Abstract
The nuclear genome harbours hundreds to several thousand copies of ribosomal DNA. Despite their essential role in cellular ribogenesis few studies have addressed intrapopulation, interpopulation and interspecific levels of rDNA variability in wild plants. Some studies have assessed the extent of rDNA variation at the sequence and copy-number level with large sampling in several species. However, comparable studies on rDNA site number variation in plants, assessed with extensive hierarchical sampling at several levels (individuals, populations, species) are lacking. In exploring the possible causes for ribosomal loci dynamism, we have used the diploid genus Anacyclus (Asteraceae) as a suitable system to examine the evolution of ribosomal loci. To this end, the number and chromosomal position of 45S rDNA sites have been determined in 196 individuals from 47 populations in all Anacyclus species using FISH. The 45S rDNA site-number has been assessed in a significant sample of seed plants, which usually exhibit rather consistent features, except for polyploid plants. In contrast, the level of rDNA site-number variation detected in Anacyclus is outstanding in the context of angiosperms particularly regarding populations of the same species. The number of 45S rDNA sites ranged from four to 11, accounting for 14 karyological ribosomal phenotypes. Our results are not even across species and geographical areas, and show that there is no clear association between the number of 45S rDNA loci and the life cycle in Anacyclus. A single rDNA phenotype was detected in several species, but a more complex pattern that included intra-specific and intra-population polymorphisms was recorded in A. homogamos, A. clavatus and A. valentinus, three weedy species showing large and overlapping distribution ranges. It is likely that part of the cytogenetic changes and inferred dynamism found in these species have been triggered by genomic rearrangements resulting from contemporary hybridisation.
Highlights
In most plant species, repetitive DNA constitutes a large fraction of the nuclear genome and is a major contributor to plant chromosome structure
A summary of the results found in the analysis of the number and chromosomal location of the 45S ribosomal DNA (rDNA) sites in 196 individuals from 47 populations is shown in Tables 1 and 2, and S2 Table
The 45S rDNA site number, their chromosomal distribution and structure have been assessed in a significant sample of seed plants, which usually exhibit rather consistent features, except for polyploid wild plants
Summary
Repetitive DNA constitutes a large fraction of the nuclear genome and is a major contributor to plant chromosome structure. The nuclear genome harbors hundreds to several thousand copies of each ribosomal family, 45S (18S, 5.8S, 25S/26S) and 5S, which are usually arranged in distinct arrays of tandemly-repeated units [2]. Despite their essential role in cellular ribogenesis and organismal growth and integrity, few studies have addressed intrapopulation, interpopulation and interspecific levels of rDNA variability in wild plants (i.e., site number and genomic location, copy number, and sequence divergence). There have been studies assessing the extent of rDNA variation at the sequence and copy-number level with large hierarchical sampling in several species [3, 4]. Comparable studies on rDNA site number variation in plants, assessed with extensive sampling at several levels (individuals, populations, species) are lacking
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