cheilanthoids

Introduction

Cheilanthoids are a diverse clade most notable for their ability to colonize xeric habitats, including deserts of the New and Old World, as well as rocky habitats worldwide. These are generally smallish ferns (large individuals may be over a meter in length, but that is unusual) with leaves that are densely scaly or hairy (many Cheilanthes, Astrolepis, Bommeria), covered beneath with a wax-like flavonoid deposit (“farina”; Notholaena, Argyrochosma, Pentagramma), or protected with a thick cuticle (Pellaea; Doryopteris).

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Left: Fertile leaf of Cheilanthes lendigera, abaxial surface, showing pronounced false indusia and hairy indument. © 2008 Amanda Grusz. Center: When water-stressed, Notholaena standleyi leaves roll inwards, exposing their farina-covered under-surfaces. © Carl Rothfels. Right: The leaves of Pellaea truncata are protected with a thick cuticle. © Carl Rothfels

The cheilanthoid clade includes approximately 20 genera (as currently circumscribed) and 500 species. Generic concepts within the group have been particularly challenging, inducing Rolla and Alice Tyron to pronounce cheilanthoids the “most contentious group of ferns with respect to a practical and natural generic classification” (Tryon and Tryon, 1982). Much of the challenge of developing stable morphology-based classifications appears to be a result of convergent evolution driven by adaptation to xeric habitats (Gastony and Rollo, 1998; Kirkpatrick, 2007; Lellinger, 1989; Prado et al., 2007; Rothfels et al., 2008; Tryon and Tryon, 1973; Windham et al., 2008)—there seems to be a limited number of morphologies that allow ferns to survive and reproduce in extremely xeric conditions (basically the hairy, waxy, and thick-leaved morphologies mentioned above), and cheilanthoid ferns have explored each of them multiple times independently.

Characteristics

Cheilanthoids show a great variety of form, both in terms of gross morphology (e.g., leaf shape) and in specifics of their reproductive structures (e.g., position of the sporangia, presence or absence of a false indusium). These latter characters, in particular, have often been used as the basis for delimiting cheilanthoid genera, but are increasingly shown to be homoplasious (and not indicative of evolutionary relationships).

Almost all of the “farinose” ferns—those that have the undersurfaces of their leaves covered with a dense layer of wax-like deposits—are cheilanthoids, including members of the genera  Argyrochosma, Cheilanthes, Notholaena, and Pentagramma. Outside of the cheilanthoids this feature is rare, with Pityrogramma (in the pteridoids) being the main exception. Farina can be composed of different compounds, with certain compounds or combinations of compounds being distinctive for particular groups of ferns (Wollenweber, 1984; Wollenweber and Schneider, 2000).  See A Quick Guide to Farinose Ferns.

Discussion of Phylogenetic Relationships

Of the primary subclades of Pteridaceae, the cheilanthoids are the least well supported. This apparent lack of support is due to some uncertainty about the position of what we here interpret to be the earliest-diverging cheilanthoid branch (the Doryopteris ludens clade). While the remaining taxa are strongly supported as monophyletic, and the Doryopteris ludens clade has never been resolved in a position other than that presented here (as sister to the rest of the cheilanthoids), this placement is rarely strongly supported (Schuettpelz et al., 2007; Zhang et al., 2005).

The remainder of the cheilanthoids fall into six strongly supported primary clades that we treat (following Windham et al., 2008) as the: bommeriids; skinneri clade; myriopterids; pellaeids; notholaenids; and hemionitids.

Other Names for cheilanthoids

• Vernacular Names: cheilanthoid ferns

References

Gastony, G. J., and D. R. Rollo. 1995. Phylogeny and generic circumscriptions of cheilanthoid ferns (Pteridaceae: Cheilanthoideae) inferred from rbcL nucleotide sequences. American Fern Journal 85:341-360.

Kirkpatrick, R. E. B. 2007. Investigating the monophyly of Pellaea (Pteridaceae) in the context of a phylogenetic analysis of cheilanthoid ferns. Systematic Botany 32:504-518.

Lellinger, D. B. 1989. The ferns and fern-allies of Costa Rica, Panama, and the Chocó (Part 1: Psilotaceae through Dicksoniaceae). Pteridologia 2A:1-364.

Prado, J., C. D. N. Rodrigues, A. Salatino, and M. L. F. Salatino. 2007. Phylogenetic relationships among Pteridaceae, including Brazilian species, inferred from rbcL sequences. Taxon 56:355-368.

Rothfels, C. J., M. D. Windham, A. L. Grusz, G. J. Gastony, and K. M. Pryer. 2008. Toward a monophyletic Notholaena (Pteridaceae): Resolving patterns of evolutionary convergence in xeric-adapted ferns Taxon 57:712-724.

Schuettpelz, E., H. Schneider, L. Huiet, M. D. Windham, and K. M. Pryer. 2007. A molecular phylogeny of the fern family Pteridaceae: assessing overall relationships and the affinities of previously unsampled genera. Molecular Phylogenetics and Evolution 44:1172-1185.

Tryon, R. M., and A. F. Tryon. 1973. Geography, spores, and evolutionary relations in the cheilanthoid ferns. Pages 145-153 in The phylogeny and classification of ferns. (A. C. Jermy, J. A. Crabbe, and B. A. Thomas, eds.). Academic Press, New York.

Tryon, R. M., and A. F. Tryon. 1982. Ferns and Allied Plants with Special Reference to Tropical America. Springer-Verlag, New York.

Windham, M. D., L. Huiet, E. Schuettpelz, C. J. Rothfels, J. Beck, A. L. Grusz, G. Yatskievych, and K. M. Pryer. 2008. Using plastid and nuclear DNA sequences to redraw generic boundaries and demystify species complexes in cheilanthoid ferns. American Fern Journal (in review).

Wollenweber, E. 1984. Exudate flavonoids of Mexican ferns as chemotaxonomic markers. Rev. Latinoamer. Quim. 15:3-11.

Wollenweber, E., and H. Schneider. 2000. Lipophilic exudates of Pteridaceae -- chemistry and chemotaxonomy. Biochemical Systematics and Ecology 28:751-777.

Zhang, G., X. Zhang, and Z. Chen. 2005. Phylogeny of cryptogrammoid ferns and related taxa based on rbcL sequences. Nordic Journal of Botany 23:485-493.