Objectives of the proposed research A. Phylogenetic questions. Recent analyses of rbcL and morphological data, both alone and together, have clearly demonstrated that basal relationships among ferns remain virtually unknown. We have designed a revised dataset that includes critical basal taxa that were not included in earlier studies (while concurrently reducing the number of taxa in the well-supported derived "polypodiaceous" clade), and that will also include information from four additional genes - two from the chloroplast (SSU cpDNA [16S] and atpB) and two from the nuclear genome (SSU nrDNA [18S] and LSU nrDNA [26S]). Morphological characters that are potentially informative for the taxa under consideration will be taken primarily from Pryer et al. (109). Specific phylogenetic questions that we hope to resolve with these additional data are (cf. Fig. 1): A.1. Is Osmundaceae the most basal leptosporangiate family? A.2. How are the gleichenioid and dipteroid ferns related and what is their relationship to Matonia? A.3. Although the monophyly of the heterosporous ferns (Marsileaceae and Salviniaceae) is now clearly resolved (53, 109), to which group of homosporous ferns are they most closely related? The Schizaeaceae and Hymenophyllaceae have been postulated as close relatives to the Marsilaeaceae and Salviniaceae, respectively; is there any support for a close association of the heterosporous ferns to either of these families? A.4. Recent rbcL and morphology analyses place Plagiogyria as sister group to the tree ferns. Such an affinity has never before been proposed; rather, it has been thought to be more closely related to Osmunda. Is this alliance supported? A.5. Are tree ferns the sister group to the derived "polypodiaceous" clade and can additional data better resolve relationships within the tree ferns (Cyatheaceae, Dicksoniaceae, Lophosoriaceae, Metaxyaceae)? A.6. What are the relationships of the Hymenophyllopsidaceae and Lophosoriaceae, which were not included in earlier studies, and do they bring better resolution to the base of the fern topology? A.7. What can a better resolved fern phylogeny tell us about "deep" tracheophyte relationships? Which pteridophyte group is sister taxon to seed plants? B. Character evolution and species diversification. In addition to addressing phylogenetic questions, this study will subsequently use the resulting best estimate of fern relationships to assess the evolution of selected homologous characters, especially those that may have been correlated to the radiation and diversification of ferns. Of particular interest to this study are causal interpretations of radiations within ferns, when they occurred, and the events leading up to them. Due to the lack of a phylogeny for ferns, questions regarding character evolution and diversification, such as those below, have never been posed and tested in a rigorous fashion. B.1. What is the sequence of apomorphies that might have promoted diversity in ferns and where did they arise? B.2. What specific features of ferns may have promoted their diversity and/or ecological success relative to other groups of plants, in particular, to non-angiospermous seed plants? B.3. Were these radiations triggered by changes in the physical environment, or by the origin of new features in the plants themselves (key adaptations to existing environments vs. new features with incidental effects on speciation rate)? It will be instructive to consider the relative timing of intrinsic and extrinsic events, especially whether a particular climatic or ecological setting existed or was widespread before or after the origin of particular features. B.4. Why did ferns radiate so vigorously in the Cretaceous? Did they gain some key intrinsic feature that favored diversification? N.B. Numbers in parentheses refer to literature citations listed in References. Top of page