Concerted evolution of 18-5.8-26S rDNA repeats in Nicotiana allotetraploids

Abstract

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 2 n = 4 x = 48) and one synthetic N. tabacum line (Th37, Ǩ N. sylvestris (2 n = 24) ¥ ǩ N. tomentosiformis (2 n = 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.

Ribosomal DNA in the GrasshopperPodisma pedestris: Escape From Concerted Evolution

“…This result is in contrast to the expectation that functional arrays would be particularly prone to homogenization through concerted evolution (Kovarik et al 2004). What could explain a low rate of homogenization in Podisma?…”

“…These large differences, however, may be attributed to divergence that has accumulated in different species and has then been brought together by hybridization (Lim et al 2000;Hartmann et al 2001;Muir et al 2001). The rDNA of hybrids can diverge rapidly from either parent within two or three generations (Kovarik et al 2004). For this reason, and because of the divergence between different rDNA arrays and pseudogenes, some authorities consider rDNA to be a poor choice for phylogenetic analysis (Á lvarez and Wendel 2003).…”

Concerted Evolution of rDNA in Recently Formed Tragopogon Allotetraploids Is Typically Associated With an Inverse Correlation Between Gene Copy Number and Expression

“…Frequent dominance of low-copy rDNA: In many allopolyploids, concerted evolution has homogenized rDNA repeats to variable degrees and directions, altering the copy number of inherited genes (Alvarez and Wendel 2003;Kovarik et al 2004;Volkov et al 2007). In most cases, expression is dominated by a high-copy rRNA gene family (Flavell 1986;Lim et al 2000;Joly et al 2004).…”

Molecular Evidence for Transcription of Genes on a B Chromosome in Crepis capillaris

“…Sequence divergence may occur due to the relaxation of the constraints of concerted evolution due to lack of pairing of the B chromosome with the A's. However, there are a number of examples of either concerted evolution of different arrays of rRNA genes ( Joly et al 2004;Kovarik et al 2004) or maintenance of different arrays in various polyploid species. Thus either maintenance of sequence identity or sequence divergence is equally likely even in the absence of detectable pairing.…”