Evolutionary dynamics across discontinuous freshwater systems: Rapid expansions and repeated allopolyploid origins in the Palearctic white water-lilies (Nymphaea)

White water-lilies (Nymphaea) provide an excellent model for studying evolutionary diversification in a discontinuous habitat system, which has experienced dramatic differences in connectivity during different climatic periods. Here we explore cytogenetic and molecular variation and phylogenetic relationships in the Eurasian clade of this genus based on relative nuclear DNA contents, AFLP fingerprints, and nuclear (ITS) and cpDNA sequences. In line with some recent taxonomic treatments, our results support that the Eurasian clade consists of three species. We show that the Eurosiberian N. candida is an allopolyploid that originated at least twice by hybridization in the wide overlap zone between the mainly European N. alba lineage and the mainly Asian N. tetragona lineage. The N. alba lineage served as the chloroplast donor except for the Karelian N. candida, but the ITS region of the allopolyploid was homogenized in the direction of N. alba in all cases. The southern, widespread lineage of N. candida probably originated first, whereas the northwestern lineage, restricted to a small, previously glaciated area in Karelia, may have originated as late as in the current interglacial. We observed only low levels of DNA sequence variation within N. alba and N. tetragona, indicating rapid range expansions possibly preceeded by strong bottlenecks in glacial refugia. Our results suggest that populations of freshwater organisms found now in disjunct aquatic habitats across Eurasia were connected during previous climatic periods. These connections promoted rapid expansions of divergent lineages and repeated formation of evolutionary novelty via hybridization and polyploidization.