Snow White and Rose Red: Studies on the contrasting evolutionary trajectories of the genera Leucanthemum Mill. and Rhodanthemum B.H. Wilcox & al. (Compositae, Anthemideae)
Plant systematics, the study of taxonomy, phylogeny and evolutionary processes in plants has undergone considerable progress in the last decades. The application of modern molecular approaches and DNA-sequencing techniques in the field has led to an improved inventory of biodiversity and a better understanding of evolutionary processes shaping the biological diversity on our planet. The increased availability of molecular and genomic data has particularly facilitated the investigation of shallowly diverged and taxonomically complex taxon-groups, which is challenging due to minor morphological differences, low genetic differentiation and/or hybridization among taxa. The present thesis investigates species delimitation, hybridization and polyploidization in the recently diverged genera Leucanthemum Mill. and Rhodanthemum B.H. Wilcox & al. of the subtribe Leucantheminae K.Bremer & Humphries (Compositae, Anthemideae) by applying Sanger-, 454-pyro-, and restriction site associated DNA (RAD) sequencing, as well as AFLP-fingerprinting and morphometric analyses. The first two parts are focusing on species delimitation and hybridization in the closely-knit taxon groups around L. ageratifolium Pau and R. arundanum B.H. Wilcox & al., respectively. Various analyses based on AFLP fingerprinting, RADseq and multi-locus sequence data demonstrate that the robustness of species delimitation results is considerably influenced by the intensity of hybridization among species and the number of hybrid individuals included. Therefore, a step-by-step approach is performed in both studies, with an initially step of identification and subsequent removal of hybrid individuals, followed by application of different species-delimitation methods. This strategy results in the reliable identification of independent species, subspecies and nothospecies in both taxonomically complex plant groups. The third part of the present thesis compares the contrasting evolutionary trajectories of diploid representatives of both genera in a more comprehensive phylogenetic study. Specific hypotheses for the formation of polyploids in plants are proposed and evaluated to find factors that promote polyploidization in certain plant groups (e.g., Leucanthemum) and not in others (e.g., Rhodanthemum). Multi-locus sequence data from 127 accessions of the subtribe Leucantheminae unveil a significantly higher genetic divergence and hybridization signal among diploid lineages of Leucanthemum compared to Rhodanthemum, in spite of a similar crown age and diversification pattern during the Quaternary. The study demonstrates the importance of genetic differentiation among diploid progenitors and their concurrent affinity for natural hybridization for the formation of a polyploid complex. Furthermore, the role of climate-induced range overlaps on hybridization and polyploid speciation during the Quaternary is discussed
