Molecular evolution and organization of ribosomal DNA in the hawkweed tribe Hieraciinae using ITS, ETS and 5S markers.

The Asteraceae subtribe Hieraciinae (tribe Cichorieae) consists of four genera: Hieracium s.str., Pilosella, Andryala and Hispidella. Their evolutionary relationships have been addressed previously based on ITS/ETS rDNA, chloroplast and low-copy nuclear markers. Here, we studied variation in the organization and dynamics of rDNA loci in diploid representatives of genera, species and, in some cases, populations of the Hieraciinae using a cytogenetic approach. The most common pattern showed two 45S rDNA loci and one locus of 5S rDNA per haploid genome; the latter was usually situated on the same chromosome bearing also a 45S rDNA locus. A number of deviations from this pattern were observed in Hieracium, some of them even occurred at the population level. These differences concerned additional or lost 45S rDNA loci and an additional locus of 5S rDNA; some of them occuredoccurred in the hemizygous state. These results were compared with molecular phylogenies based on non-coding sequences of the respective loci (ITS and ETS for 45S rDNA loci, and the non-transcribed spacer NTS for 5S) that are sufficiently variable to allow inferring interspecific relationships. More specifically, we asked whether the patterns of primary sequence divergence correlate with the number and position of loci on chromosomes, whether phylogenies based on 5S and 45S rDNA are topologically congruent or reflect different aspects of the evolutionary history of the subtribe, and finally whether the direction of homogenization via concerted evolution is the same for both multi-copy arrays. The most prominent example of differential homogenization was found in Hieracium intybaceum, an ancient hybrid formed by wide hybridization with a nowadays extinct genus. All nuclear markers investigated so far belonged to the extinct taxon, however, here we observed homogenization of 5S-NTS towards the genus Hieracium despite the co-occurrence of the locus with one of the 45S rDNA loci on the same chromosome. Generally, primary sequence divergence did not well match the number or position of loci organization, the latter being far more dynamic indicating that molecular sequence evolution is relatively independent of the particular localization on chromosomes.