Distribution of 45S rDNA sites in chromosomes of plants: Structural and evolutionary implications

Fernando Roa,Marcelo Guerra

doi:10.1186/1471-2148-12-225

Abstract

Background45S rDNA sites are the most widely documented chromosomal regions in eukaryotes. The analysis of the distribution of these sites along the chromosome in several genera has suggested some bias in their distribution. In order to evaluate if these loci are in fact non-randomly distributed and what is the influence of some chromosomal and karyotypic features on the distribution of these sites, a database was built with the position and number of 45S rDNA sites obtained by FISH together with other karyotypic data from 846 plant species.ResultsIn angiosperms the most frequent numbers of sites per diploid karyotype were two and four, suggesting that in spite of the wide dispersion capacity of these sequences the number of rDNA sites tends to be restricted. The sites showed a preferential distribution on the short arms, mainly in the terminal regions. Curiously, these sites were frequently found on the short arms of acrocentric chromosomes where they usually occupy the whole arm. The trend to occupy the terminal region is especially evident in holokinetic chromosomes, where all of them were terminally located. In polyploids there is a trend towards reduction in the number of sites per monoploid complement. In gymnosperms, however, the distribution of rDNA sites varied strongly among the sampled families.ConclusionsThe location of 45S rDNA sites do not vary randomly, occurring preferentially on the short arm and in the terminal region of chromosomes in angiosperms. The meaning of this preferential location is not known, but some hypotheses are considered and the observed trends are discussed.

Highlights

45S rDNA sites are the most widely documented chromosomal regions in eukaryotes
Lima-de-Faria [7] showed that the secondary constrictions of mitotic chromosomes, which bear the 18S-5.8S25S ribosomal RNA genes transcribed in the previous interphase, called the nucleolus organizer regions (NORs) or 45S rDNA sites, were preferentially distributed on the short arms and in the subterminal region in most species of plants and animals
By using the fluorescence in situ hybridization (FISH) technique, it is known that the secondary constrictions represent only the expression of rRNA genes which were active during the last interphase and that other functional sites may not form secondary constrictions, especially if located too close to the terminal end of the chromosomes [8]

Summary

Introduction

45S rDNA sites are the most widely documented chromosomal regions in eukaryotes. The analysis of the distribution of these sites along the chromosome in several genera has suggested some bias in their distribution. Lima-de-Faria [7] showed that the secondary constrictions of mitotic chromosomes, which bear the 18S-5.8S25S ribosomal RNA genes transcribed in the previous interphase, called the nucleolus organizer regions (NORs) or 45S rDNA sites, were preferentially distributed on the short arms and in the subterminal region in most species of plants and animals. The author compared the frequency of secondary constrictions in the proximal, interstitial and terminal regions of the chromosome arm, taking into account the size of the arms (arm frame method). The fact that the 18S-5.8S-25S rRNA genes are arranged in hundreds or thousands of tandem repeats and are highly conserved between species made of this chromosome region the most widely investigated by FISH, being the position of these sites known in hundreds of plant species, especially in angiosperms

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