Since its separation from the dryopteroid ferns as a distinct group, about 60 years ago, Thelypteridaceae has been treated as a natural group comprising nearly 1000, mostly tropical species. Although generally recognized as a natural monophyletic group, there is a wide divergence of views about generic circumscription. Morton placed all species in a single genus Thelypteris; Holttum characterized 25 genera in the Old World alone; Pichi Sermolli, largely following Holttum, accepted 32 genera; while Smith adopted an intermediate view, recognizing 5 genera. Most of the paleotropical segregates, and several of the neotropical ones, have been recently revised or monographed, making this one of the best known fern families morphologically, cytologically, and distributionally. Little, however, is known about relationships between these segregates. Data from four chloroplast genes (rps4 + rbcL + trnS spacer, + trnL spacer; 2600 base pairs) for 23 of the recognized segregates show the family to be monophyletic and sister to an unresolved alliance of blechnoid, athyrioid, onocleoid, and woodsioid ferns. The family itself comprises two primary lineages, one phegopterid, the other thelypterid. The phegopterid lineage includes those elements within Thelypteridaceae that are the most dissected and morphologically distinct: Macrothelypteris, Pseudophegopteris, and Phegopteris. Within the thelypterid lineage, three predominantly north-temperate subgroups, including Thelypteris s.s., form a free-veined clade that is in turn sister to the rest of the family. All segregates possessing x =36 (Cyclosorus sensu Smith, with predominantly anastomosing veins) form a strongly supported clade. Those groups with dysploid base chromosome numbers (x=27, 29, 30, 31, 33, 34, 35) form a series of smaller clades basal to Cyclosorus s.l. Although our sampling is as yet insufficient to favor one classification over another, the present analysis suggests that recognition of an intermediate number of genera may be the most reasonable taxonomic course.

Key words: and trnS spacer, phylogeny, rbcL, rps4, Thelypteridaceae, trnL spacer