The highest frequency of polyploidy among plants is considered to occur in the Pteridophytes. In this study, we focused on polyploidy displayed by a specific fern taxon, the genus Pteris L. (Pteridaceae), comprising over 250 species. Cytological data from 106 Pteris species were reviewed. The base number of chromosomes in Pteris is 29. Polyploids are frequently found in Pteris, including triploids, tetraploids, pentaploids, hexaploids, and octoploids. In addition, an aneuploid species, P. deltodon Bak., has been recorded. Furthermore, the relationship between polyploidy and reproductive biology is reviewed. Among these 106 Pteris species, 60% exhibit polyploidy: 22% show intraspecific polyploidy and 38% result from polyploid speciation. Apogamous species are common in Pteris. Diploids are the most frequent among Pteris species, and they can be sexual or apogamous. Triploids are apogamous; tetraploids are sexual or apogamous. Most Pteris species have one to two ploidy levels. The diverse ploidy levels suggest that these species have a complex evolutionary history and their taxonomic problems require further clarification. 1. Introduction Polyploidy provides a rapid route for species evolution and adaptation [1, 2]. Taxa arising from polyploidy are usually characterized by divers gene expression [3]. This variation in gene expression also has effects on ecological traits, which play an important role in speciation because a specialised niche is a key factor in the formation of new taxa [4–8]. For example, ecological isolation can allow taxa with genetic variation to become segregated [9]. It is estimated that the highest frequency of polyploidy is exhibited in ferns. The frequency of polyploid speciation in ferns is 31%, which is much higher than 15% in angiosperms [10]. In ferns, a special form of asexual reproduction known as apogamy is common [11, 12]. Apogamy provides a bypass to crossover mispairing of chromosomes and stabilises the reproduction of polyploids [13–15]. During metaphase I of meiosis, these polyploids present multivalents, which may have difficulty separating equally. Apogamous species are clonal hybrid genotypes, and, as a result, apogamy creates reproductive barriers that prevent gene flow among closely related taxa, thereby facilitating sympatric speciation [16]. Each taxon maintains an independent genetic lineage, leading eventually to a new species. Pteris L. (Pteridaceae) is a cosmopolitan fern genus with over 250 species. Some Pteris species have several different ploidy levels and are found in several geographical areas, such as
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