Two-dimensional RFLP analyses reveal megabase-sized clusters of rRNA gene variants in Arabidopsis thaliana, suggesting local spreading of variants as the mode for gene homogenization during concerted evolution |
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Authors: | Gregory P Copenhaver Craig S Pikaard |
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Institution: | Biology Department, Washington University, Campus Box 1137, One Brookings Drive, St Louis, MO 63130, USA |
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Abstract: | Eukaryotic genes encoding the precursor of 18S, 5.8S and 25S ribosomal RNA (rRNA genes or rDNA) are virtually identical within a species, yet they evolve rapidly between species, a phenomenon known as concerted evolution. The mechanisms by which sequence homogenization and fixation of new rRNA gene variants occurs within a genome are not clear. In diploid Arabidopsis thaliana , approximately 1500 rRNA genes are tandemly arrayed at two nucleolus organizer regions, one on chromosome 2 ( NOR2 ), the other on chromosome 4 ( NOR4 ). This paper shows that NOR2 and NOR4 are similar in size, each spanning approximately 3.5–4.0 Mbp. Using two-dimensional mapping techniques involving a combination of pulsed-field and conventional gel electrophoresis, the distributions of four distinct rRNA gene variants at NOR2 and NOR4 have been determined. rRNA genes at NOR4 are homogeneous with respect to a Hin dIII site occurring once per gene. In contrast, fewer than 10% of the rRNA genes at NOR2 are Hin dIII-bearing variants. A single intergenic spacer length is found among rRNA genes at NOR2 but three classes of spacer length variants are present at NOR4 . The NOR4 variants are not intermingled with one another; instead, they are highly clustered over distances as large as 1.5 Mbp. These data suggest that in the concerted evolution of rRNA genes, homogenization is a consequence of local spreading of new rRNA gene variants. |
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