Ribosomal RNA genes evolution in Tragopogon: A story of New and Old World allotetraploids and the synthetic lines
Tragopogon mirus Ownbey and Tragopogon miscellus Ownbey are recent allotetraploids that have formed recurrently within the last 80 years, following the introduction of the diploids T dubius Scop., T pratensis L. and T porrifolius L. from Europe to North America. In some areas, the progenitor diploids still occur along with expanding populations of the allotetraploids, and the polyploids at those locations likely represent descendents of the nearby diploids. In most natural populations of T mirus and T miscellus, there are far fewer rDNA units of the common parent (T dubius) than there are of the other diploid parent and in some rarely occurring individuals, one parental locus was >90% deleted. Nevertheless, in contrast to several ancient Old World allotetraploids, gene copies from both parents are readily detected by molecular methods in both T mirus and T miscellus. In one population of T minis and herbarium specimens collected at the time of species origin the gene ratios were balanced. Several lineages of T mints and T miscellus were recently successfully resynthesized from diploid species. Among 181 synthetic individuals analyzed, we observed frequent deviations from copy-number additivity; that is in most cases, the T dubius homeologs were reduced in copy number. At the epigenetic level, the genes of T. dubius origin dominate expression in most natural and synthetic allopolyploids. The fact that some rDNA genotypes seen in 80-year-old allopolyploids are already evident in the first generation of synthetic lines supports the hypothesis that the extent and tempo of rDNA homogenization in older allopolyploids is largely influenced by genetic and epigenetic changes in the early generations. Thus, T mints and T miscellus that formed repeatedly in the wild within the last century represent a unique system to study the early stages of genome evolution following interspecific hybridization and genome duplication. This review summarizes recent works on the rDNA chromosomal organization, repeat and loci inheritance and gene expression.