Frozen ploidies: a phylogeographical analysis of the Leucanthemopsis alpina polyploid complex (Asteraceae, Anthemideae)

Abstract

Polyploidization plays an important role in the diversification processes of many alpine plant groups. The spatial distribution of cytotypes studied in conjunction with phylogeographical analyses based on DNA sequencing and fingerprinting can provide valuable insights into the evolutionary history of plant groups during the glacial cycles of the Pleistocene by pinpointing candidate areas for glacial refugia and assisting in the discussion of temporal and spatial patterns of polyploidization and expansion processes. With its distribution range covering the whole Alpine range and other European mountain systems (Pyrenees, Apennines, Carpathians), the extensive morphological variability exhibited and the diversity of ploidy, Leucanthemopsis alpina (Asteraceae, Anthemideae) is a suitable model system for analysing polyploid diversification in high mountains. We have studied ploidy for populations throughout the distribution range of the species by 4',6-diamidino-2-phenylindole (DAPI) flow cytometry, used sequence information from plastid intergenic spacer regions, and AFLP fingerprinting for phylogeographical analyses and multivariate statistics for inference of patterns in morphological characteristics. Diploid and tetraploid plants were observed throughout the Alps, whereas populations in the Pyrenees harboured only tetraploid and hexaploid cytotypes. An overwhelming number of populations were shown to contain just one cytotype ('homoploid sites'), and only single populations with either a mixed constitution ('heteroploid sites') or with a triploid individual were encountered. Plastid sequence data revealed potential glacial refugia in the Maritime Alps, central Switzerland (Pennine and Lepontine Alps), the Dolomites and the Lower Tauern (Austria). Extensive sharing of plastid haplotypes across ploidies, the geographical restriction of haplotype groups to distinct regions of the Alps irrespective of ploidy and the similarity of geographically adjacent but cytologically different populations in AFLP fingerprint patterns and morphology argue for the recurrent local formation of tetraploids by autopolyploidy. The continuous but geographically correlated morphological and genetic variation observed across cytotypes is considered to be best suited by a classification of L. alpina comprising seven allopatrically distributed, heteroploid subspecies.