Gaga, a New Fern Genus Segregated from Cheilanthes (Pteridaceae)

Ongoing molecular phylogenetic studies of cheilanthoid ferns confirm that the genus Cheilanthes (Pteridaceae) is polyphyletic. A monophyletic group of species within the hemionitid clade informally called the “C. marginata group” is here shown to be distinct from its closest relatives (the genus Aspidotis) and phylogenetically distant from the type species of Cheilanthes. This group is here segregated from Cheilanthes as the newly described genus, Gaga. In this study, we use molecular data from four DNA regions (plastid: matK, rbcL, trnG-R; and nuclear: gapCp) together with spore data to circumscribe the morphological and geographical boundaries of the new genus and investigate reticulate evolution within the group. Gaga is distinguished from Aspidotis by its rounded to attenuate (vs. mucronate) segment apices, minutely bullate margins of mature leaves (vs. smooth at 40 +), and less prominently lustrous and striate adaxial blade surfaces. The new genus is distinguished from Cheilanthes s. s. by its strongly differentiated, inframarginal pseudoindusia, the production of 64 small or 32 large spores (vs. 32 small or 16 large) per sporangium, and usually glabrous leaf blades. A total of nineteen species are recognized within Gaga; seventeen new combinations are made, and two new species, Gaga germanotta and Gaga monstraparva, are described. Keywords—Cheilanthoid ferns, hybridization, reticulate evolution, matK, Mexico, polyploidy, spore measurements. Fern systematics has changed radically in thepast decadedue, in largepart, to the rapidpace ofmolecular phylogenetic studies. Family-level classifications have been overhauled (Smith et al. 2006; Christenhusz et al. 2011; Rothfels et al. 2012), and substantial effort has been directed toward reassessing generic boundaries. This has resulted in the recognition of many new genera across the polypod ferns (e.g.MoranopterisR. Y.Hirai & J. Prado; Alansmia M. Kessler, Moguel, Sundue & Labiak; Mickelia R. C. Moran, Labiak& Sundue; LeucotrichumLabiak; SerpocaulonA. R. Sm.; Araiostegiella M. Kato & Tsutsumi). One notable exception to this trend involves cheilanthoid ferns (Pteridaceae), a monophyletic group of 400 + species that have diversified in semi-arid habitats normally avoided by ferns.Morphological convergence is rampant in this group (Gastony and Rollo 1998), which has been referred to as “the most contentious group of ferns with respect to a practical and natural generic classification” (Tryon and Tryon 1982: 248). Despite a growing number of phylogenetic studies indicating that most of the major genera are not monophyletic (Gastony and Rollo 1995, 1998; Schuettpelz et al. 2007; Kirkpatrick 2007; Prado et al. 2007; Zhang et al. 2007; Rothfels et al. 2008; Windham et al. 2009; Yesilyurt and Schneider 2010; LinkPerez et al. 2011; Eiserhardt et al. 2011), the boundaries of most cheilanthoid genera remain unchanged. The main impediment to generic reorganization of cheilanthoid ferns is the large and loosely defined genus Cheilanthes Sw. Recent phylogenetic studies have documented the extensive polyphyly of Cheilanthes s. l., with species traditionally assigned to this genus appearing in every major clade of cheilanthoid ferns (Gastony and Rollo 1995, 1998; Schuettpelz et al. 2007; Zhang et al. 2007; Kirkpatrick 2007; Prado et al. 2007; Rothfels et al. 2008; Eiserhardt et al. 2011). These run the gamut from single, morphologically anomalous species nested within or among other cheilanthoid genera to diverse, well supported clades whose relationships to Cheilanthes s. s. are tenuous at best. Among the latter is what has been called the “Cheilanthes marginata group” (Mickel and Smith 2004; Li et al. 2011), a clade of about twenty species that represents an early diverging lineage within the hemionitid ferns (Windham et al. 2009; Windham et al. in prep.). This group is sister to the small (four species) genus Aspidotis (Nutt. ex Hook.) Copel. and phylogenetically isolated from the type species of Cheilanthes, C. micropteris Sw. (Kirkpatrick 2007; Eiserhardt et al. 2011; Windham et al. in prep). Given our current understanding of phylogenetic relationships among cheilanthoid ferns, inclusion of the “C. marginata group” in Cheilanthes is untenable because the resultant clade also encompasses thegenusHemionitis,whichhaspriority overCheilanthes. Such an expansion of Hemionitis would require hundreds of new combinations, subsume several diverse, well-characterized genera (e.g.Doryopteris J. Sm. andAdiantopsis Fée), andwould be undefinable morphologically (Windham et al. in prep.). Other less disruptive options include subsuming the “C. marginata group” under Aspidotis (its sister clade) or recognizing it as a distinct genus. BecauseAspidotis is a relatively distinct andmorphologically cohesive group, we have chosen to segregate the “C. marginata group” as a new genus herein named Gaga (see Taxonomic Treatment). Although about twenty species have been attributed to this group (Mickel and Smith 2004), only one has been sampled in previous phylogenetic studies (Kirkpatrick 2007; Rothfels et al. 2008; Eiserhardt et al. 2011), and the actual number of taxa involved is uncertain due to extensive hybridization, polyploidy, and apomixis. In this study, we use spore measurements to estimate the ploidy level and reproductive mode of each species in the genus. Sequence data from three plastid regions (matK, rbcL, and trnG-R) are used to assess the monophyly of the group, estimate species numbers and relationships, and assist in identifying maternal progenitors of hybrids. We also use sequences from the nuclear gapCp region, in conjunction with the spore data, to examine patterns of reticulate evolution within the genus and propose a new species level taxonomy for this group of cheilanthoid ferns. Materials and Methods Taxon Sampling—For spore analyses, a total of 44 samples from the “Cheilanthes marginata group” and three Aspidotis specimens were included, 42 of which were also sampled for the molecular study (Appendix 1). A total of 75 specimens (Appendix 1) representing all known species from the “Cheilanthes marginata group” (including one holotype and five isotypes) were sampled for the molecular phylogenetic study; 58 of these were previously included in a DNA barcoding study by Li et al. (2011). Materials for DNA extraction were obtained either from accessioned herbarium sheets or from the vouchered Fern Lab Database silica-dried tissue archive (fernlab.biology.duke.edu). Sequences for three plastid loci (matK, rbcL, and trnG-R) and one nuclear locus (gapCp) were