of the rRNA genes in individual NORs by studying the number and size of the primary nucleoli present in individual nuclei.
The synthetic activity of NORs translocated to new positions in the chromosome complement by interchanges has previously been studied (Nicoloff et al. 1977, 1979; Anastassova-Kristeva et al. 1977). The data obtained showed that the activity of the rRNA cistrons may become characteristically impaired when all NORs are combined in a single chromosome pair. Under these circumstances, two of the four NORs are more or less completely "repressed" and only produce micronucleoli ("nucleolar dominance"). However, it was impossible to establish definitely which of the NORs were the active ones, either the transposed NORs or those remaining in their original position.
The investigations presented here provide further information on this problem. The methods used have previously been described (cf. Anastassova-Kristeva et al. 1979).
Four reciprocal translocations between the barley SAT-chromosomes 6 and 7 resulting in a chromosome pair with two NORs in each of the homologues and two combinations (with duplications) due to crossing of these translocations, described in detail by Hagberg and Hagberg (1978), have been studied in the present paper as to the peculiarities of nucleolus formation:
T 6 - 7a with a large part of the short arm of chromosome 6 translocated to the long arm of chromosome 7 giving rise to a SAT-chromosome with both NORs in opposite arms (see Fig. la);
T 6 - 7d in which the long arm of chromosome 7 is replaced by a smaller part of the short arm of chromosome 6 once more giving rise to a SAT-chromosome with both NORs in opposite arms (see Fig. la);
Duplication line D1 from the cross T 6 - 7a x T 6 - 7d with a duplication (not involving the satellite region) in the chromosome containing both satellites and NORs in opposite arms (see Fig. la);
T 6 - 7ae in which the NOR of chromosome 7 is translocated to the satellite of chromosome 6 yielding a chromosome with two tandemly arranged NORs (see Fig. lb);
T 6 - 7k in which the NOR 6 is translocated to the satellite of chromosome 7 once more yielding a chromosome with two tandemly arranged NORs (see Fig. lb);
Duplication line D 3 from the cross T 6 - 7k x T 6 - 7ae with two duplications involving the satellites and 8 NORs tandemly arranged in two SAT-chromosome pairs (see Fig. lb).
Figure 1 a and b. The satellite chromosomes and
the number and relative size of primary nucleoli produced by the barley
standard karyotype, four translocation lines with both satellites in opposite
arms or tandemly in the same arm of one chromosome, and two duplication
lines (dl and D3) from appropriate crosses between the translocation lines.
c. The pattern of nucleolus formation at early telophase translocation lines with tandemly arranged satellites in duplication line D3 and 8 NORs.
The maximum number of nucleoli per cell in translocation lines T 6 - 7a and T 6 - 7d is 4, two of them being micronucleoli. Root tip cells of the homozygous duplication line D l from T 6 - 7a x T 6 - 7d exhibit the same pattern of nucleolus formation (Fig. la). Similar results have previously been obtained in translocation lines T 505, T 506, T 548, T 571 and T 2052 (Nicoloff et al. 1977, 1979; Anastassova-Kristeva et al. 1979). The NORs in all these karyotype show similar functional behavior when combined in one pair of SAT-chromosomes, two are active and two are strongly suppressed and give rise to micronucleoli.
For duplication line D 1, it is well established that NOR 6 was translocated to the long arm of SAT-chromosome 7. Irrespective of the change of its normal position, NOR 6 being now part of chromosome 7 is characterized by a larger secondary constriction and satellite. This is taken to indicate that the larger nucleoli are formed by NOR 6, while the weaker NOR 7 becomes suppressed under the influence of the "stronger" NOR 6 now localized in the same chromosome. In the root tip cells of translocation lines T 6 - 7ae and T 6 - 7k with tandem satellites only two large primary nucleoli of equal size were observed. In cells of the homozygous duplication line D 3 (8 NORs) obtained from the cross T 6 - 7k x T 6 - 7ae a maximum number of 4 large (not always of the same size) nucleoli per nucleus was observed once more substantiating "nucleolar dominance" (Fig. lb).
In AgNO3 stained preparations of T 6 - 7ae it was possible to locate four silver positive NORs in early telophase. In more advanced telophase cells, it became obvious that only one of the NORs in chromosome T 6 - 7ae was synthetically active, the second one remained strongly suppressed and gave rise to only a micronucleolus which was found to be closely associated with the larger nucleolus. The active NORs were situated close to the centromere and therefore represent NOR 6, which in chromosome T 6 - 7ae takes this position (Fig. lc). With the start of interphase the micronucleoli are no longer visible since they become totally incorporated into the mass of the larger nucleoli. Thus only two (after fusion only one) large nucleoli are present in interphase cells. In T 6 - 7k in which the stronger NOR 6 takes a position distal to the weaker one of chromosome 7, the larger nucleolus appears in a position more distant from the centromere (Fig. lc).
These findings suggest that the stronger NOR 6 is the one which forms the large nucleoli. NOR 7 remains suppressed in all cases of NOR combination, by interchange, in a single chromosome pair. These observations provide additional evidence for the rule previously postulated: In reconstructed barley chromosomes with 2 NORs only one of these is fully active. Under these circumstances NOR 6 exerts "nucleolar dominance" over NOR 7.
Anastassova-Kristeva, M., H. Nicoloff, G. Künzel and R. Rieger. 1977. Nucleolus formation in structurally reconstructed barley karyotypes with six satellite chromosomes. Chromosoma (Berl.), 62:111-117.
Anastassova-Kristeva, M., H. Nicoloff, S. Georgieff. 1978. SAT-chromosome numbers in Triticum monococcum with respect to nucleolar organizer activity. Theoret. & Appl. Genetics, 53:229-231.
Hagberg, P. and A. Hagberg. 1978. Segmental interchanges in barley. III. Translocations involving chromosome 6 and 7 used in production of duplications. Z. Pflanzenzuchtung 81:111-117
Nicoloff, H., M. Anastassova-Kristeva, G. Künzel, and R. Rieger. 1977. The behavior of NORs in structurally changed karyotypes of barley. Chromosoma (Berl.), 62:103-109.
Nicoloff, H., M. Anastassova-Kristeva, G. Künzel, and R. Rieger. 1979. "Nucleolar dominance" as observed in barley translocation lines with specifically reconstructed SAT-chromosomes. Theoret. & Appl. Genetics (in press).
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