Rapid concerted evolution of nuclear ribosomal DNA in two Tragopogon allopolyploids of recent and recurrent origin.
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We investigated concerted evolution of rRNA genes in multiple populations of Tragopogon mirus and T. miscellus, two allotetraploids that formed recurrently within the last 80 years following the introduction of three diploids (T. dubius, T. pratensis, and T. porrifolius) from Europe to North America. Using the earliest herbarium specimens of the allotetraploids (1949 and 1953) to represent the genomic condition near the time of polyploidization, we found that the parental rDNA repeats were inherited in roughly equal numbers. In contrast, in most present-day populations of both tetraploids, the rDNA of T. dubius origin is reduced and may occupy as little as 5% of total rDNA in some individuals. However, in two populations of T. mirus the repeats of T. dubius origin outnumber the repeats of the second diploid parent (T. porrifolius), indicating bidirectional concerted evolution within a single species. In plants of T. miscellus having a low rDNA contribution from T. dubius, the rDNA of T. dubius was nonetheless expressed. We have apparently caught homogenization of rDNA repeats (concerted evolution) in the act, although it has not proceeded to completion in any allopolyploid population yet examined. (+info)
Evolution and expression of homeologous loci in Tragopogon miscellus (Asteraceae), a recent and reciprocally formed allopolyploid.
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On both recent and ancient time scales, polyploidy (genome doubling) has been a significant evolutionary force in plants. Here, we examined multiple individuals from reciprocally formed populations of Tragopogon miscellus, an allotetraploid that formed repeatedly within the last 80 years from the diploids T. dubius and T. pratensis. Using cDNA-AFLPs followed by genomic and cDNA cleaved amplified polymorphic sequence (CAPS) analyses, we found differences in the evolution and expression of homeologous loci in T. miscellus. Fragment variation within T. miscellus, possibly attributable to reciprocal formation, comprised 0.6% of the cDNA-AFLP bands. Genomic and cDNA CAPS analyses of 10 candidate genes revealed that only one "transcript-derived fragment" (TDF44) showed differential expression of parental homeologs in T. miscellus; the T. pratensis homeolog was preferentially expressed by most polyploids in both populations. Most of the cDNA-AFLP polymorphisms apparently resulted from loss of parental fragments in the polyploids. Importantly, changes at the genomic level have occurred stochastically among individuals within the independently formed populations. Synthetic F(1) hybrids between putative diploid progenitors are additive of their parental genomes, suggesting that polyploidization rather than hybridization induces genomic changes in Tragopogon. (+info)
Concerted evolution of rDNA in recently formed Tragopogon allotetraploids is typically associated with an inverse correlation between gene copy number and expression.
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We analyzed nuclear ribosomal DNA (rDNA) transcription and chromatin condensation in individuals from several populations of Tragopogon mirus and T. miscellus, allotetraploids that have formed repeatedly within only the last 80 years from T. dubius and T. porrifolius and T. dubius and T. pratensis, respectively. We identified populations with no (2), partial (2), and complete (4) nucleolar dominance. It is probable that epigenetic regulation following allopolyploidization varies between populations, with a tendency toward nucleolar dominance by one parental homeologue. Dominant rDNA loci are largely decondensed at interphase while silent loci formed condensed heterochromatic regions excluded from nucleoli. Those populations where nucleolar dominance is fixed are epigenetically more stable than those with partial or incomplete dominance. Previous studies indicated that concerted evolution has partially homogenized thousands of parental rDNA units typically reducing the copy numbers of those derived from the T. dubius diploid parent. Paradoxically, despite their low copy number, repeats of T. dubius origin dominate rDNA transcription in most populations studied, i.e., rDNA units that are genetic losers (copy numbers) are epigenetic winners (high expression). (+info)
Rapid chromosome evolution in recently formed polyploids in Tragopogon (Asteraceae).
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