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Karyotype evolution and diversification in Pulsatilla Mill. (Ranunculaceae)

Karyotypic changes such as polyploidy, disploidy and chromosomal rearrangements are amongst the major drivers in plant evolution and speciation. Their role is investigated in the North hemisphere genus Pulsatilla Mill. (Ranunculaceae). Chromosome numbers, karyotype morphology, banding pattern, 5S and 35S rDNA localization in the species were investigated and interpreted in a phylogenetic context drawn from sequences of the nuclear ribosomal internal transcribed spacer (ITS) and plastid accD-psaI, psbM-trnD and trnL-trnF intergenic spacers. The phylogenetic analysis of concatenated sequences identified the main phylogenetic lineages within the genus, which mostly corresponds to the classic series-level classification. The cytogenetic data obtained in this study agree with the molecular phylogeny of the Pulsatilla obtained here for the first time ; the lineages in the phylogenetic tree correspond with the number and distribution of rDNA loci. All species had a chromosome base number of x=8. The karyotype showed a significant variation among diploids with one or two 35S and one to three 5S rDNA loci. Hemizygous 5S rDNA sites have been widely observed in many Pulsatilla species. The position of the 35S rDNA loci is relatively conservative, as the 35S rDNA loci are located terminally on the short arm of acrocentric chromosomes. 5S rDNA sites showed much more variability both in number and physical location than did 35S rDNA sites. For 5S rDNA there is no preferential chromosomal position as it exhibits terminal, subterminal, interstitial or pericentromeric position, and is located either on acrocentric or metacentric chromosomes. Greater variation in the number and position of 35S and 5S rDNA loci is found in the polyploid taxa analysed. Tetraploids exhibit changes such as rDNA loss/gain. The dispersed 35S rDNA signals occurred at centromeric positions which are hotspots for chromosomal breakpoints and are also enriched for transposable elements. Therefore we postulate that the seeding of rDNA repeats to ectopic locations in the genome could be the result of transposable element activity in polyploids. Genome restructuring, especially involving 5S rDNA characterizes the evolution of Pulsatilla. Ancestral state reconstruction of number of rDNA signals brings evidence that these characters have followed increases and decreases during evolution in Pulsatilla.

karyotype evolution; phylogeny; Pulsatilla; rDNA

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