Genome Evolution, Polyploidy and Gene Conversion in Nicotiana
It is well recognised that autopolyploidy, and polyploidy associated with interspecific hybridisation (allopolyploidy) are major forces in plant evolution. However little is known about the genetic consequences of polyploidy. Using fluorescent in situ hybridisation (FISH) we present a phylogenetic scheme of the following plant species: Nicotiana tomentosiformis, N. kawakamii, N. tomentosa, N. otophora, N. setchellii, N. glutinosa (diploids, section Tomentosae), and N. tabacum (tobacco, sect. Genuinae). N. tabacum is an allotetraploid thought to be derived from ancestors of N. sylvestris (S genome donor) and N. tomentosiformis (T genome donor). In the evolution of the section Tomentosae we show: (1) how 5S rDNA families have evolved, diverged and homogenised; (2) at what point in evolution the tandem repeats GRS and NTRS evolved; (3) how this coincides with the de-novo integration and evolution of geminiviral related DNA (GRD) and; (4) how the distributions of these repeats and 18S-5.8S-26S rDNA changes over c. 75-100 million years of evolution. In the evolution of N. tabacum (c. 6 million years ago), and associated with allopolyploidy, there has been (a) chromosomal translocations between the S and T genomes, especially involving the HRS60 repeat of N. sylvestris origin; (b) conservation in the locations of the repeat NTRS of N. tomentosiformis origin and in the distribution of 18S-5.8S-26S and 5S rDNA; (c) amplification, mobility and divergence of GRD; (d) intra- and inter-locus gene conversion of 18S-5.8S-26S rDNA and; (e) no homogenisation of two 5S rDNA families. Our data suggests that active undermethylated 18S-5.8S-26S rDNA units may be vulnerable to gene conversion, perhaps due to associations within the nucleolus, while heavily methylated, largely condensed rDNA may be less vulnerable.
A.R. Leitch, Y.K. Lim, C.P Lichtenstein, R. Matyasek1, J. Fulnecek1, M. Bezdek1, A. Kovarik1,
School of Biological Sciences, Queen Mary and Westfield College,