# The Chromosome Number and rDNA Loci Evolution in _Onobrychis_ (Fabaceae)
## 3 Discussion
Wild and domesticated plants are shaped by evolutionary responses to different selection pressures, which may lead to genetic diversification between these species forms [62, 63]. The origin of tetraploid _O. vicifolia_ (allo- or autopolyploidy) and its genome composition are unknown. In allopolyploids, 355 rDNA often undergoes homogenisation towards either the maternal or paternal ribotype, and both scenarios are well-documented [34, 58, 64]. In addition, the extent of homogenisation and conversion can differ between older and recently formed polyploid accessions of the same species [65, 66], and thus, the presence of _O. vicifolia_ lineages that differed in ribotype variants might suggest multiple origins of this tetraploid similar to the case in other polyploids [67, 68]. The analyses of basic chromosome numbers in the phylogenetic context showed that \(x=8\) was reconstructed as ancestral for all analysed species as well as for species from subgenera _Sisyrosema_ and _Onobrychis_; however, the patterns of basic chromosome number evolution were different in these two subgenera. In both of them, species with \(x=8\) and \(x=7\) were revealed, but most _Sisyrosema_ subgenus species have \(x=8\), whereas those belonging to the subgenens _Onobrychis_ have \(x=7\). One event of descending dysploidy at a relatively deep node was inferred in the subgenus _Onobrychis_. In contrast, three independent events of descending dysploidy were reconstructed at the tips of the tree in the subgenus _Sisyrosema_. The recurrent events of dysploidy were recovered for several genera. Often the derived basic chromosome number appeared several times during the diversification and evolution of a genus, e.g., in _Crepis_ and _Artemisia_[16; 69]. On the other hand, in taxa, such as _Helianthemum_ or _Chenopodium_, the basic chromosome number is a genus-specific feature [18; 70]. Three events of whole genome duplication were reconstructed for the analysed _Onobrychis_, one for the species with \(x=8\) (subgenus _Sisyrosema_) and two for the species with \(x=7\) (subgenus _Onobrychis_). In subgenus _Onobrychis_, the reconstructed polyploidisation events might suggest a common tetraploid ancestor for \(O\). _transcauasia_, \(O\). _bibersteinii_ and cultivated \(O\). _vicitiglia_, as well as a common tetraploid ancestor for \(O\). _aliissima_, \(O\). _inermis_, \(O\). _cyri_ and the wild accession of \(O\). _vicitiglia_. Since only one type of nrITS was amplified from the polyploid _Onobrychis_, this only suggests that the tetraploids, which group together, probably share one parental taxon. Thus, the data do not allow inferences of independent or common origins of analysed groups of polyploids. Moreover, [PERSON] et al. [1] suggest that the _Onobrychis_ taxonomy is over-complicated by the existence of synonyms and spurious subspecies. For example, \(O\). _pyrenaica_, \(O\). _aliissima_, \(O\). _arenaria_, \(O\). _inermis_ and \(O\). _montana_ might all be synonyms for \(O\). _vicitiglia_. In some cases, rDNA loci are robust chromosome markers, allowing comparative analyses of karyotypes, especially of wild and neglected species [22; 71; 72; 73]. In angiosperms, most often 35S rDNA loci are placed in the subterminal while most 5S rDNA loci are in the interstitial region of the chromosome [21; 74]. In contrast, the chromosomal distribution of these loci is usually opposite, with 35S rDNA localised interstitial and 5S rDNA terminal in _Onobrychis_. Regarding diploid _Onobrychis_, both these from the subclade Ia and Ib showed diverse patterns of rDNA loci number and distribution. While in subclade Ib relatively few events of rDNA loci reorganisation (one duplication of 35S rDNA and one or two repositionings of 5S rDNA loci) were retrieved, in subclade Ia nearly every species showed different patterns of rDNA loci organisation with duplication or even triplication of their number being the most common. The patterns of rDNA loci evolution often differ between evolutionary lineages [75].