Sequence Copy Number Changes Associated With Nicotiana Tabacum Allopolyploidy

KY Lim1, A Kovarik2, R Matyasek2, K Skalická2, B Koukalova2, M-A Grandbastien3 and A Leitch1

1 School of Biological Sciences, Queen Mary University of London, E1 4NS, UK
2 Institute of Biophysics, Brno, Czech Republic
3 Laboratoire de Biologie Cellulaire, Institut Jean-Pierre Bourgin, France

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Allopolyploidy has been associated with DNA copy number increases and decreases, although genome downsizing seems to be an overall trend (1). The DNA C-value of N. tabacum (tobacco, 2n = 4x = 48) of 1C = 5.85 pg is not materially different, although slightly larger, than the sum of the values of the putative parents N. sylvestris (2n = 2x = 24, maternal S-genome donor, 1C = 2.88 pg,) and N. tomentosiformis (2n = 2x= 24, paternal T-genome donor, 1C = 2.83 pg (DNA C-values database cval/homepage/html). Recent evidence (Chase pers. comm.) suggests that tobacco may be only a few thousand years old. We compared copy numbers of (i) four non-coding tandem repeat sequences, of which three are found in N. tomentosiformis and tobacco (GRD, NTRS and GRS) and one is found in N. sylvestris and tobacco (HRS60), (ii) one dispersed repeat (Tnt1) and (iii) two tandem repeat coding sequences (5S and 45S rDNA). There was little deviation from expectation in the abundance of the non-coding tandem repeat sequences, or of 5S rDNA. Interestingly, substantial copy number variation has been observed between related diploids. In contrast the copy numbers of Tnt1 and 45S rDNA in tobacco were 67% and 28% of expectation respectively. This might reflect either losses in the polyploid or gains in the diploid following tobacco speciation.

1. Leitch IJ, Bennett MD. (2003). Genome downsizing in polyploid plants. Biological Journal of the Linnean Society, submitted.