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Repeated reunions and splits feature the highly dynamic evolution of 5S and 35S ribosomal RNA genes (rDNA) in the Asteraceae family

DOI: 10.1186/1471-2229-10-176

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Abstract:

Dominant linked rDNA genotype was found within three large groups in subfamily Asteroideae: tribe Anthemideae (93% of the studied cases), tribe Gnaphalieae (100%) and in the "Heliantheae alliance" (23%). The remaining five tribes of the Asteroideae displayed canonical non linked arrangement of rDNA, as did the other groups in the Asteraceae. Nevertheless, low copy linked genes were identified among several species that amplified unlinked units. The conserved position of functional 5S insertions downstream from the 26S gene suggests a unique, perhaps retrotransposon-mediated integration event at the base of subfamily Asteroideae. Further evolution likely involved divergence of 26S-5S intergenic spacers, amplification and homogenisation of units across the chromosomes and concomitant elimination of unlinked arrays. However, the opposite trend, from linked towards unlinked arrangement was also surmised in few species indicating possible reversibility of these processes.Our results indicate that nearly 25% of Asteraceae species may have evolved unusual linked arrangement of rRNA genes. Thus, in plants, fundamental changes in intrinsic structure of rDNA units, their copy number and chromosomal organisation may occur within relatively short evolutionary time. We hypothesize that the 5S gene integration within the 35S unit might have repeatedly occurred during plant evolution, and probably once in Asteraceae.It is generally considered that in prokaryotes and in some species of early diverging eukaryote groups e.g. yeast, Saccharomyces cerevisiae, rRNA genes are organised in a single operon, clustered in tandem and transcribed by the same RNA polymerase. In eukaryotes, the 35S (encoding 18S-5.8S-26S rRNA) and 5S genes are transcribed by different polymerases, RNA polymerase I and III, respectively. Independent control of transcription probably enabled physical separation of both loci in chromosomes, an arrangement that is typical for most eukaryotic organisms. Nevertheless,

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