Polyploidization is an important speciation and evolution mechanism in ferns. Initially, new cytotypes face challenges in maintaining themselves within the majority cytotype populations. Unlike in most even-ploidy cytotypes, fern triploids are often apomictic or infertile, due to genetic imbalance. An interesting opportunity to study these phenomena has emerged with the discovery of triploid bracken (Pteridium aquilinum), a triploid fern that is fertile but not apomictic. Originally found in one Welsh population, the distribution of this cytotype in Europe is unknown as is its origin and how it maintains itself among the presumed diploid majority. We sampled 135 populations of P. aquilinum, focusing on Central Europe. Ploidy level of all samples was analyzed by flow cytometry. We compared the two cytotypes via micromorphological characters (spore and stomata size), fertility characteristics (spore abortion and proportion of populations with sporangia-bearing fronds). Additionally, genetic difference between ploidy levels was tested as well. The diploid cytotype of P. aquilinum is dominant in continental Europe with 121 entirely diploid populations found, but we also found 9 mixed and 5 entirely triploid populations. Fertile diploid and triploid plants were found only in 17.7% and 21.4% of populations, respectively. The cytotypes are distinguishable using both tested micromorphological characters, but stomata are more reliable due to overall reduced fertility. Unlike the Welsh specimen, our tested triploid has most spores aborted, ca 97.4%, compared to mean 6.0% of spores aborted in diploids. The triploid cytotype is rare and likely originated multiple times from the diploids and relies on clonal and possibly limited sexual reproduction to maintain itself. However, diploids and triploids are often genetically different within a population, indicating that the triploid may migrate between populations. Due to its vegetative growth and presumed continuous formation, the triploid cytotype is likely to remain established in Central Europe, although in small numbers.
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