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AOBPreview originally published online on February 29, 2008
Annals of Botany 2008 101(6):815-823; doi:10.1093/aob/mcn019
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© The Author 2008. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Evolution of rDNA in Nicotiana Allopolyploids: A Potential Link between rDNA Homogenization and Epigenetics

Ales Kovarik1, Martina Dadejova1, Yoong K. Lim2, Mark W. Chase3, James J. Clarkson3, Sandra Knapp4 and Andrew R. Leitch2,*

1 Institute of Biophysics, Academy of Sciences of the Czech Republic, CZ-61265 Brno, Czech Republic
2 School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, UK
3 Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK
4 Department of Botany, Natural History Museum, London SW7 5BD, UK

* For correspondence. E-mail A.R.Leitch{at}qmul.ac.uk

Received: 6 July 2007    Returned for revision: 26 October 2007    Accepted: 15 January 2008    Published electronically: 29 February 2008

Background: The evolution and biology of rDNA have interested biologists for many years, in part, because of two intriguing processes: (1) nucleolar dominance and (2) sequence homogenization. We review patterns of evolution in rDNA in the angiosperm genus Nicotiana to determine consequences of allopolyploidy on these processes.

Scope: Allopolyploid species of Nicotiana are ideal for studying rDNA evolution because phylogenetic reconstruction of DNA sequences has revealed patterns of species divergence and their parents. From these studies we also know that polyploids formed over widely different timeframes (thousands to millions of years), enabling comparative and temporal studies of rDNA structure, activity and chromosomal distribution. In addition studies on synthetic polyploids enable the consequences of de novo polyploidy on rDNA activity to be determined.

Conclusions: We propose that rDNA epigenetic expression patterns established even in F1 hybrids have a material influence on the likely patterns of divergence of rDNA. It is the active rDNA units that are vulnerable to homogenization, which probably acts to reduce mutational load across the active array. Those rDNA units that are epigenetically silenced may be less vulnerable to sequence homogenization. Selection cannot act on these silenced genes, and they are likely to accumulate mutations and eventually be eliminated from the genome. It is likely that whole silenced arrays will be deleted in polyploids of 1 million years of age and older.

Key words: Diploidization, epigenetics, nucleolar dominance, polyploidy, rDNA, ribosomal DNA, sequence homogenization


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