Concerted evolution of 18-5.8-26S rDNA repeats in Nicotiana allotetraploids
We review and extend data showing concerted evolution of parental 18-5.8-26S nuclear ribosomal DNA (18-26S rDNA) gene families in three natural Nicotiana allotetraploids (N. tabacum, N. rustica and N. arentsii, each 2n = 4x = 48) and one synthetic N. tabacum line (Th37, female N. sylvestris (2n = 24) x male N. tomentosiformis (2n = 24)). The origin of the gene families was analysed by sequence polymorphisms in the intergenic spacer (IGS) region and the number of chromosomal loci by fluorescence in situ hybridization (FISH). FISH revealed that the number and locations of 18-26S rDNA in the natural allopolyploids was the sum of those found in the diploid progenitors. However, the rDNA restriction patterns showed polymorphisms in the IGS that were not additive, suggesting that parental rDNA clusters were partially (N. tabacum, N. rustica) or completely (N. arentsii) overwritten by hybrid-specific units. Thus the Nicotiana allotetraploids show evidence of concerted evolution, including both intralocus and interlocus gene conversion. A feral N. tabacum collected in Bolivia had a higher proportion of unconverted parental rDNA units than cultivated tobacco varieties, suggesting either that rDNA homogenization is accelerated by inbreeding or multiple origins of tobacco. There is no evidence for the elimination of N. sylvestris-derived rDNA units in the synthetic Th37 tobacco line as occurred in natural tobacco, although several novel rDNA unit variants were found in most but not all the hybrid plants. Factors that may control the occurrence and extent of rDNA homogenization are discussed for allopolyploids in Nicotiana and other taxa. (C) 2004 The Linnean Society of London.