II. 4. Nucleolus organizer activity as affected by chromosome repatterning in barley.
M. Anastassova-Kristeva, Institute of Morphology, Academy of Sciences, Sofia, Bulgaria; H. Nicoloff, Institute of Genetics, Academy of Sciences, Sofia, Bulgaria; G. Künzel, Zentralinstitut fur Genetik und Kulturpflanzenforschung der Akademie der Wissenschaften der Wissenschaften der DDR, 4325 Gatersleben; and A. Hagberg, The Swedish Seed Association, S-26800 Svalöf, Sweden. "R"
It has previously been reported (Nicoloff et al., 1977) that the combination by means of interchanges of all four nucleolus organizing regions (NORs) in a single barley chromosome pair may affect transcription of rDNA. In the barley standard karyotype, chromosome pairs 6 and 7 represent satellite chromosomes and are involved in nucleolus formation which results in a maximum number of four primary nucleoli per nucleus. If the NORs of chromosome pairs 7 become translocated onto the long arms of SAT chromosome pair 6 or vice versa, this results in the repression of the transposed RNA cistrons as inferred from impaired nucleolus formation by two of the four NORs. Under these circumstances, the maximum number of primary nucleoli is either two or, if the transposed NORs show residual activity in some of the translocation lines, two of normal size plus two micronucleoli.
In connection with these observations we were interested to study translocation lines in which the NORs are not located in different arms of the same chromosome pair but tandemly arranged in the same arm since such lines might give additional clues as to the influence of chromosomal position on the activity of translocated NORs.
Hagberg and Hagberg (1969) described two barley translocation lines
(T 130 and T 412) with all four NORs tandemly arranged in one chromosome
pair (7 or 6, respectively). T 130 is due to a reciprocal translocation
between standard chromosome 6 (translocation point in the short arm) and
standard 7 (translocation point in the satellite) which resulted in a new
type of chromosome 7 with tandemly arranged NORs in the same chromosome
arm. In T 412, the NOR of chromosome 7 is part of chromosome 6 and tandemly
positioned with respect to the original NOR of this chromosome (Figure
1).
Number and size of primary nucleoli in these translocation lines were determined by the nucleolar test described previously (cf. Anastassova-Kristeva et al., 1977; Nicoloff et al., 1977). The maximum number of primary nucleoli of normal size (7 to 10 mm in diameter dependent on cell size) was two per nucleolus in both translocation lines (Figure 1), as compared with four in the standard karyotype. In addition, the translocation lines regularly showed two tiny micronucleoli (diameter about 1 mm) of weak pyroninophily which is indicative of a low content of ribosomal RNA. These two micronucleoli may associate with one another or with the nucleoli of normal size. Probably these micronucleoli consist of only the bright and dark fibrillar zones of nucleoli but are devoid of the granular part.
The micronucleoli are indicative of reduced transcriptional activity of the translocated NORs. In telophase chromosomes, it was observed that the nucleoli of normal size are located proximally and the micronucleoli distally in the same chromosome arm. This may be taken as evidence that it is the transposed NORs which are of only residual transcriptional activity. The differential length of segments between the two secondary constrictions of the translocation lines was without measurable influence on both transcriptional activity of the transposed NORs and position of the micronucleoli. All NORs were found to be stainable by the silver staining (Ag AS) technique developed by Goodpasture and Bloom (1975) which is assumed to be specific for active ribosomal DNA in a wide variety of animal and plant mitotic chromosomes. Normal and residual NOR activity (as inferred from the formation of micronucleoli) in barley did not result in recognizable differential silver staining of the NORs in question (manuscript in preparation).
The results reported here substantiate the earlier findings which showed that the activity of translocated barley NORs may be affected by karyotype reconstructions resulting in the combination of all NORs in one chromosome pair. As far as the evidence goes, only one NOR per chromosome seems to be able to function normally (cf. Nicoloff et al., 1977; Anastassova-Kristeva, 1977). In all cases studied translocation of NORs to a SAT-chromosome active in nucleolus formation resulted in either complete inactivation or grossly reduced activity of transposed rDNA. Though the mechanism underlying this phenomenon is presently unknown, the cases so far studied represent what may be called position effects with respect to the transcription of rDNA.
Acknowledgment:
We are grateful to B. Sellnick for excellent technical assistance.
References:
Anastassova-Kristeva, M., H. Nicoloff, G. Künzel, and R. Rieger.
1977. Nucleolus formation in structurally reconstructed barley karyotypes
with six satellite chromosomes. Chromosoma 62:111-117.
Goodpasture, C. and S.E. Bloom. 1975. Visualization of nucleolar organizer regions in mammalian chromosomes. Chromosoma 53:37-50.
Hagberg, A. and G. Hagberg. 1969. Use of induced structural rearrangements in plant breeding. In Induced mutations in plants. Proc. of a symposium. Int. Atomic Energy Agency, Vienna, pp. 647-653.
Nicoloff, H., M. Anastassova-Kristeva, G. Künzel, and R. Rieger. 1977. The behavior of nucleolus organizers in structurally reconstructed karyotypes of barley. Chromosoma 62:103-lO9.
