Molecular phylogenetics, morphology and a revised classification of the complex genus Saxifraga (Saxifragaceae)

Saxifraga, the most species-rich and taxonomically complex genus of Saxifragaceae, is a characteristic component of temperate to polar climatic zones and of montane to alpine vegetation belts in mountain ranges of the Northern Hemisphere. The genus encompasses more than 440 species, which display notable diversity in growth form, vegetative and reproductive characters, as well as in micromorphology of pollen grains and seeds. Using a large taxon set including representatives attributed to nearly all recognised subgenera, sections and subsections of Saxifraga (altogether 254 species), as well as a broad coverage of outgroup taxa, we inferred evolutionary relationships within Saxifraga, explored the reasons of the striking inconsistencies between previous taxonomic treatments of Saxifraga, estimated the degree of homoplasy in characters frequently employed for classification, assessed the role of hybridisation in the evolution of the genus and, finally, provided a phylogeny-based framework upon which to revise its taxonomy. The phylogenetic trees based on nuclear ribosomal internal transcribed spacer (ITS) and plastid trnL-trnF DNA markers, including more than 460 newly generated sequences, were generally highly congruent, except within a single clade (sect. Saxifraga), in which incomplete lineage sorting and presumably allopolyploid speciation plays an important evolutionary role. Major lineages of Saxifraga are concordantly resolved by the DNA markers from both organelles with mostly strong node support. Our molecular phylogenetic results support the recognition of at least 13 sections and 9 subsections within Saxifraga. A part of these lineages agrees well with previously recognised infrageneric groupings, whereas others are differently delineated. Some of the groups identified by molecular phylogenetics are characterised only by the combination of different morphological characters. Frequently, micromorphological characters support the clades in the phylogenetic tree comparatively well. We here provide new ranks for three infrageneric epithets, namely for sect. Saxifraga subsect. Androsaceae and subsect. Arachnoideae and sect. Porphyrion subsect. Squarrosae. Finally, our study identifies remaining taxonomic uncertainties within Saxifraga.