Does the chromosomal position of 35S rDNA sites influence their transcription? A survey on Nothoscordum species (Amaryllidaceae)

Abstract

35S ribosomal DNA (rDNA) sites are the regions where the ribosomal genes 18S, 5.8S and 25S, responsible for the formation of the nucleoli, are found. The fact that rDNA sites have non-random distribution on chromosomes suggests that their positions may influence their transcription. To identify if the preferentially transcribed rDNA sites occupy specific position, six species (nine cytotypes) of the genus Nothoscordum were analyzed using two different techniques to impregnate the nucleolar organizer regions (NORs) with silver nitrate. Both techniques strongly stained NORs, but one of them also stained the proximal region of all chromosomes, suggesting the existence of another group of argentophilic proteins in this region. In species with rDNA sites in acrocentric and metacentric chromosomes, sites located on the short arms of the acrocentric chromosomes were preferentially activated. On the other hand, in species with rDNA sites restricted to the short arms of the acrocentrics, all of them were activated, whereas in those species with sites restricted to the terminal region of metacentric chromosomes, the frequency of active sites was always lower than expected. This indicate that, at least in Nothoscordum, the transcription of an rDNA site is influenced by its chromosomal position, and may explain, at least partially, the strongly non-random distribution of these sites in plant and animal chromosomes.

The fate of 35S rRNA genes in the allotetraploid grass Brachypodium hybridum

“…In support of this, the 'always' dominant D-genome NORs are located at the chromosome termini. There is some evidence that the chromosomal position of the rDNA locus may influence its activity (Schubert and K€ unzel, 1990;Neves et al, 2005;B aez et al, 2019). For instance, the 5S rDNA arrays that are located proximally to the centromeres are more methylated than the units that are located in distal positions in A. thaliana (Mathieu et al, 2002).…”

Intense proliferation of rDNA sites and heterochromatic bands in two distantly related Cuscuta species (Convolvulaceae) with very large genomes and symmetric karyotypes

“…In spite of the much higher number of 35S rDNA sites in C. monogyna than in C. indecora, the expression of these sites, as estimated by the maximum number of nucleoli per nucleus, was relatively small and similar in both species, possibly because some of them were permanently inactivated, as in Arabidopsis thaliana (Chandrasekhara et al, 2016), or temporarily inactive, as observed in other species (Grabiele et al, 2018;Báez et al, 2020). Thus, the exceeding number of rDNA sites is most likely an accidental consequence of the genome expansion rather than a selective advantage fixed during the evolution of these species.…”

Nuclear DNA content and comparative FISH mapping of the 5s and 45s rDNA in wild and cultivated populations of Physalis peruviana L.

“…The 45s rDNA locus is known to be involved in the creation of nucleoli during the cell cycle, especially if it is positioned in the satellite sections of the chromosomes since it correlates with the existence of nucleolar organizing areas (Long and Dawid 1980;Lopez et al 2021). If the species has a large number of 45S sites, however, not all of them will be functional (Grabiele et al 2018;Báez et al 2020). In the current investigation, no satellites correlated with an active 45s rDNA locus were found in the analyzed chromosomes, however, the N2 chromosome is likely to contain a functional locus because it is conserved in all goldenberry populations and is the only type of chromosome that has a 45s rDNA locus in the Ayacucho cultivated population.…”