| There are approximately 60 gnetalean joint-firs, Ephedra, morphologically highly reduced shrubs. Most species occur in true deserts, but a few occur in semi-arid habitats. Phylogenetic relationships among the morphologically diverse species of Ephedra are poorly understood. New World species of Ephedra are reviewed through fieldwork, herbarium study, micromorphological examination of stem and seed cuticles, scanning electron microscopy of pollen, and DNA sequence data. The validity of the original subdivision of the genus Ephedra into three sections as proposed by Stapf in 1889 was tested with these different types of information. While stomatal opening shape and epidermal cell shape has some limited concordance with Stapf s system, particularly section Alatae, other characters appear more tied to plant habit, especially those found in extreme arid environments. A considerable variation in seed shape, micromorphology, and bract morphology was also found. Pollen in Ephedra is typically ellipsoidal, polyplicate with straight ridges, but dimorphic grains have a highly folded ectexine. Pollen dimorphism is documented for Ephedra trifurca, Ephedra torreyana, Ephedra boelkei and an intermediate form between Ephedra torreyana and Ephedra funerea, all of section Alatae. Revisionary study of New World Ephedra recognizes 22 species and three interspecific hybrid taxa. Sequence data from the nrDNA ITS-1 and the plastid rps4 gene were used to infer phylogenetic relationships in Ephedra. Both maximum parsimony and maximum likelihood analyses of these sequences resulted in a well resolved phylogeny that supports the monophyly of Ephedra, but not its subdivision into the three sections traditionally recognized. Character reconstruction of ovulate strobilus types indicates that fleshy bracts are ancestral within Ephedra, with shifts to dry, wing-bracted strobili having occurred multiple times. Low levels of sequence divergence suggest that either Ephedra underwent a recent and rapid ecological radiation in North America, or that ribosomal DNA evolution in North American Ephedra has been highly conservative. |
