A unique diploid – triploid contact zone provides insights into the evolutionary mechanisms of cytotype coexistence in flowering rush (Butomus umbellatus)

Polyploidisation is an important evolutionary force in land plants. Due to its recurrent incidence, many plant species retain individuals of two or more different ploidy levels. However, particular ecological and evolutionary mechanisms facilitating intraspecific cytotype coexistence have been identified for just a handful of species and cannot yet be generalised. Our pilot data have revealed a unique complex of mixed diploid-triploid populations of the marshland perennial Butomus umbellatus in the Eastern Slovak Lowland (eastern Central Europe). Intensive flow-cytometric ploidy screening of 1,230 individuals sampled from 72 populations was conducted at both regional and local (within-population) scales to assess cytotype distribution patterns. Vegetation assessments along with phenotypic comparisons of cytotypes directly in the field and later under common garden cultivation served to provide insight into mechanisms of cytotype coexistence. Altogether 42 % of the sampled populations were mixed-ploidy, pointing to unexpectedly high rates of diploid-triploid coexistence. While the cytotype distribution was random at the regional scale, significant spatial clustering occurred at the local scale. No ecological niche differences between the cytotypes were detected. Triploids attained greater values of several morphological characters both in the field and under cultivation, differences in the shape of inner tepals even show potential for cytotype discrimination. Both cytotypes exhibited high and comparable investments into clonal traits, reproductive assurance provided by asexual reproduction likely plays a key role in cytotype coexistence and triploid predominance. The common cytotype coexistence in this region seems to be also facilitated by periodical seasonal floods promoting transport of vegetative propagules among populations (i.e. metapopulation dynamics) and providing long-term continuity of favourable sites by recurrent disturbances.