Cystopteris Bernh.

Smith, Alan R. & Kessler, Michael, 2018, Prodromus of a fern flora for Bolivia. XXVIII. Cystopteridaceae, Phytotaxa 344 (1), pp. 75-79 : 76

Cystopteris Bernh. in Schrader, Neues J. Bot. 1(2): 26. 1805.

Cystopteris is monophyletic, and characterized by the hoodlike indusia attached at the proximal side of sorus bases and arching over the sporangia. The genus includes the rare segregate genus Cystoathyrium Ching , in China, a monotype now known to nest within Cystopteris ( Wei & Zhang 2014) . In mature sori the indusia are often partially hidden and difficult to see. Cystopteris may also be recognized by its pale green, often seasonal leaves; it occurs at high elevations. Cystopteris is often confused with Woodsia , which usually has less divided blades, equilateral (vs. slightly inequilateral) pinnae, denser, more conspicuous indument, and veins that terminate in submarginal hydathodes ( Sundue & Rothfels 2014). Athyrium differs in being more robust and having elongate or J-shaped sori. Members of Thelypteridaceae usually have more regularly 1-pinnate to 1-pinnate-pinnatifid lamina, more rounded pinna margins, persistent leaves, usually conspicuous, acicular, hyaline hairs on all parts of the leaves, and stiffer, less succulent petioles ( Smith & Kessler, 2017). Cystopteris is sister to Acystopteris ( Rothfels et al. 2012a, 2013), and these two genera, in turn, are sister to Gymnocarpium . Cystopteris is of nearly worldwide distribution with at least 26 species (Rothfels 2012, PPG I 2016), two of which occur in Bolivia.

Cystopteris (at the time including Acystopteris ) was monographed by Blasdell (1963), who treated 10 species and six hybrids (some of which are now regarded as good species; see Haufler et al. 1993). Since then, additional species, mostly segregates of C. fragilis s.l., have been recognized, especially in North America north of Mexico (for a total of nine species; Haufler et al. 1993) and Mesoamerica, as well as eastern Asia (11 species; Wang & Haufler 2013). In the Neotropics, and within a floristic context, Cystopteris was treated by Stolze et al. (1994) for Ecuador, by Tryon & Stolze (1991) for Peru, and by Mickel & Smith (2004) for Mexico. All of these authors subsumed C. diaphana within C. fragilis s.l. More recently, Arana & Mynssen (2015) presented a taxonomic treatment for Brazil and Cono Sur ( Argentina and Chile), recognizing three species; two of these are not known from Bolivia: C. ulei Christ (known only from Goiás) and C. apiiformis Gand. (mainly southern Argentina, to Tierra del Fuego, and southern Chile, also Kerguelen). It seems highly likely, from both descriptions and specimens we have seen, that Cystopteris , at least in the Andes, is mostly C. diaphana , as circumscribed here.

Cystopteris species are often polyploid—tetraploid or hexaploid—and hybridity is probably rampant, with co-occurring taxa. Allopolyploidy has been shown to be common, resulting in numerous allotetraploids of independent origin ( Rothfels et al. 2014, 2017) and largely without names, if strict definitions of species were to be applied.A sterile allopolyploid intergeneric hybrid with Gymnocarpium , × Cystocarpium Fraser-Jenk. , from the French Pyrenees has recently been discovered ( Fraser-Jenkins 2008, Rothfels et al. 2015); the two parental genera were estimated to have diverged ca. 60 million years ago. This remarkable discovery suggests that ferns, and perhaps other plants with abiotically mediated fertilization, may have evolved reproductive incompatibilities more slowly than seed plants, perhaps because they lack many of the premating isolation mechanisms that characterize seed plants ( Rothfels et al. 2015).

Many species of Cystopteris are adapted to extreme habitats by somewhat succulent trophopods (persisting petiole bases) combined with rapid development of thin, herbaceous, energetically “cheap” leaves. In the dry season, the plants survive by water stored in the trophopods, while the leaves dry and wither. A study on C. fragilis in Switzerland ( Gämperle & Schneller 2002) found that local populations are genetically differentiated and have different timing of leaf development, pointing to local adaptations and adding a further level of complexity in the genus.