‘Nucleolar dominance’ as observed in barley translocation lines with specifically reconstructed SAT chromosomes

Abstract

Diploid homo- and heterokaryotypes of barley translocation lines with only one satellite chromosome pair containing two nucleolus organizer regions (NORs) in opposite arms were found to show repressed nucleolus formation by the transposed NOR as evident from the formation of only micronucleoli. The same was true for auto-tetraploid homokaryotypes and for translocation lines with all NORs tandemly arranged into the same chromosome arm. When NORs were transposed to chromosomes without NOR in the standard karyotype, the normal pattern of nucleolus formation remained unaffected. The modified mode of nucleolus formation after the combination of all NORs in one chromosome pair is interpreted to be due to intrachromosomal nucleolar dominance analogous to interchromosomal nucleolar dominance observed in certain interspecific hybrids.

Nucleolar organiser activity in wheat-barley chromosome addition lines

“…The maximum number of nucleoli observed agrees with the number of Ag-NORs present in the cells (table 1). Our results are in agreement with those of Nicoloff et al, (1979).…”

Transcriptional Activity of Translocated NORs in Barley (Hordeum VulgareL.)

“…the percent of cells containing the maximum number of nucleoli depends on the rate of association after mitosis and is different in the various studied samples (1). there is a positive correlation between the size of the secondary constriction of a particular noR and the size of the nucleolus formed on it (1,26,41). Reciprocal translocations somehow concerning the nucleolar organizers result in modification of the normal path of rRNA gene expression and the morphology and size of nucleoli ( Table 2).…”

“…in standard barley karyotype the size of the primary nucleoli varied pairwise indicating that both chromosome pairs differed in their nucleolus forming capacity (2,28,41). As tsuchiya (56) and Anastassova-Kristeva et al (2) mentioned, the noR6h is functionally stronger (shows a wider secondary constriction) and produces a larger nucleolus than chromosome 5h although noR 5h contains 70% more rDnA than noR 6h (54,55).…”

“…in plants, noRs include two different structural and functional domains -transcriptionally active rRnA genes, which give rise to the secondary constriction on a metaphase chromosome, and silent rRnA genes, packed into heterochromatin body (3,4,5,42,43,44). the nucleolus, as the site of ribosomal assembly, is a phenotypic manifestation of rDnA expression; therefore only actively transcribed genes form nucleoli (32,41,45). As a rule, the number of transcriptionally active rDnA-encoding loci in a cell is related to the number of the formed nucleoli.…”

“…As a rule, the number of transcriptionally active rDnA-encoding loci in a cell is related to the number of the formed nucleoli. the size of the nucleolus formed in association with a particular noR is proportional to rRnA gene dosage and the level of gene activity (1,2,26,27,39,41). Due to translocation with breakpoints within or close to the noRs, the localization of all or part of their ribosomal genes may be changed in the genome, which results in modification of the expression patterns of the transposed genes as well as of those in the original location.…”