The widely distributed Cystopteris fragilis brittle bladder-fern, complex is a challenging system for studying polyploid evolution, cytotype diversity, distribution and ecology. Hanušová et al. screen cytotypes over the global range of the species: common and widespread tetraploids and hexaploids coexist, with 40% mixed-ploidy populations.
High ploidy-level diversity matches the well-documented patterns in some angiosperms. The ploidy coexistence is not driven by environmental factors, but it could be facilitated by the perennial life-form of the species, its reproductive modes and efficient wind dispersal of spores. Monoploid genome size (Cx) comparisons indicate independent origins of hexaploids and/or inter-ploidy gene flow in mixed-ploidy populations.