Species boundaries and hybridization in central-European Nymphaea species inferred from genome size and morphometric data.

Aquatic plants often pose considerable taxonomic problems. The genus Nymphaea (water lily) in central Europe is a good example of this in that their morphological similarity blurs the boundaries between species, which in addition are highly phenotypically plastic and possibly hybridize. The situation is further complicated by the occurrence of many garden cultivars. We used DNA flow cytometry and multivariate morphometrics (both distance-based and geometric) to obtain an insight into their phenotypic variation, identify taxon-specific characters and assess the frequency of hybridization in water lilies collected from 72 localities in the Czech Republic. For comparative purposes, we also included 34 garden cultivars. Flow cytometric measurements revealed a 45% difference in the holoploid genome sizes of N. alba and N. candida, which makes it easy to reliably separate them. In addition, the great majority of garden cultivars have distinctly smaller genomes than their native counterparts. Interspecific hybridization under natural conditions was quite rare (only ~1.8% of the individuals cytotyped corresponded to N. ×borealis), and involved both reduced and unreduced gametes. Discriminant analyses revealed cultivar- and species-specific morphological characters, which allow accurate determination of the samples. Gynoecium and stamen characters had the greatest taxonomic value. The recognition of N. ×borealis on the basis of morphological characters is uncertain. Our study shows that genome size may help to resolve the long-standing taxonomic complexities in this important component of the temperate aquatic flora.