Ribosomal RNA Gene Loci and Their Nucleolar Activity in Brassica alboglabra Bailey

Abstract

Fluorescence in situ hybridization (FISH) and silver staining techniques were used to reveal the number, chromosomal sites and nucleolar activity of ribosomal RNA (rRNA) gene loci in Brassica alboglabra Bailey (genome: CC, 2n = 18). Two major and two minor rRNA gene loci were disclosed using FISH. The two major rRNA gene loci were located on the secondary constrictions of the satellited chromosome pair 9, the two minor rRNA gene loci, on the terminal region of the short arms of the non‐satellited chromosome pair 8. Two silver‐stained nucleolus organizer regions (Ag‐NORs) were detected on the secondary constrictions of chromosome pair 9. Two major nucleoli were recorded in telophase nuclei. In interphase nuclei, two micronucleoli were observed in addition to the two major nuceoli. Thus, it is suggested that the satellited chromosome pair 9 with Ag‐NORs contains major active rRNA gene loci leading to the formation of major nucleoli, while the non‐satellited chromosome pair 8 without detectable Ag‐NORs carries minor active rRNA gene loci responsible for the formation of micronucleoli.

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

“…Similar results were observed in translocation line ab (T [6][7] ab) where the presence of the tandemly arranged noR6h and part of noR5h on the short arm of one chromosome results in strong suppression of noR5h activity. Analysis of somatic cells of ab revealed four AgnoRs (Fig.…”

“…According Medina et al (33) "the number of active AgnoRs per metaphase cell corresponds to the maximum number of nucleoli per nucleus, and only the functionally active noRs during interphase are stained by silver at the next mitotic metaphase". However, the number of detectable noR-bearing chromosomes and the numbers of nucleoli may be more variable than generally recognized (7,26,27,28,31,54).…”

“…this denotes the presence of chromosomes with a nucleolus-forming activity which is too low to produce visible secondary constrictions (26,27,54) or to be detected by silver nitrate (26,51), since the intensity of staining depends on the transcriptional activity of the genes (7,22,35,51,53). hence, not always does the number of nucleoli correspond to the number of AgnoRs, and not always does the number of noRs correspond to the number of secondary constrictions (related to the silencing of noRs or to activity being too low).…”

Characterization of the A and C Genomes of Brassica Campestrisand B. Alboglabra

“…In situ hybridization also disclosed the presence of 2 major and 1 minor rDNA loci in each haploid set of B. oleracea ( MALUSZYNSKA and HESLOP-HARRISON 1993). However, silver staining and in situ hybridization studies on B. alboglabra have revealed 2 rDNA sites, a major one on the secondary constriction of chromosome 9, and a minor one at the terminal region of the short arm of chromosome 8 (CHENG et al 1995b). Probes that are specific to gene families other than ribosomal genes have also been used for studies on the localization and linkage of their determining loci ( KIANIAN and QUIROS 1992a;QUIROS et al 1994).…”

Number and Sites of rDNA Loci of Guizotia Abyssinica (L. f.) Cass. as Determined by Fluorescence in Situ Hybridization

“…Since secondary constrictions often denote the sites of nucleolar organizer regions, in situ hybridization with an rDNA probe would give a reliable answer to questions relating to potential number and location of nucleolar organizer regions and potential number of satellited chromosomes. The number and location of rRNA gene loci have been determined by the in situ hybridization method for several crop species including wheat (MUKAI et al 199 l), barley (LEITCH and HESLOP-HARRISON 1992;PEDERSEN and LINDE-LAURSEN 1994), and Brassicu species (MALUSZYNSKA and HESLOP-HARRISON 1993;CHENG et al 1995).…”