Flora of North America species comparison

Long recognized as Cystopteris fragilis var. mackayi, C. tenuis was returned to species status by R. C. Moran (1983b). It is probably an allotetraploid originating from C. protrusa and an extinct diploid related to C. fragilis (C. H. Haufler 1985; C. H. Haufler and M. D. Windham 1991).

Cystopteris tenuis is common in eastern North America and less frequent at the northern and western perimeter of its range. In the center of its distribution (Minnesota, Iowa, Illinois, Wisconsin, Indiana, Ohio, Pennsylvania), the narrow, elliptic pinnae angled toward the blade apex and the rounded teeth make C. tenuis relatively distinct from C. fragilis and C. protrusa (although the early season, sterile leaves of C. protrusa often resemble those of C. tenuis). In the west and especially in the northeast, C. tenuis and C. fragilis are difficult to distinguish. For the most part, C. fragilis is confined to higher latitudes and elevations than C. tenuis, but the two species can be sympatric and occasionally form sterile tetraploid hybrids. Cystopteris protrusa and C. tenuis are infrequently sympatric, but where they are, sterile triploid hybrids can occur. Hybrids between C. tenuis and C. tennesseensis are recognized as C. × wagneri (R. C. Moran 1983). Hybridization between C. tenuis and C. bulbifera has also been reported (R. C. Moran 1982b). This hybrid, C. × illinoensis R. C. Moran, is known only from the type and needs to be studied further.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Cystopteris fragilis is most often confused with C. tenuis in the east and C. reevesiana in the southwest. Habitat and geography, as well as the morphologic features discussed in the key, usually serve to separate these taxa. For instance, C. fragilis is more likely to be found on cliffs whereas the other species prefer boulders and soil (C. fragilis occurs at higher elevations and/or latitudes than the other species). These distinctions can be confounded when C. fragilis forms hybrids with sympatric species. Sterile pentaploid plants have been discovered where C. fragilis overlaps with C. laurentiana, tetraploid hybrids are likely where C. fragilis occurs with C. tenuis, and triploids may form where C. fragilis is found with C. reevesiana. Even after segregating relatively distinct elements such as Cystopteris protrusa, C. reevesiana, and C. tenuis, and identifying sterile hybrids, C. fragilis still remains a polymorphic and complex taxon that probably contains a number of natural, cryptic evolutionary units. For example, morphologically distinct hexaploid cytotypes have been reported (C. H. Haufler and M. D. Windham 1991). These occur as isolated and disjunct populations in Ontario, Alaska, Arizona, Colorado, Montana, and Wyoming. Isozymic profiles of each of these populations indicate that the hexaploids are polyphyletic and should not be accorded species status.

The presence of verrucate spores (as opposed to the normal spiny spores) has been used to circumscribe Cystopteris dickieana. Although genetic analyses have not been undertaken, we think the verrucate spore is probably a recessive feature controlled by one or a few genes. While present at low frequency in much of the range of C. fragilis, verrucate spores are particularly prominent in the Great Plains. Perhaps in this region the genetic combinations specifying verrucate spores have been fixed. Following R. F. Blasdell (1963), C. dickieana is also considered here to be conspecific with C. fragilis because (1) early stages in the development of spiny spores can appear verrucate (A. C. Jermy and L. Harper 1971), (2) the hexaploid cytotypes discussed above always have verrucate spores, regardless of their parentage, (3) individuals with verrucate spores can be found in populations that are otherwise uniformly spiny-spored, and (4) individuals and populations that have verrucate spores are not otherwise (morphologically, ecologically, or genetically) distinct from those that have spiny spores.

Especially in the western portion of its North American range (British Columbia, Washington, Montana, Idaho, Oregon, California), Cystopteris fragilis appears to be developing morphologically and ecologically distinctive variants. Hybrid individuals with aborted spores have been discovered, and plants from these areas increasingly tend to grow on both soil and rock and to have slightly different morphologies on the two substrates. These variants intergrade, however, and are not sufficiently distinct to warrant species status. This polymorphic polyploid is probably actively speciating at the tetraploid level, perhaps through gene silencing (C. R. Werth and M. D. Windham 1991).

(Discussion copyrighted by Flora of North America; reprinted with permission.)