The family Dryopteridaceae includes some of the most species-rich genera in the ferns, including Athyrium, Dryopteris, and Polystichum–the subject of this inquiry. In this work we identified the divergence events leading up to the origin of Polystichum and discerned the major lineages within this large genus. Our approach was to use molecular data from the chloroplast gene rbcL to infer the phylogenetic history of selected species of Polystichum and its traditional allies. We sampled 23 Polystichum species representing at least 11 of the 22 described sections and 10 species from allied genera. The Polystichum species came from seven of the eight world centers of diversity for the genus. Ten additional rbcL sequences came from Genbank. We found support for a monophyletic narrowly defined Polystichum, excluding a group of Asian species, P. tripteron, P. deltodon, and P. lepidocaulon. All species of Polystichum can be included in a monophyletic genus Polystichum sensu lato if Cyrtomium is included in the larger genus. The sister group of Polystichum sensu lato is a tropical American group including Phanerophlebia and Polystichopsis. At the limits of our immediate inquiry we found a trichotomy leading to 1) a monophyletic group including all the aforementioned taxa, 2) Dryopteris plus Arachniodes, and 3) the single New-World Ctenitis included in the sample. We found that Rumohra and Didymochlaena were both remote from Polystichum in our analysis. Within the genus, the taxa sampled from a continent often constitute a monophyletic group, most notably those from Africa and tropical America. We intend to pursue the more recent phylogenetic history of the genus by addressing diversification within continents using the intergenic spacer between rbcL and atpb.

Key words: Dryopteridaceae,phylogeny,Polystichum