Cheilanthoid Ferns (Pteridaceae: Cheilanthoideae) in the Southwestern United States and Adjacent Mexico-A Molecular Phylogenetic Reassessment of Generic Lines

Cheilanthoids are the most commonly encountered fe rn species of the arid southwest and other xeric habitats throughout the world. Cheilanthes, Notholaena, Pellaea, and Bommeria are the best known southwestern genera, but some authors recognize segregate genera such as Argyrochosma, Aspidotis, Astrolepis, and Pentagramma. Others reject distinctions among some of these genera as artificial , leaving cheilanthoid generic concepts in a state of flux. This unsettled taxonomy is often attributed to morphological homoplasy associated with adaptation to xeric habitats , suggesting the need for new analyses that do not depend on potentially misleading morphology. Nucleotide sequences of the maternally inherited, chloroplast-encoded rbcL gene from 57 species that bear on the relationships of the cheilanthoids of the southwest were c1adistically analyzed under the optimality criterion of maximum parsimony. The results provide new insights into phylogenetic re lationships and generic circumscriptions of these ferns . Mexican Llavea cordifolia is rejected from the cheilanthoids, traditional Cheilanthes, Notholaena, and Pellaea are polyphyletic, and the segregations of A rgyrochosma, Aspidotis, Astrolepis, and Pentagramma are supported. To assess confidence in these conclusions, results of the rbcL-based analys is are compared with those based on ITS sequences of biparentally inherited nuclear ribosomal DNA (nrDNA) for a subset of cheilanthoid taxa. These two data sets yield remarkably congruent topologies at shallower phylogenetic levels, suggesting that previous taxonomic problems in this group may indeed be attributable to diffi culties in interpreting the taxonomic significance of morphological characters . Disagreement at deeper levels of the topologies suggests the need to incorporate data from less rapidly evolving nrDNA regions.