Karyotype of Araucaria angustifolia and the decondensation/activation mode of its nucleolus organiser region

Abstract

The chromosomes of the gymnosperm Araucaria angustifolia (Bertol.) Kuntze were analysed with the fluorochromes chromomycin A3 (CMA) and 4′,6-diamidino-2-phenylindole (DAPI) and with C-banding. This species contains a diploid complement made of 26 chromosomes, with 18 larger metacentric, four smaller metacentric and four submetacentric chromosomes. The only CMA+/DAPI– region observed corresponded to the nucleolus organiser region (NOR) localised at the proximal portion of a large metacentric chromosome pair. C-banding marked the NOR as well as a terminal region of another chromosome pair. In addition, small C-bands were occasionally seen interspersed in many chromosomes. The NOR appeared to condense at approximately the same rate as the rest of the chromosome from prophase throughout metaphase. In interphase nuclei, NOR decondensation and activation was characterised by the formation of CMA+ blocks that resembled a string of beads inside the nucleolus. The number and size of beads was inversely proportional to the size of the nucleolus, suggesting that transcriptional activation of the nucleolar cistrons starts simultaneously at several points of the NOR. The mode of NOR activation in A. angustifolia differs from that observed in most species, providing a unique opportunity to study activation and transcriptional control of rRNA genes.